Compound growth is where intuition fails
These calculators are not about tax rules — they are the same arithmetic everywhere, in any currency. But the reason they are worth running rather than estimating is that compound growth is not linear, and human intuition is reliably wrong about it. Doubling the time does not double the outcome. Halving the term does not halve the interest. The examples below all come from the formulas these calculators use, so you can reproduce every one of them.
A longer mortgage term can cost more than the house
Stretching a mortgage lowers the monthly payment, which is exactly why it is tempting. Here is the same £300,000 loan at 5% over three terms:
| Term | Monthly payment | Total repaid | Total interest | Interest as % of loan |
|---|---|---|---|---|
| 25 years | £1,754 | £526,131 | £226,131 | 75% |
| 30 years | £1,610 | £579,767 | £279,767 | 93% |
| 35 years | £1,514 | £635,906 | £335,906 | 112% |
Going from 25 to 35 years saves £240 a month — and costs £109,775 in extra interest. On the 35-year term the interest alone exceeds the amount borrowed: you pay for the house roughly twice. That trade is sometimes the right one, if the lower payment is what makes the purchase possible at all. But it should be a decision, not a default. Run your own numbers in the mortgage calculator.
Small overpayments work harder than they look
Because interest is charged on the outstanding balance, every extra pound goes straight against the principal and stops accruing interest for the rest of the term. On the same £300,000 at 5% over 25 years, adding just £100 a month:
- Term falls from 25.0 years to 22.5 years — two and a half years earlier
- Interest falls from £226,131 to £200,126 — a saving of £26,005
- Total extra paid over those years: £27,000
Put differently, £27,000 of overpayments removes £26,005 of interest — close to a pound saved for every pound advanced, before you even count the two and a half years of payments you no longer make at all. The overpayment calculator shows the effect for your own balance and rate.
Ten years of delay costs far more than ten years of contributions
The most expensive financial decision most people make is postponement, and the numbers are not intuitive. Saving £200 a month at 7% until age 65:
| Start age | Total contributed | Value at 65 | Of which growth |
|---|---|---|---|
| 25 | £96,000 | £524,963 | £428,963 |
| 30 | £84,000 | £360,211 | £276,211 |
| 35 | £72,000 | £243,994 | £171,994 |
| 45 | £48,000 | £104,185 | £56,185 |
Starting at 25 instead of 35 means contributing £24,000 more — and ending with £280,968 more. The extra money you put in accounts for less than a tenth of the difference; the rest is time. Note too how the growth column overtakes the contribution column: start at 25 and over 80% of the final balance is growth, while starting at 45 leaves growth at barely half. Compound interest rewards duration far more than it rewards amount. See it in the compound interest calculator.
The FIRE number is a multiple, not a target
Financial independence is usually framed as a savings goal, but the arithmetic runs the other way: your target is set by your spending, divided by the withdrawal rate you are willing to rely on.
| Annual spending | At 4% withdrawal | At 3.5% withdrawal |
|---|---|---|
| £24,000 | £600,000 | £685,714 |
| £30,000 | £750,000 | £857,143 |
| £40,000 | £1,000,000 | £1,142,857 |
Two things follow. First, cutting annual spending by £6,000 lowers the target by £150,000 at a 4% rate — reducing what you need is roughly 25 times more powerful per pound than earning more. Second, being more cautious is expensive: moving from 4% to 3.5% raises the target by 14% at every spending level. Whether 4% is prudent is a genuine debate; what is not debatable is that the choice of rate moves the goalpost more than most people expect. The FIRE calculator lets you set both.
What these calculators do and don't account for
- Universal arithmetic, no tax. These tools ignore income tax, capital gains tax and any tax-advantaged wrapper. For tax on employment income, use the country calculators.
- Currency-agnostic. Pick a symbol; the mathematics is identical. The examples above use £ purely for readability.
- Nominal, not real. Returns and repayments are not adjusted for inflation, which materially reduces the purchasing power of a distant future balance.
- Constant rates. Real mortgage rates change at the end of a fixed period and real investment returns are volatile. A steady 7% is a modelling assumption, not a forecast.
- No fees. Arrangement fees, early-repayment charges, platform charges and fund costs all reduce the outcome. Check whether your lender caps annual overpayments before relying on the overpayment figures.
These are planning tools, not advice. If a figure looks wrong, please tell us — our methodology explains how each calculator is checked before publication.