The formula
How to calculate mortgage interest paid
Over a full term, mortgage interest often approaches or exceeds half the amount borrowed. Seeing the total, and how much of it lands in the early years, is what makes the case for overpaying or shortening the term.
The period figure is derived from the balance: whatever you paid in during those years that did not reduce the debt was interest. On a 25-year mortgage the first five years are roughly three quarters interest.
The inputs, one by one:
- Mortgage amount
- Interest rate (%)
- Term (years)
- Interest paid in the first (years)
Everything recalculates as you type, and the numbers in the address bar update with it, so a link to this page carries your figures with it.
Where a figure is not immediately to hand — a precise interest rate, an exact balance — a reasonable estimate is a perfectly good starting point. Because every result updates instantly, refining a rough guess into the real figure once you have it takes a moment, and nothing about the calculation depends on getting it exactly right on the first attempt.
Why mortgage interest paid matters
The formula behind mortgage interest paid is standard and has not changed in decades; what changes is the situation it gets applied to. Two households can run the identical calculation and land on very different conclusions once their own numbers — income, rate, term, balance — are dropped in, which is why a generic textbook example is less useful than a calculator you can adjust to match your own circumstances.
It is also useful as a sense check before signing anything. A quote, an offer letter or a spreadsheet from someone else can contain an error, an optimistic assumption, or simply a different convention for rounding — running the same inputs through an independent calculator is a quick way to confirm a number before relying on it.
The reason a page like this exists at all, rather than leaving the calculation to a spreadsheet or a textbook appendix, is that the formula behind mortgage interest paid is fiddly enough to get wrong by hand but not complicated enough to need specialist software. That middle ground — real enough maths to matter, simple enough to check instantly — is exactly what a dedicated calculator is for, and it is why the same figure recalculated here should match a careful manual calculation almost exactly.
Where the same calculation needs to be run for several different scenarios side by side — three loan offers, two savings plans — the fastest approach is usually to open the calculator in a second browser tab for each one, so that the results can be compared directly rather than overwriting each other in a single set of fields.
Worked example
Work through the defaults on this page:
- Mortgage amount: 250,000
- Interest rate: 4.5 %
- Term: 25 years
- Interest paid in the first: 5 years
That gives:
- Total interest over the term: 166,874.36
- Interest in that first period: 53,019.63
- Interest as a share of total repaid: 40.03 %
These figures are only the calculator's own starting values, included so the working is visible rather than hidden inside the tool above. Replace them with your own numbers and the same arithmetic applies — nothing about the method changes, only the inputs feeding it.
Reading the result
The share output is the blunt summary. At 4.5% over 25 years, roughly a third of everything you repay is interest; over 35 years it is closer to half.
Where this goes wrong. Treating the total as fixed. It assumes the rate holds for the whole term, which is almost never true in the UK — most borrowers move through a series of two- and five-year deals at different rates.
A useful check on any unfamiliar result is to compare it against a rough mental estimate first — round the inputs to convenient numbers and see whether the calculator's answer lands in roughly the same territory. A wildly different figure usually means one of the fields was entered in the wrong unit, most often a percentage typed as a whole number where a decimal was expected, or the reverse.
At 4.5% over 25 years, roughly £167,000, making about £417,000 repaid in total. Cutting the term to 20 years reduces the interest by around £37,000 for about £150 a month more.
Not on your own home. Landlords no longer deduct it from rental income either — since 2020 they receive a basic-rate tax credit on finance costs instead.
The headline figure is total interest over the term. With 250,000 mortgage amount, 4.5 % interest rate and 25 years term, that comes to 166,874.36. Change any field and the figure moves with it.
Whenever one of the underlying figures changes — a new interest rate, a different balance, an updated term — since the result only reflects what is currently in the fields. There is no need to keep a separate record of past results; the web address for a filled-in version already carries the figures used to produce it.
Not unless a tax rate or a fee is explicitly one of the inputs above. Where it is not, the figure shown is a gross calculation, and any tax due depends on your personal circumstances and current tax rules, which are worth checking separately.
The arithmetic itself is exact — the calculator applies the formula shown above precisely, with no rounding until the final figure is displayed. The uncertainty, where it exists, is entirely in the inputs: an estimated rate or an approximate balance carries that same approximation through to the result.