How we calculate

Formulas from standard references

Every calculator uses the textbook formula for its question: the amortizing payment M = P·r ÷ (1 − (1 + r)^−n) for loans and mortgages, A = P(1 + r/n)^(nt) and the future value of an annuity for savings, and so on. Each calculator page prints its formula and works it through with your numbers, so it can be checked by hand or against a spreadsheet's PMT and FV functions.

Rounding

• Calculations run at full precision. Rounding happens only when a figure is displayed.
• Amortization schedules are built in whole cents, the way lenders keep accounts: each month's interest is rounded to the cent and the final payment absorbs any difference, so the principal column adds up exactly to the loan.
• Fractions are calculated exactly with whole-number arithmetic and are never rounded; repeating decimals are marked.
• Unit conversions use exact definitions (1 inch = 2.54 cm, 1 pound = 0.45359237 kg).

Conventions we state rather than assume

• Whether payments and deposits happen at the start or the end of each period.
• How often interest compounds, and how deposits made at a different frequency are converted to the same effective rate.
• How dates are counted: whether the end date is included, how month ends are handled, and that weekends are the only non-working days.
• What is left out: taxes, fees, holidays, or anything else the calculator does not model.

Testing

The calculation code is separate from the page code and covered by automated tests that run before every release: reference values worked out by hand, rules that must always hold (a schedule's principal adds up to the loan; adding then subtracting a period returns the same date; converting there and back returns the same value), and edge cases such as a 0% rate, leap days and month ends.

Limits

Results are estimates for information. Lenders, banks, employers and tax authorities apply their own terms and rounding, so their figures can differ by a few cents or more. Financial results are not advice and do not guarantee any return, rate or approval; see the terms.

Corrections

If a result looks wrong, use Share on the calculator and send the link to the address on the about page. Confirmed errors are fixed and noted here.