01 / START WITH YOUR NUMBERS
How to use the mortgage overpayment calculator
Use your remaining balance and term
Enter what is outstanding now, not the original mortgage amount. The remaining term is the number of years left under the current schedule.
Add the extras you want to explore
A one-off payment is applied immediately. The regular extra is then added to every monthly payment until the mortgage is cleared. You can use either type on its own or combine them.
Compare time and interest saved
The result compares the overpayment plan with paying only the original scheduled payment. It assumes the scheduled payment stays the same so the mortgage finishes earlier.
02 / THE NUMBERS IN PRACTICE
A worked example
The starting example uses a £200,000 repayment mortgage with 25 years remaining, a constant 4.5% annual interest rate and an extra £200 each month. No lump sum or early repayment charge is included.
£36,280.39
- Original monthly payment
- £1,111.66
- Payment including monthly extra
- £1,311.66
- Time saved
- 6y 1m
- Interest with overpayments
- £97,219.09
- Total paid including lump sum
- £297,219.09
Fixed illustration using the starting inputs, separate from the live calculator above. Displayed amounts are rounded.
Repayment mortgages only. Keeps your original scheduled payment, adds monthly overpayments and shortens the term. Applies the lump sum before the first month's interest. No overpayment penalties included.
03 / UNDERSTAND YOUR RESULT
Why earlier payments can reduce later interest
Each month's interest is calculated on the remaining balance. Paying down that balance sooner means less interest is added in following months. The same scheduled payment can then clear more capital, which helps shorten the term.
A lump sum reduces the balance before the first month of this projection. If the lump sum equals the whole balance, the calculator shows immediate repayment and no future interest. In reality, a lender's settlement figure can include accrued interest or charges.
04 / UNDERSTAND YOUR RESULT
Check the lender's treatment of an overpayment
An overpayment may reduce the term or lead a lender to recalculate the monthly payment. This model keeps the original payment unchanged. If your lender reduces that payment, the interest and time savings can be different.
Some mortgages have overpayment limits or early repayment charges. Those charges are not subtracted from the savings shown. Check your agreement before making an extra payment; MoneyHelper's early repayment guide explains considerations to review.
PLAIN ENGLISH, PLEASE
Mortgage overpayment glossary
The terms behind the inputs and results, explained without the guesswork.
- Outstanding balance
- The capital still owed at the starting point of the calculation.
- Scheduled payment
- The regular payment calculated to clear the balance over the remaining term without extras.
- Monthly overpayment
- An additional amount paid on top of the scheduled payment every month.
- Lump sum
- A one-off capital payment applied at the start of this projection.
- Interest saved
- Baseline interest minus the interest calculated with overpayments. It excludes any charges.
- Time saved
- The difference in whole monthly payment periods between the two schedules.
BEHIND THE ESTIMATE
How the calculation works
First we calculate a baseline repayment using the opening balance, annual interest ÷ 12 and the remaining number of months. We then subtract the lump sum and simulate each month, adding interest and subtracting the scheduled payment plus the regular extra.
The last payment is capped at the remaining balance plus that month's interest. We stop once the balance is cleared and compare interest and payment count with the baseline. Rates stay constant and penalties are excluded. At zero interest, extra payments save time but no interest.
Sources and further reading
Examples use Switcha’s calculation model above. Supporting information and linked guidance checked on 10 September 2026.