A calculator that only prints a single number asks you to trust it. The calculators here show how they got there: the formula, the inputs it used, and a breakdown you can check line by line — an amortisation schedule for a loan, a year-by-year balance for savings, the exact calendar arithmetic for a date. Each tool page also works through real examples, and every figure in those examples is produced by the same code that runs the calculator and checked by automated tests, so the numbers in the text and the numbers on screen cannot drift apart.
Rounding is part of the answer
Money is paid in whole cents, but interest is calculated on fractions. A loan schedule that rounds each month independently will not end at exactly zero; a naive one ends at minus a few cents or leaves a tiny balance forever. These calculators round every payment to the cent and settle the difference in the final period, the way lenders do. Percentages have a similar trap: 0.1 × 3 is not exactly 0.3 in binary floating point, so results are formatted to a sensible number of significant digits instead of showing 0.30000000000000004.
Dates are not just numbers of days
Months have different lengths, leap years add a day, and "one month after January 31" does not have an obvious answer. The date calculator works on calendar dates rather than timestamps, so a result never shifts by a day because of the time zone your computer is set to, and it explains the rule it applies to month-end dates and to people born on 29 February.
What these results are for
The results are estimates for planning and comparison. Real loans include fees, insurance and rate changes; real investments do not grow at a fixed rate. Use the figures to understand the shape of a decision — how much interest a shorter term saves, how much a regular contribution adds — and confirm the final numbers with the lender or the contract before you sign anything.
- Mortgage calculator: monthly payment, total interest and a full schedule, with both equal-payment and equal-principal methods and extra repayments.
- Compound interest calculator: growth of a lump sum and of regular contributions at any compounding frequency.
- Date calculator: days between dates, business days, adding or subtracting days, and exact age.
- Percentage calculator: percent of a number, what percent one number is of another, and percentage change.
Prepay the mortgage or invest the money?
The question these calculators are most often used for is whether spare cash should go into the mortgage or into savings. The honest answer rests on one comparison: the loan rate against the return you can reasonably expect after tax. The guide on this question runs the numbers for a 300,000 loan at 6% over 30 years. Paying 50,000 extra after the first year cuts the monthly payment by 303.50, and investing that difference every month ends almost exactly level with investing the 50,000 at once when both earn 6%. Below the loan rate, prepaying comes out ahead; above it, investing does, with more risk attached.