🔓 openBIM Standards, Open-Source & Interoperability Tools

IfcOpenShell is an open-source software library for working with Industry Foundation Classes files. It exposes the full IFC schema to C++ and Python, converts IFC geometry into meshes with the Open CASCADE kernel, and provides a high-level authoring API that Bonsai is built on. It is the de facto engine behind many free IFC viewers, converters and validation services. Developers, BIM managers and researchers use it to automate model checks, extract quantities, batch-fix files and build custom openBIM pipelines.

Vendor

IfcOpenShell project (community, led by Thomas Krijnen and Dion Moult)

Current version

0.8.5 (stable); 0.9.0 alpha builds · 2026

First release

2011

Official site

ifcopenshell.org

Platforms

WindowsmacOSLinux

Deployment

desktopserverweb

Pricing model

Open source

Free tier

Fully free; LGPL-3.0-or-later (core library), community funded via Open Collective

Native formats

.ifc.ifcxml.ifczip

Exchange formats

IFCOBJDAEglTFSVGCSVODSXLSXBCFIDS

IFC

import: Yes · export: Yes

openBIM notes

Complete parsing for IFC2x3 TC1, IFC4 ADD2 TC1, IFC4x1, IFC4x2 and IFC4x3 ADD2. Ships IfcTester (IDS 1.0 validation to HTML, ODS, JSON and BCF), IfcCSV (schedule export/import), IfcPatch (scripted model fixes), IfcConvert (geometry conversion) and BCF-XML libraries.

Automation & APIs

Python API (ifcopenshell, ifcopenshell.api)C++ libraryIfcConvert CLIIfcPatch recipesIfcCSV CLIIfcTester CLI

Learning curve

High — Requires Python skills and a working understanding of the IFC schema.

Typical users

BIM developersBIM coordinatorsBIM managersResearchersInformation managers

Works with

Bonsai (formerly BlenderBIM Add-on)FreeCAD (BIM workbench)BlenderAutodesk RevitGraphisoft ArchicadBIMserver (open source)Specklexeokit SDK

Alternatives

xBIM ToolkitThat Open Engine (@thatopen/components + web-ifc)BIMserver (open source)

Best for: Programmatic IFC processing: checking, extracting, transforming and generating models in Python without a commercial authoring tool.

Market notes: Widely used as the IFC engine inside other tools; the PyPI package targets Python 3.10 to 3.14.

Official training & certification

• IfcOpenShell documentation · documentation
• IfcTester (IDS) documentation · documentation
• OSArch community forum · community

Sources: github.com/IfcOpenShell/IfcOpenShell · pypi.org/project/ifcopenshell · github.com/IfcOpenShell/IfcOpenShell/releases · docs.ifcopenshell.org/ifctester.html · Last verified 2026-09-15

Bonsai is a free and open-source Blender extension that turns Blender into a native IFC authoring tool. Instead of translating a proprietary model into IFC on export, Bonsai edits the IFC file directly, which makes it a reference implementation for openBIM workflows. It covers architectural modelling, structural analysis models, cost schedules, 4D sequencing, drawings and issue tracking. It is used by architects, engineers and researchers who want full control of IFC data, and by BIM managers as a free IFC editor for fixing models produced elsewhere. The project still describes itself as alpha software.

Vendor

IfcOpenShell project / OSArch community

Current version

Bonsai 0.8.5 (stable); 0.8.6 and 0.9.0 alpha builds · 2026

First release

2019

Official site

bonsaibim.org

Platforms

WindowsmacOSLinux

Deployment

desktop

Pricing model

Open source

Free tier

Fully free; GPL-3.0-or-later; optional community donations

Native formats

.ifc

Exchange formats

IFCBCFIDSCSVODSSVGPDFDXFglTFOBJ

IFC

import: Yes · export: Yes

openBIM notes

Native IFC authoring: the IFC file is the model, so there is no export step. Supports IFC2x3 and IFC4 authoring with IFC4x3 support developing, BCF issue management, IDS authoring and checking through IfcTester, bSDD classification lookup and 2D drawing generation from IFC.

Automation & APIs

IfcOpenShell Python APIBlender Python APIBlender Extensions platform

Learning curve

High — Blender navigation plus IFC concepts must be learned together.

