Monthly payment, total interest and a year-by-year amortisation, plus what an extra monthly payment or a lump sum actually saves you. No adverts, no email box, no “compare rates” form — every number stays in your browser.
| Year | Interest paid | Principal paid | Balance at year end |
|---|
Runs entirely in your browser. Nothing is uploaded, stored, or sent anywhere.
Search for a mortgage calculator and you will find pages carrying a dozen adverts, a “get your rate” form above the numbers, and a privacy policy explaining that your figures may be shared with lending partners. The calculator is the bait; your details are the product.
This one is a page of arithmetic. There are no adverts, no tracking beyond the site's own basic analytics, no email box, no “compare rates” button, and nothing is sent anywhere — every number stays in your browser. We built it because we wanted one, and it costs us nothing to leave it here.
It sits on a printed-circuit-board design consultancy's site, which is admittedly an odd place for it. That is the honest reason it has no adverts: we are not trying to monetise it.
payment = P · r / (1 − (1+r)−n) — with r the
monthly rate and n the number of payments
The standard amortisation formula, then a month-by-month simulation so extra payments and lump sums behave correctly rather than being approximated. The year-by-year table shows the split between interest and principal, which is the part that surprises people.
Interest is charged on the outstanding balance, so at the beginning nearly all of your payment is interest and almost none of it reduces what you owe. On a 30-year loan at a typical rate, the first year's payments are overwhelmingly interest, and the balance barely moves.
That is also why extra payments early are worth so much more than the same money later. A pound of principal removed in year two never accrues interest again for twenty-eight years; the same pound in year twenty-eight saves almost nothing. Put a modest extra payment in above and watch both the interest total and the payoff date move — the effect is usually much larger than people expect.
For anything that involves a decision, talk to a broker or an adviser who can see the whole picture. This is here to answer “roughly what does that cost, and what happens if I pay a bit more” without three adverts and a form.
Each of these runs entirely in your browser. Nothing is uploaded, stored or sent anywhere, and none of them needs an email address.
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We add tools here fairly often and write up the things worth writing up — a stackup that behaved oddly, a standard that turned out to be obsolete, a calculator that was quietly wrong. Join and you get told when something new lands.
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