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For most existing Qt applications, the safest route to Qt 6 is to first bring the project to Qt 5.15, clear deprecated-API warnings, then port and test against a specific Qt 6 release. Treat the work as both a source-code and runtime migration: module changes, build configuration, Qt Quick rendering, high-DPI behavior and supported platforms can all affect the result.
Start with a Qt 5.15 baseline
Qt’s porting guidance recommends updating a Qt 5 application to 5.15 before upgrading. It is the latest Qt 5 version and gives you a smaller jump to Qt 6 while exposing APIs that may be removed. See The Qt Company’s Porting to Qt 6 guide.
- Make the current build reproducible. Build and test the application with Qt 5.15. Record the operating systems, architectures, compilers, Qt modules, third-party Qt integrations and QML imports the project actually uses.
- Enable deprecation diagnostics. Fix warnings while the application still builds on Qt 5.15. You can define
QT_DISABLE_DEPRECATED_UP_TO=0x050F00to disable APIs deprecated through Qt 5.15 and make remaining use fail at build time. - Save a working baseline. Keep the Qt 5.15 build and test results available as a reference when diagnosing later compile or runtime regressions.
Inventory APIs and modules for your target Qt 6 release
Do not treat Qt 6 as a single, unchanging target. Module availability and changes differ among Qt 6 minor releases. Compare the modules, C++ APIs and QML imports your project uses with the changes documented for the exact release you plan to adopt. The official Qt module changes index identifies removed modules and replacements; the porting guide covers broader porting changes.
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- Check whether a module was removed, moved or otherwise changed in your target release, and identify the replacement before changing includes or imports.
- Review obsolete and deprecated C++ and QML APIs as well as module names.
- Recheck the list if you change the planned Qt 6 minor version.
Choose a build-system path deliberately
CMake: target one major version or support both
If the application uses CMake, decide whether to migrate directly to Qt 6 or keep one source tree that can build with Qt 5.15 and Qt 6. Qt 5.15 introduced versionless targets and commands, which allow compatible project code to refer to targets such as Qt::Core.
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For a dual-version build, Qt documents trying Qt 6 first and falling back to Qt 5.15 when Qt 6 is unavailable:
find_package(Qt6 COMPONENTS Core QUIET)
if (NOT Qt6_FOUND)
find_package(Qt5 5.15 REQUIRED COMPONENTS Core)
endif()
target_link_libraries(myapp PRIVATE Qt::Core)Here, myapp represents your existing CMake target. The Qt 5 and Qt 6 CMake compatibility guide documents the pattern and its constraints:
- A versionless target resolves to the first Qt major version found in a given context. Configure and build separate Qt 5 and Qt 6 variants; do not mix the two major versions in one library or executable.
- Versionless targets are not always the best choice. Versioned targets can be safer if you must support Qt 5 earlier than 5.15, if tool definitions suppress versionless names, or if your library exports targets to consumers.
- Check what your project exports and what its consumers need before changing public CMake target names.
qmake: an application does not have to switch immediately
Qt continues to support qmake for application builds, so moving an application to Qt 6 does not by itself require an immediate switch to CMake. The distinction matters for code that builds Qt components: Qt 6 changed Qt’s own build system to CMake, and custom plugins or libraries tied to Qt 5 build-system internals need to be migrated. See Qt’s build-system changes documentation.
Qt’s Windows instructions for building Qt 6.10 from source specify CMake 3.22 or later and recommend Ninja. That is a requirement for that Qt source-build path, not a blanket minimum for application projects. Check the build requirements for your own application and target release; the Qt 6.10 Windows source-build guide covers the source-build case.
Port and test graphics, QML and display scaling
A successful compile does not establish that a Qt Quick interface renders or behaves the same way. Qt Quick has a new graphics backend in Qt 6, and OpenGL is not guaranteed to be the default on every target platform. Test the application on the graphics backends and devices it actually supports.
- If application code calls OpenGL APIs directly, account for the Qt OpenGL module and check the target release’s API and module requirements.
- Exercise Qt Quick scenes, effects and custom rendering on each supported platform rather than assuming one platform’s result applies everywhere.
- Check Widgets layouts and assets at representative display scales, including fractional scaling. Qt 6 changed the default high-DPI rounding policy from
RoundtoPassThrough; Qt documents settingRoundto restore Qt 5 behavior where Widgets glitches occur.
These changes may not produce the same symptoms in every app. Record the platform, graphics path and scale factor when reporting a visual difference so it can be reproduced.
Validate the application’s real support matrix
Check the supported configurations for the specific Qt minor release you select, then build and run the application on the operating systems, architectures and compilers you intend to ship. Qt describes its supported configurations as actively maintained and tested, and notes that patch releases can drop or replace configurations. The Qt 6.10 supported platforms page is specific to that release.
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- Build and run automated tests on each shipping operating system and compiler combination.
- Exercise the project’s actual QML imports, plugins, graphics paths and display scales.
- Check third-party Qt integrations for compatibility with the chosen major and minor release.
- Keep Qt 5.15 and Qt 6 build results separate when maintaining a dual-version source tree.
Documentation defines Qt’s supported configurations; it cannot predict which regressions a particular codebase will encounter. Those require testing the application itself.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Select a Qt release with support terms in view
Qt’s release and support details are time-sensitive and eligibility depends on the release and license. As listed by The Qt Company on 2026-10-07, Qt 6.10.3 standard support ended on that date, while Qt 6.8.6 LTS standard support ran through 2029-10-08 for commercial users only. Qt 5.15 support is also license-dependent. These dates describe the status shown on that lookup date, not a guarantee of what is available now. Check the Qt 6.10 release table and applicable license terms before choosing a long-term maintenance target.
| Release as listed | Standard-support detail | What to verify |
|---|---|---|
| Qt 6.10.3 | Support ended 2026-10-07, according to The Qt Company’s table on that date | Check the live release table for current status and terms. |
| Qt 6.8.6 LTS | Commercial-only standard support through 2029-10-08, according to The Qt Company’s table on 2026-10-07 | Confirm commercial eligibility, current availability and applicable terms. |
| Qt 5.15 | Support eligibility depends on license; a single universal support period is not established here | Check the release table and your license terms. |
Can one codebase support Qt 5 and Qt 6?
Yes. A shared source tree can use CMake’s conditional package discovery and versionless targets, provided the Qt 5 baseline is at least 5.15 and each build selects one Qt major version. It does not mean one executable or library can combine Qt 5 and Qt 6. If the project exports libraries or must support older Qt 5 releases, assess versioned targets and consumer compatibility before adopting the versionless pattern. Qt’s CMake compatibility guide explains the trade-offs.
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