How is this different from a single overpayment calculator?

A standard calculator models one overpayment against a baseline. This tool runs three independent strategies at once on the same mortgage, so you can directly compare, for example, "a steady amount every month" versus "an annual bonus lump sum" versus "a big lump now plus a small monthly top-up" — and instantly see which clears the debt soonest and saves the most interest.

How is the interest saved calculated?

The tool simulates the mortgage month by month. Each month it charges interest on the outstanding balance, applies your contractual repayment, then applies any overpayment straight to the principal. It runs the same simulation with no overpayments as a baseline, and the interest saved is simply the baseline total interest minus each strategy's total interest.

Is a monthly overpayment or a one-off lump sum better?

It depends on timing and amount. A lump sum paid early removes principal sooner, so it usually saves more interest per pound than the same money spread out — but a steady monthly overpayment is easier to sustain and compounds over the whole term. Enter both as separate strategies and the chart and table will show you the exact difference for your numbers.

Does overpaying always shorten the term?

In this model, yes. Every extra pound goes straight to the principal while the contractual repayment stays fixed, so the balance reaches zero sooner and the term shortens. In real life some lenders instead reduce your monthly payment unless you ask to keep it level, and some cap penalty-free overpayments at around 10 percent of the balance per year, so check your mortgage terms.

Are early repayment charges included?

No. Many fixed-rate deals charge an early repayment charge (ERC) if you overpay beyond an annual allowance, often a percentage of the amount overpaid. This tool models the pure interest mathematics, so subtract any ERC your lender would apply before deciding. Once a deal's fixed period ends, overpayments are usually penalty-free.

Is my financial information private?

Completely. Every figure is calculated locally in your browser using JavaScript. Nothing you type is uploaded, logged or sent to any server, and your inputs are saved only in this browser's local storage so the page remembers them next time.

Mortgage Overpayment Scenarios

Most mortgage calculators answer one question at a time: “what happens if I overpay by this much?” This tool answers three at once. Enter a single mortgage, then line up three different overpayment strategies and watch them race to clear the debt. You get a ranked table of interest saved and years cut, a clearly marked winner, and a chart that draws every balance curve against the no-overpayment baseline so the differences are obvious at a glance.

It is built for the real decisions people agonise over: should I drip-feed a fixed amount every month, throw my annual bonus at it once a year, or drop a big lump sum now and add a small top-up? Those choices move the finish line by years and the interest bill by tens of thousands, and eyeballing them is almost impossible — small differences in when money hits the principal compound across decades. Seeing all three side by side makes the trade-off concrete.

How it works

The tool models a standard repayment (capital-and-interest) mortgage. From your balance, rate and term it derives the fixed contractual monthly payment using the amortising-loan formula. Then it simulates the mortgage one month at a time: it charges a month of interest on the outstanding balance, applies the contractual payment, and then applies any overpayment from the strategy straight to the principal. Because overpaid money never accrues future interest, the balance falls faster and the term shrinks.

Each strategy can combine two levers: a regular overpayment (a set amount every month, or once a year to model a bonus) and a one-off lump sum that lands in a month you choose. The same simulation runs with no overpayments at all to produce the baseline. Interest saved is the baseline’s total interest minus the strategy’s total interest; time cut is the difference in months to clear. Everything recomputes instantly as you type, and all three strategies plus the baseline are drawn on one balance-over-time chart.

Example

Take a £240,000 mortgage at 4.5% over 25 years — a contractual payment of about £1,334/month and roughly £160,000 of total interest if you never overpay.

Steady £200/month clears it around 4 years 8 months early and saves about £28,000 in interest.
£15,000 lump sum now, plus £100/month removes a big chunk of principal immediately, clearing the mortgage even sooner and saving substantially more interest than the same money trickled in later.
An annual £3,000 bonus overpayment lands in fewer, larger hits and lands somewhere between the two.

The table ranks them, stars the winner, and the chart shows exactly where each curve peels away from the baseline. Every figure is computed in your browser — nothing is uploaded or stored on a server.