Kotlin Multiplatform: Sharing Code Across Android, iOS, and Web

We’ve all been there—building a mobile app for Android, and just when you think the logic is polished, it’s time to build the same thing again for iOS. Then someone suggests adding a web version too, and before you know it, your core business logic is duplicated across three platforms. It’s tedious, error-prone, and time-consuming. Enter Kotlin Multiplatform—a game-changer that lets you write once and run (almost) anywhere.

In this guide, we’ll break down everything you need to know about Kotlin Multiplatform (KMP), especially if you’re curious about how it lets you share code across Android, iOS, and Web. Whether you’re a startup founder looking to speed up development or a developer tired of rewriting the same logic, this one’s for you.

What is Kotlin Multiplatform?

Kotlin Multiplatform is a feature of the Kotlin language developed by JetBrains that allows you to share common code between different platforms such as Android, iOS, web (via JavaScript), and even desktop apps (via JVM or native binaries). Unlike solutions like Flutter or React Native, which rely on a single codebase and custom UI rendering engines, KMP encourages code reuse in the logic layer while keeping the native UI layer intact.

This hybrid approach respects platform conventions and offers a lot of flexibility. You get to keep your native UI (so your app feels just right on every platform) while skipping the rewrite of shared business logic like networking, database access, and domain models.

Why Kotlin Multiplatform is a Big Deal

Let’s say you’re building a note-taking app. With Kotlin Multiplatform, you can write your core logic—like how notes are saved, synced, encrypted, and parsed—once. Then, you plug that logic into Android using Jetpack Compose, into iOS using SwiftUI, and even into a web app using Kotlin/JS. That’s a lot of time and cost saved.

This is exactly what Kotlin Multiplatform excels at: sharing code across Android, iOS, and Web while still delivering a native experience.

Setting Up a Kotlin Multiplatform Project

Starting a KMP project might seem a bit overwhelming at first, but JetBrains has made it increasingly developer-friendly with tools like IntelliJ IDEA and Android Studio. Here’s a basic overview of the structure:

project-root/
│
├── shared/                   // Shared module
│   ├── src/
│   │   ├── commonMain/       // Common code (Kotlin)
│   │   ├── androidMain/      // Android-specific code
│   │   ├── iosMain/          // iOS-specific code
│   │   └── jsMain/           // JS-specific code
│
├── androidApp/              // Android UI
├── iosApp/                  // iOS UI
└── webApp/                  // Web frontend

You use Gradle (Kotlin DSL) to configure your build, and the kotlin {} block inside your build.gradle.kts defines your targets.

kotlin {
    android()
    ios()
    js(IR) {
        browser()
    }

    sourceSets {
        val commonMain by getting {
            dependencies {
                implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.7.0")
            }
        }
        // other source sets omitted for brevity
    }
}

This structure allows you to maintain one codebase for all shared logic and plug in platform-specific code when needed.

Sharing Code Across Android, iOS, and Web

Here’s where it gets exciting. Imagine writing your business logic, validation functions, or API calls in one place (commonMain) and using them seamlessly in your Android, iOS, and Web projects.

Here’s a real-world example: Suppose you’re building a weather app. You need to fetch weather data from an API and parse it.

In commonMain:

class WeatherService {
    suspend fun getWeather(): WeatherData {
        val response = httpClient.get("https://api.weather.com/data")
        return parseWeather(response)
    }
}

In Android, you can use it like this:

val weather = weatherService.getWeather()
displayWeather(weather)

On iOS, using Kotlin/Native interop with Swift:

let weatherService = SharedWeatherService()
weatherService.getWeather { weather in
    updateUI(weather)
}

On Web, via Kotlin/JS:

window.onload = {
    GlobalScope.launch {
        val data = WeatherService().getWeather()
        displayWeatherInBrowser(data)
    }
}

One function. Three platforms. That’s the magic of Kotlin Multiplatform.

Dependency Management in KMP

You’re not alone in the shared code—KMP plays well with many popular libraries like:

• Ktor for HTTP client/server
• SQLDelight for cross-platform SQLite
• Kotlinx.coroutines for async operations
• Kotlin Serialization for parsing JSON

You’ll add these as dependencies in your shared module and KMP handles the rest.

Kotlin Multiplatform vs Flutter vs React Native

It’s tempting to lump Kotlin Multiplatform with Flutter and React Native, but they serve different goals.

Feature	Kotlin Multiplatform	Flutter	React Native
UI Framework	Native UI	Custom	Custom
Code Sharing	Business Logic	Full UI + Logic	Full UI + Logic
Performance	Native	Near-native	JS Bridge
Language	Kotlin	Dart	JavaScript
Best For	Native devs wanting shared logic	Rapid UI dev	Web devs moving to mobile

The bottom line? If your team has native Android/iOS devs and you want code reuse without sacrificing native performance or design, Kotlin Multiplatform is the way to go.

Real-World Use Cases and Companies Using KMP

Companies like Netflix, Philips, and VMware are already using Kotlin Multiplatform in production. Here are a few real-world cases:

• VMware uses it to build internal tools with shared logic across web and desktop.
• Netflix experimented with KMP for their studio apps, which needed shared logic across platforms.
• Touchlab (a major contributor) uses KMP in client projects to unify Android and iOS development.

The takeaway? It’s no longer an experiment—it’s a production-ready solution.

Challenges You Might Face

Kotlin Multiplatform isn’t perfect. You might run into:

• Build Configuration Complexity: Multi-target Gradle scripts can be tricky.
• Tooling Maturity: Xcode integration is still evolving.
• Library Compatibility: Not all Android libraries are multiplatform-ready.
• Learning Curve: You’ll need to learn how to split code across common and platform-specific modules.

Still, the benefits—especially in large-scale or long-term projects—often outweigh the hiccups.

Kotlin Multiplatform in the Web Ecosystem

Kotlin/JS has come a long way. You can compile Kotlin code to JavaScript and build full-featured web apps. Combine it with frameworks like KVision or integrate with React wrappers via kotlin-wrappers.

Imagine writing your form validation logic once and reusing it in both your Android app and your React frontend. That’s the kind of real-world power Kotlin Multiplatform offers.

Is Kotlin Multiplatform the Future?

Kotlin Multiplatform: Sharing Code Across Android, iOS, and Web isn’t just a convenience—it’s a powerful strategy for modern cross-platform development. JetBrains continues to invest heavily in KMP, and the community around it is thriving.

As mobile teams grow, backend systems become more integrated, and frontend needs expand to web and beyond, having a shared logic layer becomes not just nice to have, but essential.

FAQs

1. Can I use Swift UI with Kotlin Multiplatform?
Yes, you can write UI in Swift and connect it to shared Kotlin logic.

2. Is Kotlin Multiplatform production-ready?
Absolutely. Many companies are already using it in live apps.

3. What’s the difference between Kotlin Multiplatform and Flutter?
KMP shares logic only and keeps native UI, while Flutter builds the entire app UI with Dart.

4. Do I need to learn Swift for iOS with KMP?
Yes, for the UI layer. KMP doesn’t replace Swift or UIKit.

5. Is Kotlin/JS good enough for web development?
It’s getting better. For logic-heavy web apps, it’s quite practical.

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