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Mortgage & Overpayment Calculator

Model a UK-style mortgage with an intro/bonus period, reversion rate, and regular monthly overpayments. See your payments, total interest, and how much earlier you could clear the loan.

Runs fully in your browser with no logins, no ads, and no tracking — ideal for quick “what if I overpay £X a month?” checks.

Inputs

Principal (£) — loan amount

Term (years)

Intro/Bonus period (years)

Intro APR (%)

Reversion APR (%)

Monthly overpayment (£)

Compounding (monthly) standard

Summary

Phase 1 monthly (intro)

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Payment during the intro/bonus period.

Phase 2 monthly (reversion)

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Payment recalculated at switch.

Total interest (no overpay)

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Across both phases, assuming £0 overpayment.

Effective blended rate

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Weighted by months at each APR.

Overpayment Impact

New term with overpay

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Estimated months to clear.

Time saved

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Interest saved

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Total paid (with overpay)

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Principal + interest, including overpayments.

Amortisation snapshot (first & last 3 months)

FAQ

What does this calculator do?: It models a repayment mortgage with an optional intro/bonus period followed by a reversion rate, and shows how a fixed monthly overpayment from month 1 changes your term and interest.
Principal: The amount you borrow (loan balance at the start). Sometimes misspelled “principle” — here we mean the loan principal.
Term: The total agreed length of the mortgage (e.g., 25 years).
APR: Annual Percentage Rate — yearly interest rate (excluding fees) used to compute monthly interest (APR ÷ 12).
Intro/Bonus period: A fixed-rate period (1–5 years is typical) before switching to a reversion/standard rate. This tool sets the initial payment over the full term using the intro APR.
Reversion rate: The APR applied after the intro period ends. At the switch, the tool recalculates the monthly payment using the remaining balance and remaining term.
Overpayment: Extra you choose to pay each month on top of the standard payment. This model applies the same overpayment from month 1 onward. Many lenders cap fee-free overpayments (for example 10% of the balance per year) or charge early repayment fees — always check your mortgage terms.
Does this include fees, taxes or insurance?: No. The results ignore arrangement fees, early repayment charges, taxes, insurance and any other costs. It’s a simplified illustration to explore “what if” scenarios.

Disclaimer

This calculator is for illustrative purposes only and does not constitute financial advice. Results are approximate and exclude fees, taxes, insurance, and lender-specific rules such as overpayment limits or early repayment charges. Always check your offer documents and speak to a qualified adviser before making decisions.