Typical users

ArchitectsBIM managersStructural engineersResearchersStudents

Works with

IfcOpenShellBlenderFreeCAD (BIM workbench)Autodesk RevitGraphisoft ArchicadBIMvisionSpeckleBIMcollab (Nexus / Zoom / BCF Managers)

Alternatives

FreeCAD (BIM workbench)Autodesk RevitGraphisoft ArchicadVectorworks Architect

Successor

Bonsai is the current name; BlenderBIM Add-on was renamed to Bonsai with the 0.8.0 release in 2024

Best for: Anyone who needs to author or repair IFC data directly without a commercial licence, or wants to learn how IFC really works.

Market notes: Distributed through the Blender Extensions platform and bonsaibim.org; adoption is strongest among openBIM advocates, academia and small practices.

Official training & certification

• Bonsai documentation · documentation
• Bonsai download and install guide · documentation
• OSArch community forum · community

Sources: bonsaibim.org/download.html · github.com/IfcOpenShell/IfcOpenShell/releases · github.com/IfcOpenShell/IfcOpenShell · docs.bonsaibim.org · Last verified 2026-09-15

FreeCAD is a general-purpose open-source parametric 3D modeller. Since version 1.0 it ships a built-in BIM workbench that combines the former Arch and BIM workbenches, offering walls, structures, windows, spaces, schedules and 2D drawing output. IFC handling relies on the bundled IfcOpenShell library, and the native IFC project mode lets users work on the IFC file itself. It is used by small practices, engineers and educators who want a free BIM-capable tool that also covers mechanical and product design.

Vendor

FreeCAD project (FPA - FreeCAD Project Association)

Current version

FreeCAD 1.1.3 · 2026

First release

2002

Official site

freecad.org

Platforms

WindowsmacOSLinux

Deployment

desktop

Pricing model

Open source

Free tier

Fully free; LGPL-2.1-or-later

Native formats

.FCStd.ifc

Exchange formats

IFCDWGDXFSTEPIGESOBJDAESTLSVGPDFglTF

IFC

import: Yes · export: Yes

openBIM notes

The built-in BIM workbench (merged Arch + BIM since 1.0) imports and exports IFC through IfcOpenShell and offers a native IFC mode where the IFC file is edited directly. FreeCAD 1.1 added basic IfcType support, BIM Views panel and drawing view tools.

Automation & APIs

Python console and macrosFreeCAD Python APIIfcOpenShell (bundled)Addon Manager

Learning curve

High — Workbench-based interface takes time; BIM-specific tutorials are limited.

Typical users

Small architecture practicesEngineersEducatorsMakersStudents

Works with

IfcOpenShellBonsai (formerly BlenderBIM Add-on)BlenderAutodesk RevitGraphisoft ArchicadSketchUp (Pro / Studio)BIMvision

Alternatives

Bonsai (formerly BlenderBIM Add-on)BricsCAD BIMSketchUp (Pro / Studio)Graphisoft Archicad

Best for: Users who need one free tool for both building modelling and precise mechanical or product modelling.

Market notes: FreeCAD 1.1 was released on 24 March 2026; the 1.1.3 maintenance release (25 July 2026) contains security fixes for file handling.

Official training & certification

• FreeCAD 1.1 release notes and news · documentation
• FreeCAD downloads and documentation · documentation
• OSArch community forum · community

Sources: blog.freecad.org/2026/03/25/freecad-version-1-1-released · github.com/FreeCAD/FreeCAD/releases · freecad.org/downloads.php · Last verified 2026-09-15

xBIM (eXtensible Building Information Modelling) is an open-source .NET toolkit for developers who need to process IFC data in C#. Xbim.Essentials handles schema parsing and writing, Xbim.Geometry generates tessellated geometry, and Xbim.WebUI (WeXplorer) renders WexBIM files in a browser. Additional packages support COBie exchange and IDS validation. It is used by software vendors, in-house development teams and researchers building Windows and cloud services around IFC.

Vendor

xBIM Team

Current version

Xbim.Essentials 6.1.605 · 2026

Official site

docs.xbim.net

Platforms

WindowsmacOSLinux

Deployment

desktopserverweb

Pricing model

Open source

Free tier

Fully free; CDDL licence (permissive, allows use in commercial products)

Native formats

.ifc.ifcxml.ifczip.xbim

Exchange formats

IFCIFC-XMLIFCZIPCOBieWexBIMIDS

IFC

import: Yes · export: Yes

openBIM notes

Supports IFC2x3 TC1, IFC4 (ADD2) and IFC4x3 through separate packages; one interface for STEP, ifcXML and ifcZip. Companion packages cover COBie (Xbim.CobieExpress) and IDS validation (Xbim.IDS.Validator).

