Colophon

One engine, copied everywhere

This site has ten calculators and sixty loan pages. All of them share a single amortization engine: a handful of pure functions with no dependencies, no network calls, and no framework underneath. The engine is copied into every page rather than loaded from one shared file, because a self-contained page paints faster and cannot half-load.

Copies drift. That is the real risk of the design, so a test holds every copy byte-identical to every other and fails the build if one character moves. More than four thousand automated checks run before any change ships.

Check the anchor yourself

Nothing below asks you to trust this site. Take a $400,000 loan at 6.5% over 30 years:

• Monthly principal and interest: $2,528.27
• Total interest over the full term: $510,178
• Total repaid: $910,178, which is more than twice what you borrowed

That payment comes from the standard amortization formula. Multiply the loan by the monthly rate, multiply the result by (1 plus the monthly rate) raised to the number of payments, then divide by that same power minus one. The monthly rate is the annual rate divided by 12. There is nothing proprietary in it. It is the same formula your lender uses, and any other calculator should give you the same $2,528.27.

Those three figures are also baked into the $400,000 at 6.5% page at the moment it is generated, so they are checkable on the live site without running a single line of script.

Where the math approximates, and where we say so

Every tool that simplifies something names the simplification on its own page. These are not disclaimers buried at the bottom. Each one is a calculation the page runs live, while you watch.

Bi-weekly payments

Most calculators model a bi-weekly plan as one twelfth of a payment added each month. That is close, but it is not what happens. A real accelerated plan makes 26 half payments on a calendar that drifts against the months. On the anchor loan, the shortcut misses the true calendar by $1,154 of interest. The bi-weekly page runs the real 26 period calendar and prints the gap instead of hiding it.

PMI removal

Extra payments move one of your two PMI dates and not the other. You can ask to cancel at 80% of the original value, and that runs on your actual balance, so paying extra pulls the date closer. Automatic termination at 78% runs on the scheduled balance by law, so extra payments do not move it at all. The PMI page shows both dates and will not let the first one flatter the second.

Recast against prepay

A recast lowers your payment. Prepaying the same money keeps the payment and kills the loan sooner. They are the same math seen from opposite ends, and the recast page proves it while you watch: recast, then keep paying the old amount, and you land on the prepay result to the cent.

Prepaying against investing

Paying down a 6.5% mortgage earns a guaranteed 6.5%. The comparison page runs both roads to the same date on the same cash, including the part most pitches skip: once the loan dies early, the whole freed payment goes into the account. Set the assumed return equal to your rate and the two roads finish to the cent apart. The page rechecks that identity every time it recalculates.

The payoff check

A payoff quote is not your balance. It is your balance plus interest for every day the wire lands after the due date. The payoff amount page prices that daily interest and then proves it: the per diem figure times 365, divided by 12, reproduces the next interest line in the schedule, to the cent.

The schedule

The schedule page adds its own columns back up in front of you. Scheduled principal plus extra equals the loan amount, to the cent. It also splits the saving, which is the finding we did not expect: of the $111,892 that a $200 monthly habit saves on the anchor loan, only $27,652 sits in the payments you deleted. The other $84,240 comes from the payments you still make, each one carrying a smaller interest line than it used to.

What this site will not do

There is no Calculate button. Results update as you type. A button exists to manufacture a moment, not because the arithmetic needs one, and you should be wary of any calculator that makes you wait for an answer it already has.

There is no sign-in, no email capture, and no saved history, because there is nothing to save. Your figures never leave your browser. They travel between tools inside the page address, which is why you can bookmark a plan or send it to someone.

There are no car loan, student loan, or body mass calculators here. Sites that sprawl into every vertical end up answering nothing particularly well. This one answers a single question: how do you get out of a mortgage sooner, and what does that cost you.

And every payoff figure on this site is principal and interest only. Paying off your mortgage does not end your housing bill. Property taxes and insurance carry on for as long as you own the house, and any tool that shows you a triumphant $0 without mentioning that is selling you something.

How it is built

One self-contained HTML file per page: design tokens, then markup, then a single inline script. No build step, no framework, nothing to keep updated. The charts are hand-written SVG rather than a charting library, because page weight is part of how a page ranks and a chart library would cost more than the chart is worth.

Three typefaces, all self-hosted so no font server learns what you read: Bricolage Grotesque for display, Public Sans for text, IBM Plex Mono for every figure and date. The figures are set in mono deliberately. Digits then line up in columns, and a number that changed is easy to spot.

The motif running through the site is a ledger correction: the payoff date the schedule had planned, struck through in red, and the date your plan actually reaches, stamped beside it in green. Red is what the loan takes from you. Green is what you keep.

It is served as static files from Cloudflare’s edge. Every deploy stamps the exact commit that shipped, and a watchdog checks the live site for the failures that stay quiet: a stalled deploy, a lost security header, an accidental instruction to search engines to forget the page.

Privacy

Nothing you type is stored or sent anywhere. The full detail, including one honest caveat about shared links, is in the privacy policy.