As cross-platform development gains traction, Kotlin continues to grow as a powerful toolkit for building applications that run across Android, iOS, desktop, and the web. Two standout technologies in this space are Kotlin Multiplatform (KMP) and Compose Multiplatform. While both are developed by JetBrains and fall under the Kotlin ecosystem, they serve fundamentally different purposes.
This blog post breaks down the key differences between Kotlin Multiplatform and Compose Multiplatform, when to use each, and how they can work together in modern software development.
What is Kotlin Multiplatform (KMP)?
Kotlin Multiplatform (KMP) allows developers to share code across different platforms. It is particularly effective for sharing non-UI logic, such as:
- Business rules
- API integrations
- Database operations
- ViewModels
- Analytics logic
KMP promotes the philosophy: “Share logic, not UI.”
Instead of rewriting the same logic in Swift, Java, JavaScript, or C#, you write it once in Kotlin and share it across Android, iOS, Web, Desktop, and beyond.
Common use cases for KMP:
- A single ViewModel powering both iOS and Android screens
- Shared API request logic using Ktor
- Shared database layer using SQLDelight
What is Compose Multiplatform?
Compose Multiplatform is a declarative UI framework for building cross-platform user interfaces using the same Kotlin-based Jetpack Compose API. Initially launched for Android, it now supports:
- Android
- Desktop (Windows, macOS, Linux)
- Web (via WASM or JS)
- iOS (currently in experimental phase)
Compose Multiplatform lets you build and reuse UI components across platforms using a single codebase.
Common use cases for Compose Multiplatform:
- A note-taking app with a consistent UI on Android, Desktop, and Web
- A dashboard app with charts and real-time updates across devices
- A design system with shared theming, typography, and components
Side-by-Side Comparison
| Feature | Kotlin Multiplatform (KMP) | Compose Multiplatform |
| Primary Purpose | Share core logic across platforms | Share UI across platforms |
| Target Layer | Business logic, networking, DB | Frontend UI |
| UI Support | Handled separately (SwiftUI, XML) | Shared Compose UI code |
| Platforms Supported | Android, iOS, Web, Desktop | Android, Web, Desktop (iOS WIP) |
| Tech Examples | Ktor, SQLDelight, Coroutines | Jetpack Compose, Material Theme |
| Code Reuse Scope | Logic only | UI and logic (if paired with KMP) |
| Use with Native UI | Yes | Optional (can still write native) |
| Recommended For | Multi-platform logic sharing | Cross-platform UI consistency |
Can KMP and Compose Multiplatform Work Together?
Yes. In fact, using KMP for logic and Compose Multiplatform for UI is a powerful architecture.
- KMP manages API calls, state, data storage, and business logic
- Compose Multiplatform renders UI across Android, desktop, and web
- On iOS, you can still use SwiftUI + KMP for logic, or experiment with Compose for iOS
This combination gives you the best of both worlds: shared code where it matters, and platform-specific code where it adds value.
When Should You Use Each?
Use Kotlin Multiplatform when:
- You want to reuse networking, ViewModel, or repository logic
- You’re building for both iOS and Android but want native UIs
- You need testable, isolated logic shared across layers
Use Compose Multiplatform when:
- You want to reuse UI components across platforms
- Your target platforms include desktop or web
- You aim to reduce UI maintenance costs and ensure consistency
Real-World Example
Scenario 1: KMP Only
A fintech app uses KMP to share login logic, financial calculations, and user session handling. Android uses Jetpack Compose; iOS uses SwiftUI. The result is platform-native UIs powered by shared logic.
Scenario 2: KMP + Compose Multiplatform
A productivity tool uses KMP for logic and Compose Multiplatform for UI across Android, macOS, and web. All platforms share ViewModels and screens, making it easier to scale, maintain, and update.
Conclusion
Kotlin Multiplatform and Compose Multiplatform are powerful in their own right, but they serve distinct roles:
- KMP helps you write and maintain shared business logic
- Compose Multiplatform helps you write and maintain shared UI
Together, they allow Kotlin developers to build modern, cross-platform apps with high efficiency and minimal redundancy.
If you’re planning a cross-platform app, understanding when and how to use each of these tools can save development time and deliver a smoother user experience across devices.
