Qt 6 is a major version, and migrating a real Qt 5 application to it is more than a recompile. The framework was modularised, the build system shifted toward CMake, some APIs were removed or replaced, and modules moved around. None of it is insurmountable, but a successful migration is planned, not stumbled into. This guide covers what changed and how to approach the move in 2026.

TL;DR

  • Qt 6 is a major release with breaking changes: a modularised structure, CMake as the primary build system, removed and replaced APIs, and relocated modules
  • The build-system move from qmake to CMake is often the biggest single piece of work
  • Migrate incrementally where possible, using the compatibility helpers Qt provides, and rely on your test suite to catch regressions
  • With Qt 5 past its active support, staying on it is an accumulating risk, so plan the move deliberately

Why Migrate to Qt 6

Qt 5 has reached the end of its active support life, so new features, fixes, and (crucially) security updates flow to Qt 6. Beyond keeping current, Qt 6 brings a modernised C++ baseline, an improved graphics stack, and ongoing investment in QML and tooling. Remaining on Qt 5 is a slowly growing liability rather than a stable resting place.

What Actually Changed

Build system: CMake first. Qt 6 makes CMake the primary build system. qmake still exists but CMake is where the investment and documentation now sit. For many projects the qmake-to-CMake conversion is the largest part of the migration.

Modularisation. Qt was restructured, and some functionality moved out of core modules or into separate ones. Code that relied on implicitly available classes may need explicit module dependencies, and some pieces now live in the Qt 5 Compatibility module.

Removed and changed APIs. Deprecated Qt 5 APIs were removed, and some classes and functions were replaced or changed signature. Common areas include changes around containers, string handling, QRegExp (replaced by QRegularExpression), enums, and parts of the graphics and multimedia stacks.

QML improvements. QML and the associated tooling advanced in Qt 6; QML-heavy apps benefit but may need adjustments.

How to Plan the Qt 5 to Qt 6 Migration

  1. Get on the latest Qt 5 first. Move to the final Qt 5 release and fix all deprecation warnings there. Qt provides compatibility helpers so you can clear most deprecations before you switch major versions.
  2. Convert the build to CMake. If you are on qmake, plan the CMake conversion as its own workstream; it is often the biggest lift.
  3. Update module dependencies. Add explicit dependencies for anything that moved, and pull in the Qt 5 Compatibility module for APIs you cannot immediately replace.
  4. Fix removed and changed APIs. Work through compiler errors from removed APIs, replacing them with their Qt 6 equivalents.
  5. Lean on your tests. A solid automated test suite is what makes this safe; it catches behavioural regressions the compiler cannot.
  6. Migrate incrementally where you can. Tackle modules or components in stages rather than attempting a single big-bang switch, especially for large applications.

Common Pitfalls

  • Underestimating the build-system conversion. Teams budget for API fixes and forget the CMake move is often larger.
  • Third-party dependencies. A dependency that is not Qt 6-ready can block the whole migration; audit these early.
  • Behavioural changes, not just compile errors. Some changes compile fine but behave differently; only tests and QA catch these.
  • Skipping the final Qt 5 step. Jumping straight to Qt 6 without first clearing deprecations on the last Qt 5 makes the move much harder.

Key Takeaways

  • Qt 6 is a major version with real breaking changes; treat migration as a planned project, not a recompile.
  • The qmake-to-CMake conversion is frequently the biggest single task.
  • Get onto the final Qt 5 and clear deprecation warnings before switching, then fix removed APIs and relocated modules.
  • Audit third-party dependencies early and rely on your test suite to catch behavioural regressions.

Get Expert Help with Your Qt Migration

A Qt 5 to Qt 6 migration goes faster and safer with deep framework experience. If you need to hire a Qt developer , Mecanik has 15+ years of Qt experience across Qt 4, 5, and 6, covering Qt Widgets and QML applications and Qt 5 to Qt 6 migrations with full source ownership. For projects with a heavy C++ component beyond Qt, a C++ developer for hire can help. See also the guide to building cross-platform desktop apps with Qt and QML .

Frequently Asked Questions (FAQ)

Is migrating from Qt 5 to Qt 6 difficult? It is a major-version migration with breaking changes, so it is more than a recompile, but it is very manageable with planning. The main efforts are the build-system move to CMake, updating relocated modules, and replacing removed APIs, with tests guarding against regressions.

Do I have to switch from qmake to CMake for Qt 6? qmake still works in Qt 6, but CMake is the primary, actively developed build system and where the documentation and investment are. For long-term maintainability, converting to CMake is recommended, and it is often the largest part of the migration.

Why should I move off Qt 5? Qt 5 has reached the end of its active support, so new features, bug fixes, and security updates go to Qt 6. Staying on Qt 5 is an accumulating risk, particularly for security, which is why a planned migration is worthwhile.

What is the Qt 5 Compatibility module? It is a Qt 6 module that provides APIs removed from the core so you can migrate incrementally. It lets you keep using certain Qt 5-era APIs while you work toward full Qt 6 equivalents, rather than replacing everything at once.

How should I approach a large Qt migration? Get onto the final Qt 5 release and clear deprecation warnings first, convert the build to CMake, update module dependencies, then fix removed APIs, migrating in stages and leaning on your automated tests. Audit third-party dependencies for Qt 6 support early.