Mortgage calculator for website visitors, with a table

The maths behind the monthly figure

A standard repayment mortgage uses the annuity formula. With a loan amount P, a monthly interest rate r (the annual rate divided by 12) and n monthly payments, the payment is P × r × (1 + r)^n ÷ ((1 + r)^n − 1). Each month, interest is charged on the remaining balance and the rest of the payment reduces it, which is why early payments are mostly interest and late ones mostly capital.

An interest-only mortgage is simpler: the monthly payment is the balance times the monthly rate, and the full loan is still owed at the end. Show the two side by side if your visitors are choosing between them, and make the end-of-term balance impossible to miss.

Inputs and outputs worth including

Property price and deposit: Ask for both and work out the loan and loan-to-value, rather than asking for the loan directly. Visitors think in prices.
Interest rate: An editable field with a default you set. Label it as an example rate, with the date you last updated it.
Term: In years, commonly from 5 to 35 or 40, with the payment updating as it moves.
Repayment type: Repayment or interest-only.
Overpayment: An optional monthly overpayment, showing the months saved and the interest saved.
Results: Monthly payment, total repaid, total interest, and the date the loan ends.

The amortisation table

A year-by-year table (opening balance, interest paid, capital repaid, closing balance) is what turns a calculator into something people trust. Offer a monthly view behind a toggle for those who want it, and a simple chart of balance over time. Round displayed money to whole units or pence consistently, but keep full precision in the calculation so the last row lands on zero.

Fixed periods and rate changes

Many mortgages have an initial fixed or discounted rate followed by a different rate. You can ask for a two-stage calculation (rate A for the first few years, then rate B), which shows visitors the payment jump to plan for. Keep the second rate clearly marked as an assumption.

Describing it to Anticode

Spell out the defaults and the wording: "A mortgage payment calculator for a broker's site. Inputs: price, deposit, rate (default 4.5%, marked as an example rate updated October 2026), term (default 25 years), repayment or interest-only, optional overpayment. Show monthly payment, total interest, end date, a yearly amortisation table and a balance chart. Under the result, say it's an estimate, not advice, and offer a call." The model writes it as one app that recalculates as the visitor types. A contact form in the app sends their enquiry, with their inputs, to you through Anticode's form pipeline.

Test it against a calculator you trust: put the same price, deposit, rate and term into both and compare the monthly figure to the penny. Then check edge cases, such as a zero deposit, a 0% rate (the formula divides by zero, so it needs a separate rule) and a term of one year.

Honest labelling

Anticode apps run with no network, so a mortgage calculator for website use can't fetch today's rates from a lender or a market feed. The rate is whatever you, or the visitor, type in. That's normal for calculators of this kind, but it must be visible. Put three things near the result:

That the rate is an example and when it was last updated.
That the figure is an estimate and doesn't include fees, insurance or tax unless you've added them.
That it isn't financial advice and doesn't mean anyone will lend that amount.

If you're a regulated adviser or broker, your regulator may have rules about how illustrations and promotions are worded; check them, and use your own approved wording in the request.

What it doesn't do

No live rates or product comparisons. It can't search lenders or show real deals.
No affordability decision. You can add a simple income-multiple illustration, but it isn't a lender's assessment.
No saved scenarios for you to see. Visitors' inputs stay in their browser unless they send the enquiry form.
No stamp duty or local tax rules unless you supply the bands; and you'll need to update them when they change.

Questions people ask

Can I update the default rate myself?

Ask for the change in a sentence ("set the default rate to 4.2% and the updated date to November") and the app is rebuilt with it.

Can it work in my currency?

Yes, ask for the currency symbol and format you want. There's no automatic currency conversion.

Can visitors download the amortisation table?

You can ask for a print-friendly view or a CSV download of the table; the browser handles the file.