Design & engineering · 2026 · active
Mainspring
You wind it up — and it drives everything.
Mainspring is a financial-independence planner that runs entirely on your machine. Your accounts, balances, and assumptions live in a single file you own; nothing is sent anywhere, and there is no account to create. It is built for the careful planner — the sort who keeps a notebook — and it is quick enough that changing one assumption redraws every projection before you lift your finger. You wind it up once, set your assumptions, and it drives everything from there.
The instrument
A projection engine you can trust
Monte-Carlo and deterministic FIRE projections run on your machine, in milliseconds. Adjust a contribution and the retirement horizon redraws before you lift your finger.
Drift and what-if, side by side
Model a sabbatical, a market decade, a paid-off house. Every scenario is a saved lathe setting you can return to — never a spreadsheet you have to rebuild from memory.
Your ledger never leaves the desk
Accounts, balances, and assumptions live in a local SQLite file you own. It runs with no network connection at all — no account, no server, no telemetry — and because the file is a plain, open format, it opens on its own even if you move on from the app.
Further views
Specification
Under the hood
- Platform
- macOS · Windows · Linux (desktop)
- Storage
- Local SQLite, single portable file
- Network
- None required · fully offline
- Core
- Rust projection engine
- Shell
- Tauri + Svelte
- Built with
- Rust · Tauri · SQLite · TypeScript · Svelte