# Mortgage Repayment Calculator

> Work out the monthly payment on a repayment mortgage, how much of the term goes on interest rather than the debt, and what paying extra each month is worth. The formula, its assumptions and its sources are published below, and the year-by-year schedule downloads as a spreadsheet.

Use it: https://tessalor.com/en/money/mortgage-repayment

This tool runs entirely in the browser. Nothing entered into it is uploaded.

## Formula

```
i = annual rate / 12          (or (1 + annual rate)^(1/12) - 1)
n = 12 x term in years

payment = P x i x (1 + i)^n / ((1 + i)^n - 1)

Then, for each month until the balance reaches zero:
  interest  = balance x i
  balance   = balance + interest - (payment + overpayment)

The final payment is whatever settles the balance, so it is
a few pounds different from all the others.

where
  P = amount borrowed, less any one-off overpayment
  i = the monthly interest rate
  n = the number of contractual payments
```

## Assumptions

- The rate never changes. A fixed deal lasts two to five years, not the whole term, so the payment shown after the fix ends is a guess and nothing more.
- Interest is charged on the balance at the start of each month and the payment is applied at the end of it. This is the ordinary-annuity convention the payment formula itself assumes, and the two have to agree or the schedule will not finish where the formula says it should.
- Interest is carried at full precision rather than rounded to the penny each month. A lender that rounds monthly will differ by a few pence over a 30-year term.
- Overpayments are assumed to shorten the term, not to reduce the payment. That is the default at nearly every lender, but it is worth checking, because the alternative saves far less.
- The one-off overpayment is credited with the first month's payment, so it earns its full saving from the start.
- Nothing else is included. Product fees, valuation fees, buildings insurance, ground rent, service charges and any tax are all on top of the figure shown.

## Inputs

| Name | Label | Type | Default | Range |
| --- | --- | --- | --- | --- |
| `amount` | Amount borrowed | currency | 300000 | 1000 to 10000000 |
| `rate` | Interest rate | percent | 6.5 | 0 to 20 |
| `years` | Term | integer | 30 | 1 to 40 |
| `overpayment` | Extra paid each month | currency | 200 | 0 to 20000 |
| `lumpSum` | One-off overpayment | currency | 0 | 0 to 10000000 |
| `interestBasis` | The monthly rate is | select | nominal |  |

## Outputs

- `monthlyPayment` — Monthly repayment (currency), primary
- `totalInterest` — Interest over the term (currency)
- `totalPaid` — Total repaid (currency)
- `firstMonthInterest` — Interest in the first payment (currency)
- `interestSaved` — Interest saved by overpaying (currency)
- `timeSaved` — Term cut by (duration)
- `payoffTime` — Cleared in (duration)

## Questions

### Are my borrowing figures sent anywhere?

No. The calculation runs in this browser tab and there is no server to send it to. Nothing about the amount you are borrowing, the rate you have been offered or what you can afford to overpay leaves the device.

### Why is nearly all of my first payment interest?

Because interest is charged on the whole balance and the balance is at its largest on day one. On 300,000 at 6.5% the first payment of 1,896.20 is 1,625.00 of interest and 271.20 off the debt. The share shifts every month as the balance falls, but on a 30-year term the two halves do not cross until month 233.

### Does overpaying reduce my monthly payment?

Usually not. Most lenders keep the payment the same and shorten the term, which is what this tool assumes and which saves considerably more. Some will recalculate the payment downwards instead if you ask; that keeps the end date and lowers the monthly cost, so it saves much less interest.

### Why does another calculator show a slightly different payment?

Almost always because of the monthly rate. Dividing the annual rate by twelve and taking its twelfth root are both defensible, and they differ: 300,000 at 6.5% over 30 years is 1,896.20 a month on the first basis and 1,859.66 on the second, a gap of about 36 a month. The advanced options let you switch between them.

### Is this the whole monthly cost of the house?

No. It is capital and interest on the loan and nothing else. Product fees, buildings insurance, property tax, service charges and any mortgage protection are all extra, and together they are frequently a fifth again on top.

## Sources

- [Appendix J to Part 1026: Annual Percentage Rate Computations for Closed-End Credit Transactions](https://www.consumerfinance.gov/rules-policy/regulations/1026/j/) — Consumer Financial Protection Bureau, applies to 2026. Retrieved 2026-07-30.
- [Directive 2014/17/EU on credit agreements for consumers relating to residential immovable property, Annex I](https://eur-lex.europa.eu/eli/dir/2014/17/oj) — Publications Office of the European Union, applies to 2026. Retrieved 2026-07-30.
- [Primary Mortgage Market Survey](https://www.freddiemac.com/pmms) — Freddie Mac, applies to 2026. Retrieved 2026-07-30.

## Variants

- [What does overpaying 200 a month do to a mortgage?](https://tessalor.com/en/money/mortgage-repayment/overpaying-200-a-month)
- [How much of my mortgage payment goes on interest?](https://tessalor.com/en/money/mortgage-repayment/how-much-of-my-payment-is-interest)

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Estimate, not advice. See https://tessalor.com/en/disclaimer.
Machine-readable catalogue: https://tessalor.com/api/tools.json