摘要
An amortization schedule is a table showing how each mortgage payment splits between principal (paying down the loan) and interest (the cost of borrowing). Early in the mortgage, most of each payment goes to interest. Over time, the balance shifts until the final payments are almost entirely principal. Understanding this split helps you see how quickly you’re building equity and how much extra payments can save.
工作原理
Every month, interest accrues on your outstanding balance. Your fixed monthly payment covers that interest first, and whatever is left reduces the principal:
- Interest this month = Outstanding balance × monthly rate
- Principal this month = Monthly payment − interest this month
- New balance = Old balance − principal this month
Because the balance shrinks each month, the interest portion of each payment decreases and the principal portion grows. This self-accelerating effect means you build equity faster as the mortgage matures.
Why early payments are mostly interest
On a £200,000 loan at 4.5% over 25 years:
- Month 1: £750 interest, £362 principal (67% goes to interest)
- Month 150 (halfway): roughly equal split
- Month 300 (final year): £24 interest, £1,088 principal (2% goes to interest)
This is why early overpayments are so powerful — every extra pound of principal you pay now prevents interest from accruing on it for the entire remaining term.
公式
Where
Monthly breakdown
For each month k:
| Step | Formula |
|---|---|
| Interest portion | Balance_(k-1) × r |
| Principal portion | M − Interest |
| Remaining balance | Balance_(k-1) − Principal |
计算示例
£200,000 at 4.5% over 25 years
Calculate monthly payment
= £1,111.66/month
Month 1 breakdown
= £361.66 principal + £750.00 interest
Month 2 breakdown
= £363.02 principal + £748.64 interest
Year 1 totals
= Balance after year 1: £195,569
Year 25 totals (final year)
= Balance after year 25: £0
Result
Total repayable: £333,499. Total interest: £133,499 (67% of the original loan). Year 1 is 67% interest; Year 25 is just 2% interest.
输入说明
- Loan amount — the total amount borrowed (property price minus deposit, or outstanding balance if remortgaging)
- Interest rate — the annual interest rate as a percentage
- Mortgage term — how long the mortgage runs, in years
输出说明
- Monthly payment — the fixed amount you pay each month
- Total repayable — monthly payment × number of months
- Total interest — total repayable minus the loan amount
- Amortization table — a month-by-month or year-by-year breakdown showing payment, principal, interest, and remaining balance for each period
- CSV export — download the full schedule as a spreadsheet
假设与局限
- The schedule assumes a fixed interest rate for the entire term. Most UK mortgages have a 2–5 year fixed period; the rate (and therefore the schedule) changes when you remortgage or revert to SVR.
- The calculator models repayment mortgages only. Interest-only mortgages pay no principal each month, so there is no amortization — the full balance is due at the end.
- No overpayments or underpayments are modelled. Use the overpayment calculator for scenarios involving extra payments.
- Monthly payment amounts are calculated to full precision. In practice, lenders may round to the nearest penny, causing tiny differences in the final payment.
- The schedule does not account for fees, insurance, or other costs — it shows the loan repayment mechanics only.
验证
| Test case | Loan | Rate | Term | Monthly payment | Total interest |
|---|---|---|---|---|---|
| Standard | £200,000 | 4.5% | 25yr | £1,111.66 | £133,499 |
| Low rate | £200,000 | 2.0% | 25yr | £847.72 | £54,315 |
| Short term | £200,000 | 4.5% | 15yr | £1,529.99 | £75,399 |
| Large loan | £500,000 | 4.5% | 30yr | £2,533.43 | £412,034 |
| Zero rate | £200,000 | 0% | 25yr | £666.67 | £0 |
Accounting identity: Total interest = (Monthly payment × n) − Loan amount. For every row: Payment = Principal + Interest. Final balance = £0.