How our calculators work

Understand inputs, formulas, rounding and the limits of an estimate.

Each standard calculator starts with example inputs, applies the formula shown on its page, and displays a result. Change the inputs to explore a scenario. Worked examples use the same calculation as the live tool.

Intermediate calculations use JavaScript numbers. Displayed amounts may be rounded, so adding displayed values can differ from an unrounded calculation. A projection assumes the rates and timing stated on its page; it does not predict a future return.

Browser interaction tests measure your device and browser as well as your response. They are not clinical, intelligence or professional assessments. Tax, legal and health estimates have additional limits.

Read the assumptions and source notes. Report a reproducible issue through our request form.