By Abdelrahman Saed — Senior Mobile Engineer. Last updated: 2026-08-10.
Choose Flutter if you want to share the entire UI across platforms. Choose Kotlin Multiplatform (KMP) if you want to share business logic, data, and networking while keeping fully native UIs per platform. KMP (especially with Compose Multiplatform sharing UI to iOS/Android/desktop) is closing the gap, but the mature, battle-tested choice for "one UI, all platforms" remains Flutter.
For a consumer app where cross-platform UI consistency and velocity matter most, I would still pick Flutter — as we did at iStoria. For a team with strong native iOS and Android expertise who want to deduplicate networking, persistence, and domain logic but keep hand-tuned native UIs, KMP is excellent.
| Feature | Flutter | Kotlin Multiplatform |
|---|---|---|
| Shares | UI + logic (everything) | Logic (UI native or Compose MP) |
| Language | Dart | Kotlin |
| UI on iOS | Flutter engine | SwiftUI (or Compose iOS) |
| UI on Android | Flutter engine | Jetpack Compose |
| Native interop | Platform channels / FFI | Direct (Kotlin ↔ Swift/ObjC) |
| App size impact | ~4-5MB engine | Smaller (no runtime engine) |
| Incremental adoption | All-or-nothing per screen | Module-by-module |
| UI maturity on iOS | Excellent | Compose MP maturing |
| Best for | Cross-platform UI sharing | Shared logic, native UI |
Flutter shares everything: UI, state, logic, navigation — one Dart codebase renders to all platforms via Flutter's engine. You get one UI.
KMP shares logic: viewmodels, repositories, networking, database (SQLDelight), domain models — written once in Kotlin, compiled to JVM and native iOS frameworks. The UI is written separately per platform (Compose on Android, SwiftUI on iOS), unless you use Compose Multiplatform which brings Compose to iOS (still maturing).
This is the fundamental architectural difference and it drives every other trade-off.
Flutter: identical UI across platforms by design. One design system, one set of widgets, one set of bugs. Great for design-driven consumer apps.
KMP: native UI per platform. iOS users get SwiftUI, Android users get Compose. Each platform feels 100% native, but you maintain two UI codebases. Compose Multiplatform reduces this but is newer and less mature than Flutter on iOS.
Flutter ships its own engine (~4-5MB app size increase) and renders everything itself. Performance is excellent and consistent.
KMP compiles Kotlin to native iOS binaries (via Kotlin/Native). There is no runtime engine — logic runs natively. The UI uses platform-native rendering, so there is no rendering overhead. App size impact is smaller than Flutter's. For pure logic sharing, KMP has a performance and footprint edge; for UI sharing, Flutter is more mature.
KMP's superpower is seamless native interop: Kotlin code calls Swift/Objective-C and Java/Kotlin APIs directly, and vice versa. This makes it ideal for incremental adoption in an existing native app — you can share one module (say, networking) and keep the rest native. Flutter's platform channels can call native code but the interop is more ceremonial and less tight.
Flutter needs Dart engineers (or mobile engineers willing to learn Dart — the ramp is short). KMP needs Kotlin engineers comfortable with both Android (Compose) and iOS (SwiftUI), plus Kotlin/Native interop. A team of senior native iOS + Android engineers who already write Kotlin is the ideal KMP adopter; a team that wants maximum sharing with minimum platform-specific expertise should pick Flutter.
// commonMain — shared across iOS and Android
class UserRepository(private val api: HttpClient) {
suspend fun fetchUser(id: String): User {
return api.get("/users/$id").body()
}
}
// androidMain — Compose UI
@Composable
fun UserScreen(vm: UserViewModel) {
val user by vm.user.collectAsState()
Text(user.name)
}
// iosMain — SwiftUI consumes the shared framework
// let repo = UserRepository(api: httpClient)
// let user = try await repo.fetchUser(id: "123")// one codebase — logic AND UI shared
class UserRepository {
final dio = Dio();
Future<User> fetchUser(String id) async {
final r = await dio.get('/users/$id');
return User.fromJson(r.data);
}
}
// same language, same UI on iOS and Android
Consumer(builder: (_, ref, __) {
final user = ref.watch(userProvider);
return Text(user.name);
});Flutter shares the UI too; KMP shares the repository but you write UserScreen twice (Compose + SwiftUI) unless you adopt Compose Multiplatform.
Choose Flutter when: cross-platform UI consistency and velocity are the priority, you want one codebase for UI + logic, your team prefers a single language, or you are building a consumer app where design consistency matters. The default for most new cross-platform apps.
Choose Kotlin Multiplatform when: you have strong native iOS and Android teams who want to keep fully native UIs, you want to deduplicate business logic/data/networking only, you are incrementally introducing sharing into an existing native app, or native interop and footprint are critical. Excellent for engineering-led teams.
Not yet for production iOS in the way Flutter is. Compose Multiplatform on iOS has improved rapidly but is less mature than Flutter's iOS story. For sharing logic now and UI later, KMP is a safe bet; for UI sharing today, Flutter is more proven.
Yes — this is one of KMP's biggest strengths. You can share a single module (e.g. networking or a data layer) in an existing native iOS+Android app and keep everything else native. Flutter is more all-or-nothing per screen.
KMP compiles to native code with no runtime engine, so shared logic has minimal overhead and the UI uses platform-native rendering. Flutter adds its own engine (~4-5MB) but renders consistently. For pure logic, KMP; for UI consistency and smoothness, Flutter is very competitive.
Available for hire. Abdelrahman Saed is a Senior Mobile Engineer (Flutter) — open to full-time, fractional, contract, or advisory work. Hire me →
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