Loan & Mortgage Amortization Calculator
This mortgage calculator works out your monthly payment, builds the complete amortization schedule, and shows exactly how much interest and time an overpayment saves. Enter the amount, rate and term and every figure recalculates as you type. The whole simulation runs in your browser, so your numbers are never sent anywhere.
- Price:
- Free, no sign-up
- Data:
- Processed locally
- Reviewed:
Interactive mortgage calculator
Loan details
Input DataOverpayments — optional
Balance over time
Results & OutputOutstanding debt, year by year
Amortization schedule
Results & Output300 payments
| Month | Payment | Interest | Principal | Balance |
|---|---|---|---|---|
| 1 | £1,918 | £1,400 | £518 | £319,482 |
| 2 | £1,918 | £1,398 | £520 | £318,963 |
| 3 | £1,918 | £1,395 | £522 | £318,440 |
| 4 | £1,918 | £1,393 | £524 | £317,916 |
| 5 | £1,918 | £1,391 | £527 | £317,389 |
| 6 | £1,918 | £1,389 | £529 | £316,860 |
| 7 | £1,918 | £1,386 | £531 | £316,329 |
| 8 | £1,918 | £1,384 | £534 | £315,795 |
| 9 | £1,918 | £1,382 | £536 | £315,259 |
| 10 | £1,918 | £1,379 | £538 | £314,721 |
| 11 | £1,918 | £1,377 | £541 | £314,180 |
| 12 | £1,918 | £1,375 | £543 | £313,637 |
| 13 | £1,918 | £1,372 | £545 | £313,092 |
| 14 | £1,918 | £1,370 | £548 | £312,544 |
| 15 | £1,918 | £1,367 | £550 | £311,994 |
| 16 | £1,918 | £1,365 | £553 | £311,441 |
| 17 | £1,918 | £1,363 | £555 | £310,886 |
| 18 | £1,918 | £1,360 | £557 | £310,329 |
| 19 | £1,918 | £1,358 | £560 | £309,769 |
| 20 | £1,918 | £1,355 | £562 | £309,206 |
| 21 | £1,918 | £1,353 | £565 | £308,642 |
| 22 | £1,918 | £1,350 | £567 | £308,074 |
| 23 | £1,918 | £1,348 | £570 | £307,505 |
| 24 | £1,918 | £1,345 | £572 | £306,932 |
| 25 | £1,918 | £1,343 | £575 | £306,357 |
How much interest will I pay on a mortgage?
On a £320,000 mortgage at 5.25% over 25 years the payment is about £1,918 a month and the total interest around £255,000 — roughly 80% of the amount borrowed. Interest is charged on the outstanding balance, so overpaying early removes interest from every remaining month, which is why small regular overpayments clear a mortgage years ahead of schedule.
How to use the mortgage calculator
- Enter the loan: Type the amount borrowed, the annual interest rate and the term in years.
- Add an overpayment: Set a monthly overpayment or a one-off lump sum to see the effect immediately.
- Read the schedule: The chart shows the balance falling; the table lists every payment split into interest and principal.
- Export or share: Download the schedule as CSV, or copy the link — your inputs are encoded in the URL.
How the mortgage calculator works
The amortization formula
A repayment loan is an annuity: a fixed payment M made n times, whose present value equals the amount borrowed P. Solving for the payment gives:
M = P × [ r(1 + r)^n ] / [ (1 + r)^n − 1 ]
where r is the periodic interest rate — the annual nominal rate divided by 12 — and n is the total number of monthly payments. When the rate is zero the formula degenerates and the payment is simply P / n.
How each payment splits
Interest for a month is the balance at the start of that month multiplied by r. Whatever is left of the payment reduces the principal. Because the balance falls every month, the interest share falls and the principal share grows — slowly at first, then sharply. On a 30-year loan at 6%, the halfway point in balance is not reached until roughly year 20.
Why overpayments compound
An overpayment is applied directly to the principal, so it removes the interest that principal would have accrued over every remaining month. This is why £100 a month extra on a 25-year mortgage typically clears it years early: the saved interest itself buys down more principal the following month.
Nominal rate vs APR
This calculator uses the nominal annual rate divided by 12, the convention almost every lender uses for monthly instalments. An APR additionally amortises arrangement fees across the term, so it will read slightly higher than the rate you enter here.
Mortgage Calculator — frequently asked questions
With the standard annuity formula: M = P x r(1+r)^n / ((1+r)^n - 1), where P is the principal, r the monthly rate (annual / 12) and n the number of monthly payments. Every figure on the page derives from a payment-by-payment simulation of that schedule, not an approximation.
Yes, and the effect is front-loaded. Interest is charged on the outstanding balance, so a pound paid in year 1 removes interest for every remaining year. The calculator reduces the balance by your overpayment each month and keeps the contractual payment fixed, which is how most lenders shorten the term rather than lower the payment.
No. The figures cover principal and interest only, so they match the loan itself rather than an escrow payment. Add property tax, insurance and any service charge separately when budgeting.
Never. The whole simulation runs in your browser in JavaScript. There is no server call, no account and no analytics on your inputs — the only place your numbers appear is the shareable URL, which you control.
Official resources & government references
Verified references, primary standards specifications, and official publications governing the rules and calculations implemented in this tool:
Official Bank Rate & Monetary Policy Decisions
The official UK central bank interest rate informing variable and fixed mortgage benchmark pricing.
Mortgage Affordability & Repayment Guidance
Impartial, government-backed mortgage guidance, budget checklists, and borrowing limits.
Consumer Mortgage Information & Rights
US federal consumer guide explaining loan estimates, APR formulas, and closing disclosures.
Important Disclaimer
Mortgage calculations, amortizations, and interest savings are mathematical estimates based on constant nominal interest rates. They do not factor in lender arrangement fees, valuation charges, private mortgage insurance (PMI), or local stamp duty / transfer taxes. This tool does not constitute a loan offer or financial advice.