Automation & APIs

.NET API (C#)NuGet packagesXbim.Geometry engineXbim.WebUI / WeXplorer WebGL viewer

Learning curve

High — Aimed squarely at C# developers familiar with the IFC schema.

Typical users

Software developersBIM developersResearchersEnterprise IT teams

Works with

IfcOpenShellAutodesk RevitAutodesk Navisworks (Manage / Simulate / Freedom)BIMserver (open source)Autodesk BIM Interoperability Tools

Alternatives

IfcOpenShellThat Open Engine (@thatopen/components + web-ifc)BIMserver (open source)

Best for: .NET shops that need an embeddable IFC engine in desktop, server or Azure applications.

Market notes: Xbim.Essentials 6.1.605 was published to NuGet on 15 July 2026.

Official training & certification

• xBIM Toolkit documentation · documentation
• xBIM Quick Start · documentation

Sources: github.com/xBimTeam/XbimEssentials · nuget.org/packages/Xbim.Essentials · docs.xbim.net · Last verified 2026-09-15

BIMserver is a Java-based open-source model server maintained by the opensourceBIM collective. It parses IFC into a database so models can be versioned, queried, merged and served to other applications through APIs. Plugins add export serialisers, model checking and integrations such as BIMvie.ws for browser viewing. It is used by research groups, universities and organisations building their own model-server or CDE-like infrastructure around IFC.

Vendor

opensourceBIM collective

Current version

BIMserver 1.5.188 · 2024

Official site

github.com/opensourceBIM/BIMserver

Platforms

WindowsmacOSLinux

Deployment

serverweb

Pricing model

Open source

Free tier

Fully free; AGPL-3.0

Native formats

.ifc

Exchange formats

IFCIFC-XMLBCFCOBieJSONglTFCollada

IFC

import: Yes · export: Yes

openBIM notes

Stores IFC2x3 and IFC4 models as objects in a database rather than files, enabling revisions, queries, merging and partial checkouts. Plugins provide serialisers, deserialisers, model checkers and BCF handling.

Automation & APIs

SOAP, JSON and Protocol Buffers APIsJava plugin frameworkBimServerClient libraries (Java, JavaScript)

Learning curve

Very high — Server administration plus API programming; not an end-user tool.

Typical users

ResearchersSoftware developersUniversitiesEnterprise IT teams

Works with

IfcOpenShellxBIM ToolkitBIMvisionAutodesk RevitGraphisoft Archicad

Alternatives

SpeckleAutodesk Construction Cloud (Autodesk Forma)Trimble Connect

Best for: Self-hosted, database-backed IFC management where full control of the server and its APIs matters more than polished UX.

Market notes: Mostly used in academia and research; the repository saw commits in March 2026 but no new release since July 2024.

Official training & certification

• BIMserver wiki · documentation
• BIMserver Get Started Quick Guide · documentation

Sources: github.com/opensourceBIM/BIMserver · github.com/opensourceBIM/BIMserver/releases · github.com/opensourceBIM/bimserver/wiki · Last verified 2026-09-15

Open IFC Viewer is a free desktop application published by the Open Design Alliance, the consortium behind the Teigha/ODA SDKs. It runs locally on Windows, macOS and Linux with no cloud dependency and reads every major IFC schema including IFC 4x3 for infrastructure. Beyond viewing, it offers measurement, validation, clash detection with BCF export and publishing to 3D PDF. It serves as a showcase for ODA's IFC SDK and as a practical free viewer for engineers who need IFC 4x3 support today.

Vendor

Open Design Alliance (ODA)

Official site

openifcviewer.com

Platforms

WindowsmacOSLinux

Deployment

desktop

Pricing model

Free

Free tier

Fully free, no licence fees

Native formats

.ifc

Exchange formats

IFCBCF3D PDF

IFC

import: Yes · export: No

openBIM notes

Reads IFC 2x3, IFC 4, IFC 4x2 (bridge) and IFC 4x3 (rail and infrastructure). Built on the ODA IFC SDK. Includes clash detection with BCF export and model validation.

