Finance⏱ 5 min read
How Mortgage Overpayments Save You Thousands in Interest
Every pound of mortgage overpayment saves around two pounds in total interest over the life of the loan. Here is the formula, the best overpayment strategy, and the 10% overpayment limit trap.
The maths of mortgage overpayment is startlingly in your favour, because any extra capital paid now saves you interest on the remaining balance for every subsequent year of the loan. The compounding works in reverse.
How Overpayments Work
Standard mortgage payment: interest + capital repayment
When you overpay, the extra goes directly to capital.
Lower capital = lower interest charged next month
= more of your regular payment goes to capital next month.
This positive spiral is why overpayments save disproportionately:
Example: £200,000 mortgage, 4.5% rate, 25-year term
Standard monthly payment: £1,100/month
Total paid over 25 years: £330,000
Total interest: £130,000
With £100/month overpayment (£1,200/month total):
Term reduced to approximately 22 years 6 months
Total paid: £324,000
Total interest: approximately £113,000
Interest saving: £17,000 (saving £17k by paying £30k extra -- net uplift)
Overpayment Savings Calculator Logic
Interest saved from a single lump sum overpayment:
For a mortgage with n months remaining at monthly rate r:
Each £1 of capital repaid today saves:
Interest saved = £1 x r x n (approximate for long-term)
More precisely: the NPV of reduced future interest payments
£5,000 lump sum overpayment:
£200,000 at 4.5% (r=0.00375/month), 200 months remaining:
Approximate interest saved: £5,000 x 0.00375 x 200 = £3,750
(actual savings is slightly higher due to compounding)
Rule of thumb: at 4-5% mortgage rates,
a lump sum overpayment saves roughly 75-100% of itself in interest
over the remaining life of a 15-20 year mortgage.
The 10% Annual Overpayment Limit
Most fixed-rate mortgages allow overpayments of up to 10% of the
outstanding balance per year without early repayment charges (ERC).
On a £200,000 mortgage:
10% allowance: £20,000/year (£1,667/month above standard payment)
Exceeding this: ERC of typically 1-5% of the excess applies
ERC example: overpay by £5,000 more than the 10% limit
ERC at 2%: £5,000 x 2% = £100 charge
In most cases: the interest saved by overpaying still exceeds the ERC.
But always check your specific mortgage terms.
Tracker and variable rate mortgages:
Usually have unlimited overpayment rights with no ERC.
Best Overpayment Strategy
Option 1: Reduce monthly payment (keep term same)
You pay less each month; money freed up can be invested or spent.
Best if: mortgage rate lower than investment return available.
Option 2: Keep payment same, reduce term (standard overpayment)
You pay off the mortgage faster; interest saved is the return.
Best if: mortgage rate higher than guaranteed savings rate.
At current mortgage rates (4-5%): the guaranteed return of overpaying
exceeds most cash savings rates -- overpayment is generally optimal.
At mortgage rates of 2-3% (when rates were low):
Stocks and shares investing at 6-7% expected return
outperformed the guaranteed mortgage overpayment benefit.