How it works
A plain-language explanation of how the calculators work, how your data is handled, and what powers the site.
The Calculators
All calculations run in your browser
No data is sent to a server when you calculate. The math happens entirely on your device using standard financial formulas — the same ones used by banks and spreadsheets.
Loan & mortgage math
Monthly payments are calculated using the standard amortization formula: M = P × [r(1+r)ⁿ] / [(1+r)ⁿ − 1], where P is the principal, r is the monthly rate, and n is the number of payments. Each row in the schedule is derived from this.
Extra payments
When you add extra monthly or yearly payments, the schedule recalculates from scratch — each extra payment reduces the balance faster, which lowers the interest charged in subsequent months and shortens the loan.
Results are estimates
Calculators use the inputs you provide. Real loans may include fees, rate adjustments, or rounding differences. Always verify with your lender before making financial decisions.
Saving Your Work
Free account required
To save calculations you need a free account. Sign up with your email — no credit card, no paid plans. Your account exists solely to associate saved calculations with you.
What gets saved
When you save a calculation we store the inputs (loan amount, rate, term, etc.) and the key results (monthly payment, total interest). The full amortization schedule is not stored — it is recalculated from the inputs when you load a saved entry.
Loading a saved calculation
Clicking Load on a saved calculation takes you back to the calculator with all your original inputs pre-filled. You can then adjust any value and recalculate.
Your Data & Privacy
Authentication via Supabase
User accounts and sessions are managed by Supabase, a hosted Postgres database platform. Passwords are hashed — we never store them in plain text. Session tokens are stored in secure, HTTP-only cookies.
Row-level security
Each saved calculation is linked to your user ID. Database-level row security policies ensure you can only read, create, or delete your own data — even if someone knew your record IDs.
No tracking or analytics
We do not run third-party analytics scripts. The only external script that may load is Google AdSense for ads, which has its own privacy policy.
Technology
Next.js 15 + React
The site is built with Next.js 15 using the App Router. Pages that need user data (like /saved) are server-rendered — your session is verified on the server before any data is returned.
Deployed on Cloudflare Workers
Every page is served from Cloudflare's global edge network, which means the server runs close to wherever you are in the world. Deployments happen automatically on every push to the main GitHub branch.
Open source
The full source code is publicly available on GitHub. You can inspect every calculation, every database query, and every piece of UI.
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Every technology, component, and design decision explained in depth.