Automation & APIs

None for end users (built on the commercial ODA IFC SDK)

Learning curve

Low — Standard viewer navigation; clash setup takes a little longer.

Typical users

Infrastructure engineersBIM coordinatorsSoftware developers evaluating ODA SDKClients

Works with

Autodesk RevitAutodesk Civil 3DBentley OpenRoads DesignerBonsai (formerly BlenderBIM Add-on)BIMvision

Alternatives

BIMvisionusBIM.viewer+Solibri Anywhere

Best for: Reviewing IFC 4x3 infrastructure models for free on any desktop OS.

Official training & certification

• Open IFC Viewer website · documentation

Sources: openifcviewer.com · Last verified 2026-09-15

usBIM.viewer+ is ACCA software's free desktop application for viewing and editing IFC files. Unlike most free viewers it can also add properties, edit classifications, federate several models and export IFC2x3, including converting DWG, SketchUp and mesh formats to IFC. It supports BCF issues and point clouds. ACCA positions it as the entry point to the usBIM cloud platform. It is used by clients, contractors and consultants who need to inspect or lightly enrich IFC data.

Vendor

ACCA software

Official site

accasoftware.com/en/ifc-viewer

Platforms

Windows

Deployment

desktopweb

Pricing model

Free

Free tier

Free desktop viewer/editor; the online usBIM platform has separate plans

Native formats

.ifc

Exchange formats

IFCBCFDWGSKP3DSOBJDAEpoint clouds

IFC

import: Yes · export: Yes · buildingSMART certification: buildingSMART International certified software (IFC2x3) per vendor page

openBIM notes

Opens, edits, federates and exports IFC2x3. Supports BCF, property assignment, classification editing and IFC file comparison. Part of ACCA's wider usBIM ecosystem, which also includes IDS and bSDD-related tools.

Automation & APIs

None documented for the free viewer

Learning curve

Low — Viewer functions are simple; editing features take a few hours to master.

Typical users

ClientsContractorsFacility managersBIM coordinators

Works with

Autodesk AutoCADSketchUp (Pro / Studio)Autodesk RevitGraphisoft ArchicadBIMvision

Alternatives

BIMvisionOpen IFC ViewerBonsai (formerly BlenderBIM Add-on)

Best for: Enriching or converting models to IFC without an authoring licence, especially in Italy and southern Europe.

Market notes: ACCA software is Italian; the tool is widely used in Italian public tenders that mandate IFC deliverables.

Official training & certification

• usBIM.viewer+ product page · documentation

Sources: accasoftware.com/en/ifc-viewer · Last verified 2026-09-15

Solibri Anywhere was the free viewing edition of the Solibri model-checking family. It let anyone open IFC and SMC files, inspect properties, review checking results and communicate through BCF, which made it a common way for clients and site teams to consume coordinated models. Solibri classified it as legacy on 13 April 2026 and discontinued the free entitlement with version 26.4.0; version 25.12.1 is the last release. Users are directed to the paid Solibri Starter or Essential tiers. It remains relevant because many organisations still have it installed.

Vendor

Solibri Inc. (Nemetschek Group)

Current version

Solibri Anywhere 25.12.1 (final) · 2025

First release

2019

Official site

solibri.com/solibri-anywhere

Platforms

WindowsmacOS

Deployment

desktop

Pricing model

Free — Legacy since 13 April 2026; free entitlement removed in Solibri 26.4.0. Successor Solibri Starter is listed at EUR 99 per year on solibri.com/products (seen September 2026).

Free tier

Was free; existing installations prior to 26.4.0 keep working but receive no updates

Native formats

.smc

Exchange formats

IFCBCFSMC

IFC

import: Yes · export: No

openBIM notes

Opened IFC and Solibri SMC files, supported BCF-based issue communication and read-only review of rule-check results produced in Solibri Office.

Automation & APIs

None

Learning curve

Low — Viewer-level use; understanding Solibri rule results takes longer.

