The Mission

Traveling should be effortless. But constantly pulling out a calculator to figure out what something actually costs in your own currency breaks the moment. Koin was built to fix that — a converter so fast and simple it gets out of the way.

Key Features

Instant conversion as you type — no submit button, no delay
150+ currencies — every major and regional currency worldwide
Offline-first — rates are cached locally for up to 24 hours
One-tap swap — reverse home and travel currencies instantly
Locale-aware formatting — comma or period decimal separator, your choice
Haptic feedback — tactile confirmation on every keypress
No account required — open the app, convert, done

Tech Stack

Koin is a full-stack mobile product, from the React Native app to the custom backend:

React Native & Expo — cross-platform iOS and Android from a single codebase
Expo Router — file-based navigation, clean and predictable
React Query & MMKV — intelligent rate caching with fast native storage
React Native Unistyles — theme-aware styling with dark mode support
React Native Reanimated — smooth, hardware-accelerated animations
Bun & Hono — lightweight backend API serving exchange rates with Redis caching
EAS Build & Submit — automated builds and store submissions via Expo’s CI

Architecture

The app lives in a Bun monorepo alongside the API and shared packages. The backend runs at koin.awinter.dev and proxies exchange rates from ExchangeRate-API with a Redis cache layer — keeping requests fast and rate limits in check. The mobile app fetches rates per base currency and persists them locally so users always have a result, even without a connection.

The Journey

Started in early 2026 as a personal tool for my own travels. What began as a quick side project grew into a fully published iOS and Android app — built, signed, and shipped entirely solo.

Available on the App Store and Google Play