How to Use the Mortgage Overpayment Calculator

The free Mortgage Overpayment Calculator helps you choose between shortening the term and lowering the monthly instalment. Enter your outstanding balance, rate and remaining term, then add a monthly extra, a lump sum, or both. Results update live with interest saved, time saved, a balance chart and a year-by-year table. You can also solve for the extra payment needed to finish by a chosen date.

Step-by-Step Guide

1. Enter Your Mortgage Details

Use figures from a recent statement where possible:

Currency: Select your preferred currency (USD, EUR, GBP and others).
Outstanding Balance: The amount still owed, not the original loan.
Annual Interest Rate: The current rate on the deal you are modelling.
Remaining Term: Years and months left on the current contract.
Current Monthly Payment: Optional. Leave blank to use the standard repayment formula. Enter your real instalment if the lender rounds the figure.
Saved inputs: values stay in this browser. Use Clear saved inputs to return to defaults.

2. Build an Overpayment Strategy

Monthly Overpayment: Extra paid every month on top of the contractual instalment.
One-off Lump Sum: A bonus, inheritance or savings sweep applied once.
Apply Lump Sum After: Delay the lump sum by a number of months if the cash is not available today.
Target payoff: Tick the box to solve the monthly extra that finishes the mortgage by a date you choose. The lump sum you entered is kept; only the monthly extra is solved.
Penalty-free cap: Typical fixed-rate deals allow about 10% of the outstanding balance per year before an early repayment charge (ERC). The calculator warns if year-one extras look larger than that cap — it does not deduct a fee.

3. Compare the Two Lender Treatments

Use the toggle above the results:

Reduce term: Keep the contractual payment and pour extras into principal. The mortgage finishes earlier and usually saves the most interest. Ask your lender to apply overpayments this way.
Reduce monthly payment: After any lump sum, the instalment is recalculated over the original remaining term. Your monthly outgoing falls, but you stay in debt for longer and save less interest. Ongoing monthly extras, if entered, are still applied on top of the new instalment.

Contractual payment: M = P × r(1+r)ⁿ / ((1+r)ⁿ − 1)

Each month, interest is charged on the remaining balance (r = annual rate ÷ 12). Anything above that interest reduces principal. That is why an extra pound paid this month saves interest on every month that follows.

4. Read the Results

Interest Saved: Baseline remaining interest minus interest under the selected strategy.
Time Saved: How much sooner the balance hits zero versus making only the contractual payment.
New Payoff: Remaining term and an estimated calendar month, measured from today.
New Monthly Outgoing: What leaves your account each month under the selected strategy.
Year-by-Year Breakdown: Paid, interest, principal and balance, with the no-overpay balance alongside for comparison.

Common Scenarios & Examples

Scenario: A modest monthly extra

Goal: See whether $200 a month is worth the cash-flow stretch on a typical repayment mortgage.

Select your currency (e.g. USD).
Outstanding Balance $200,000.
Annual Interest Rate 4.5%.
Remaining Term 25 years.
Monthly Overpayment $200, lump sum $0.
Leave the toggle on Reduce term.

Sample inputs:

Balance: $200,000
Rate: 4.5% · Term: 25 years
Monthly overpayment: $200

Approximate results (reduce term): contractual payment ≈ $1,112 · paid off about 6 years 1 month earlier · interest saved ≈ $36,280

Scenario: Lump sum vs keeping the payment

Goal: Compare how a lender can treat a $20,000 lump sum — shorter term or cheaper instalment.

Balance $200,000, rate 3%, remaining term 20 years.
Monthly overpayment $0, lump sum $20,000 immediately.
Switch the results toggle between Reduce term and Reduce monthly payment.

Same lump sum, two treatments:

Reduce term: finish about 2 years 7 months earlier and save ≈ $15,041 of interest, keeping the original instalment.
Reduce payment: term stays 20 years, monthly payment falls by about $111, interest saved ≈ $6,621.

Keeping the instalment and shortening the term almost always saves more interest. Reducing the payment is useful if you need the monthly breathing room more than the interest saving.

Scenario: Finish by a date

Goal: Work backwards from “mortgage-free in 15 years” to the extra you must pay each month.

Enter your live balance, rate and remaining term.
Tick “Find the monthly overpayment needed to finish by a target date”.
Set 15 years (and 0 months).
Read the solved monthly extra in the insight box and results grid. Add any planned lump sum first — the solver keeps it and only changes the monthly figure.

Disclaimer & Limitations

Not Professional Advice: This calculator is an estimation and educational tool and does not constitute financial, mortgage or legal advice. Speak to your lender and, if needed, a qualified adviser before changing payments.

Assumptions: Repayment mortgage only (not interest-only). The interest rate is held constant. Interest is applied monthly. Fees, product-rate ends, insurance and inflation are ignored. Early repayment charges are not deducted from the savings figure — the cap field is a warning only.

Lender rules differ: Some lenders default to reducing the payment after a lump sum unless you ask them to reduce the term. Confirm how your overpayment will be applied.

Your home may be repossessed if you do not keep up repayments on your mortgage.

Bug Reports and Suggested Improvements

Please email all suggestions for improvements and any bug reports to: mortgageoverpaymentcalc@personalfinances.me, thank you.

Mortgage Overpayment Calculator – Cut Your Term & Interest