Typical users

ClientsSite teamsConsultants reviewing Solibri results

Works with

Solibri (Starter / Essential / Advanced / Premium)Autodesk RevitGraphisoft ArchicadTekla StructuresBIMcollab (Nexus / Zoom / BCF Managers)

Alternatives

BIMvisionOpen IFC ViewerusBIM.viewer+

Successor

Solibri Starter (paid) for IFC inspection; Solibri Essential for coordination and clash detection

Best for: Historical context: understanding existing Solibri-based review workflows and planning migration to Solibri Starter or other free viewers.

Market notes: Solibri's 2026 line-up is Starter, Essential, Advanced, Premium and Security+, with Office, Site, Anywhere and CheckPoint listed as legacy.

Official training & certification

• Solibri Anywhere legacy page · documentation
• Solibri products overview · documentation

Sources: solibri.com/solibri-anywhere · solibri.com/products · Last verified 2026-09-15

That Open Engine is the family of open-source libraries published by That Open Company, the successor to the IFC.js project. web-ifc is a C++ IFC parser compiled to WebAssembly that reads and writes IFC in the browser or Node.js, and @thatopen/components adds ready-made Three.js based tools such as model loading, the Fragments format for streaming large models, BCF issue handling, measurements and floor plans. Developers use it to build custom web viewers, CDE front-ends and lightweight coordination apps without a commercial SDK.

Vendor

That Open Company

Current version

2026

First release

2020

Official site

thatopen.com

Platforms

WebWindowsmacOSLinux

Deployment

webserverdesktopmobile

Pricing model

Open source

Free tier

Fully free: @thatopen/components (MIT), web-ifc (MPL-2.0); optional paid Accelerator Program for support

Native formats

Fragments (.frag)

Exchange formats

IFCglTFDXFBCFJSON

IFC

import: Yes · export: Yes

openBIM notes

web-ifc parses and writes IFC in WebAssembly at native speed; components add IFC loading, Fragments streaming, BCF topics, measurements, plans and classification tools for browser BIM apps.

Automation & APIs

TypeScript/JavaScript APInpm packages @thatopen/components, @thatopen/components-front, @thatopen/fragments, web-ifcNode.js server use

Learning curve

High — Front-end development skills and IFC structure both required.

Typical users

Web developersBIM developersStart-upsUniversities

Works with

IfcOpenShellSpecklexeokit SDKAutodesk RevitGraphisoft Archicad

Alternatives

xeokit SDKxBIM ToolkitSpeckle

Successor

Successor of IFC.js (web-ifc-viewer / openbim-components); rebranded as That Open Company and That Open Engine

Best for: Teams building their own web-based IFC viewer or app under permissive licences.

Market notes: Widely adopted for custom web viewers; the company also offers a paid Accelerator Program for developer support.

Official training & certification

• That Open documentation · documentation
• engine_components repository · documentation
• engine_web-ifc repository · documentation

Sources: docs.thatopen.com/intro · github.com/ThatOpen/engine_components · github.com/ThatOpen/engine_web-ifc · Last verified 2026-09-15

xeokit is a JavaScript SDK developed by Creoox AG for viewing high-detail, full-precision BIM and engineering models in web browsers. Models are converted from IFC, glTF, CityJSON or point clouds into the XKT format, which loads quickly and supports double-precision coordinates for large sites. Plugins provide tree views, section planes, measurements, annotations and BCF viewpoints. It is dual-licensed under AGPL-3.0 and a commercial licence, and is used by CDE vendors and integrators who embed a model viewer in their own products.

Vendor

Creoox AG (xeolabs)

Current version

xeokit-sdk v2.6.114 · 2026

First release

2019

Official site

xeokit.io

Platforms

Web

Deployment

web

Pricing model

Mixed

Free tier

Free under AGPL-3.0; commercial licence available from Creoox for closed-source products

Native formats

.xkt

Exchange formats

IFC (via conversion to XKT)glTFCityJSONLAS/LAZOBJBCF viewpoints

IFC

import: Yes · export: No

openBIM notes

IFC is pre-converted to the compact XKT format (using xeokit-convert and IfcOpenShell-based tooling) for fast browser loading; the SDK includes a BCF viewpoints plugin for issue exchange.

Automation & APIs

JavaScript SDKxeokit-convert CLInpm package

Learning curve

High — JavaScript development plus a conversion pipeline must be set up.

