How we calculate
The rules every calculator on this site follows: what gets rounded and when, how floating-point error is handled, which defaults are assumed, and why your result may differ from your bank's by a few pence.
Rounding rules
Rounding is applied once, at the end, to the displayed figure — never partway through a calculation. Rounding intermediate values compounds error, which is how a thirty-year amortisation schedule ends up several pounds out by the final payment.
| Result type | Decimal places | Method |
|---|---|---|
| Money | 2 | Half-up, epsilon-corrected |
| Percentages | 2 | Half-up |
| Ratios (ROAS, LTV:CAC) | 2 | Half-up |
| Scientific results | 4 significant figures | Half-up unless the page says otherwise |
| Counts (bags, boxes, tiles) | Rounded up to a whole unit | Ceiling — you cannot buy 0.4 of a bag |
| BMI and body composition | 1 | Half-up, matching clinical convention |
Half-up means 2.345 displayed to two places becomes 2.35, not 2.34. Counts are the deliberate exception: material quantities are always rounded up, because a job that needs 12.2 bags of concrete needs thirteen bags.
Floating point, and why 0.1 + 0.2 is not 0.3
Computers store decimals in binary, and some ordinary decimals have no exact binary form. The classic demonstration is that 0.1 + 0.2 evaluates to 0.30000000000000004 in every mainstream programming language, JavaScript included. Left alone, that tiny error shows up in a rounding decision: 1.005 rounded to two places should be 1.01, but the stored value is fractionally below 1.005, so a naive rounding returns 1.00.
Our rounding function corrects for this by nudging the scaled value by a few units in the last place before rounding, so values that are mathematically on a boundary round the way a person expects. Where a calculation involves a long chain of multiplications — compound interest over 480 months, for example — we keep full precision throughout and round only what is printed.
Numbers are formatted with your browser's own internationalisation support, so thousands separators, decimal marks and currency symbols follow your device's locale. A German browser sees 1.234,56 where a British one sees 1,234.56; the underlying value is identical.
Default assumptions
Where a calculation needs an assumption, we use the most common convention and state it on the page. The defaults across the site are:
- Compounding. Loans and mortgages compound monthly, matching how payments are made. Savings and investment tools let you choose the frequency, because the difference between annual and daily compounding is real money over a decade.
- Year length. Interest calculations use twelve equal months rather than exact day counts, which is what most consumer lenders quote. Date calculators are the exception: they count actual days, including leap days.
- Payment timing. Regular contributions are treated as ordinary annuities — paid at the end of each period. Paying at the start (an annuity due) is worth slightly more, and pages that offer the choice say so.
- Currency. Money tools default to your locale's formatting and carry a currency switch where it matters. No exchange rate is ever applied, because that would need a live feed and this site makes no network calls.
- Time zones. Date arithmetic is done in UTC to avoid daylight-saving shifts moving a result by a day.
- Waste factors. Construction quantities default to a 10% waste allowance, which is typical for straightforward layouts. Diagonal or patterned work needs more, and the pages say so.
Why your bank's number may differ
Your lender's figure and ours can differ by a small amount, and the reason is almost always one of four things. First, day-count convention: a lender using actual days over 365 will differ slightly from twelve equal months. Second, fees: a quoted APR may fold in origination costs that a simple payment calculation does not. Third, rounding policy: some institutions round each monthly payment up to the nearest penny, which changes the final payment. Fourth, the schedule itself — a first payment date more than a month after drawdown accrues extra interest before the schedule even starts.
None of those make either figure wrong. They make the assumptions different, which is why we publish ours. If your figure and ours differ by more than a rounding difference, that is worth reporting — email info@factonisam.com with both numbers.
Checking our working
Every calculator page shows the formula it runs, defines each variable in a table, and works at least one example by hand so you can follow the arithmetic. That is deliberate: a calculator you cannot check is a calculator you have to trust, and we would rather you verified. If a hand calculation and the tool disagree, tell us — that is a bug, and our editorial policy explains how quickly we fix them.