An amortization schedule generator that builds a complete month-by-month payment table for any fixed-rate loan or mortgage. Each row splits the payment into interest and principal and shows the falling balance — so you can see precisely how a loan is paid down, and how an extra monthly payment shortens the term. For borrowers comparing offers, planning overpayments, or checking a lender’s figures.
How it works
First the fixed monthly payment is set with the amortising formula M = P · r(1+r)ⁿ ⁄ ((1+r)ⁿ − 1), where r is the monthly rate (annual rate ÷ 12 ÷ 100) and n is the term in months. Then the tool walks month by month:
- Interest = current balance × monthly rate
- Principal = (fixed payment + any extra) − interest
- New balance = balance − principal
It repeats until the balance hits zero, totalling the interest and counting the months to get the payoff time. Early rows are interest-heavy; later rows are principal-heavy.
Example
A £200,000 loan at 5.5% over 25 years has a payment of about £1,228/month:
- Month 1: interest ≈ £917, principal ≈ £311, balance ≈ £199,689
- Total interest over 25 years: about £168,000
Add £200 extra each month and the loan clears in roughly 20 years, cutting total interest by around £40,000.
| Month | Payment | Interest | Principal | Balance |
|---|---|---|---|---|
| 1 | £1,228 | £917 | £311 | £199,689 |
| 120 | £1,228 | £676 | £552 | £146,900 |
| 300 | £1,228 | £6 | £1,222 | £0 |
Everything is calculated in your browser; no loan details ever leave your device.
Reading the schedule: what each column tells you
The schedule has four columns that together reveal the full story of your loan:
- Payment: your fixed monthly payment, which stays constant for the life of the loan (with optional extra payments treated separately)
- Interest: the amount going to the lender as interest for that month — highest in month 1, lowest in the final month
- Principal: the amount actually reducing your debt — lowest in month 1, highest at the end
- Balance: the amount still owed after this payment — confirms the loan reaches exactly $0 on the final payment
The interest column shrinking and the principal column growing is called amortization in action. In the early years of a 25-year mortgage, you might pay 75% interest and 25% principal; by the final years those proportions flip completely.
The power of extra payments
The most impactful feature of the schedule is the extra payment field. Even modest additional principal payments have outsized effects because every extra pound or dollar immediately lowers the balance — and a lower balance means less interest charged the following month, which means more of every future payment goes to principal. This is a compounding benefit.
For a concrete illustration: on a £200,000 mortgage at 5.5% over 25 years:
| Extra monthly payment | Loan term | Total interest saved |
|---|---|---|
| £0 (standard) | 25 years | Baseline |
| £100/month | ~22.5 years | Significant savings |
| £200/month | ~20 years | Larger savings |
| £500/month | ~16.5 years | Substantial savings |
The exact figures depend on your rate and balance, but the direction is always the same: extra principal payments cut both time and total interest, with the savings growing non-linearly as the extra amount increases.
Comparing loan offers
The schedule is also useful for comparing loan terms side-by-side. A loan with a slightly lower rate but longer term may cost more in total interest than a higher-rate loan paid off faster. Run the schedule for each offer and compare the total interest line at the bottom — that is the real cost of the loan beyond the principal.
Why the formula works the way it does
The amortizing payment formula M = P · r(1+r)ⁿ / ((1+r)ⁿ − 1) is derived from the present value of an annuity. It sets the payment so that at the stated interest rate, paying M every month for n months results in the balance reaching exactly zero — no more, no less. This is why the final payment often differs slightly from the others: rounding of interest each month accumulates a small residual that is settled on the last payment.