Typical users

CDE and software vendorsWeb developersIntegrators

Works with

IfcOpenShellThat Open Engine (@thatopen/components + web-ifc)BIMcollab (Nexus / Zoom / BCF Managers)Autodesk Revit

Alternatives

That Open Engine (@thatopen/components + web-ifc)xBIM Toolkit

Best for: Embedding a robust, performant IFC viewer in a commercial web product where AGPL or a paid licence is acceptable.

Market notes: Used as the viewer in several commercial CDE and issue-tracking products.

Official training & certification

• xeokit-sdk repository and docs · documentation
• xeokit-sdk releases · documentation

Sources: github.com/xeokit/xeokit-sdk · github.com/xeokit/xeokit-sdk/releases · Last verified 2026-09-15

Autodesk BIM Interoperability Tools is a set of free add-ins that help Revit, Civil 3D and Autodesk Docs users meet information requirements. The Model Checker for Revit runs configurable checksets and reports compliance, the COBie Extension for Revit populates and exports COBie data, and companion tools handle shared parameters, room and area sync, standardised data and equipment data. A paid Validation Tool works on a pay-per-result basis. BIM managers use the suite to prepare deliverables for ISO 19650 and COBie handover requirements.

Vendor

Autodesk

Official site

interoperability.autodesk.com

Platforms

Windows

Deployment

desktopcloud

Pricing model

Free

Free tier

Free add-ins for Revit, Civil 3D and Docs; only the Validation Tool is pay-per-result

Native formats

.xml (Model Checker checksets).rvt

Exchange formats

COBieXLSXIFC (via Revit)

IFC

import: No · export: No

openBIM notes

The COBie Extension for Revit maps model data to COBie worksheets and the Model Checker validates models against configurable checksets; IFC export itself is handled by Revit's built-in exporter.

Automation & APIs

Model Checker checkset XMLShared Parameters ToolStandardized Data Tool

Learning curve

Medium — Revit familiarity plus understanding of COBie and checkset logic.

Typical users

BIM managersRevit usersInformation managersFacility managers

Works with

Autodesk RevitAutodesk Civil 3DAutodesk Construction Cloud (Autodesk Forma)Autodesk Navisworks (Manage / Simulate / Freedom)

Alternatives

Solibri (Starter / Essential / Advanced / Premium)xBIM ToolkitIfcOpenShell

Best for: Revit-based teams that must deliver COBie or validate models against a client checkset without buying a separate checker.

Market notes: Commonly specified in UK and US projects that require COBie deliverables from Revit.

Official training & certification

• Autodesk BIM Interoperability Tools · documentation

Sources: interoperability.autodesk.com · Last verified 2026-09-15

The buildingSMART Data Dictionary is an online service hosting data dictionaries: classification systems, object types, properties, allowed values, units and translations. It gives every concept a stable URI so that IFC models, IDS specifications and authoring tools can reference the same definitions. Dictionary owners such as national classification bodies publish through a management portal, and software reads the content via REST or GraphQL APIs. BIM managers use bSDD to standardise property naming, and IDS authors reference it for machine-readable requirements.

Vendor

buildingSMART International

Current version

2026

Official site

search.bsdd.buildingsmart.org

Platforms

Web

Deployment

cloudweb

Pricing model

Free

Free tier

Free to search and to use through the API; organisations register to publish dictionaries

Native formats

bSDD JSON import model

Exchange formats

JSONExcel (upload template)REST APIGraphQL

IFC

import: No · export: No

openBIM notes

Hosts classifications, properties, allowed values, units and translations that can be referenced from IFC (IfcClassificationReference, property sets) and from IDS specifications. Plugins and integrations exist for authoring tools; the repository is MIT licensed.

Automation & APIs

REST API (Swagger)GraphQL APIExcel and JSON upload templates

Learning curve

Medium — Searching is easy; publishing a dictionary or using the API needs data-modelling skills.

Typical users

Information managersClassification bodiesSoftware developersIDS authors

Works with

Bonsai (formerly BlenderBIM Add-on)IfcOpenShellAutodesk RevitGraphisoft ArchicadSolibri (Starter / Essential / Advanced / Premium)

Best for: Standardising property names and classifications across projects and tools.

Market notes: Used by national classification schemes and by the buildingSMART IDS ecosystem to point requirements at shared definitions.

Official training & certification

• bSDD GitHub documentation · documentation
• bSDD search · documentation

Sources: github.com/buildingSMART/bSDD · Last verified 2026-09-15