How our calculators work
Choose a calculator, enter your numbers, and select Calculate. Your answer appears with the steps used to reach it. Arithmetic tools include formulas and worked examples. Salary tools explain the deductions and assumptions used.
Calculation methods
Percentage tools use fractions out of 100. The average tool adds values and divides by their count. Date calculations use the Gregorian calendar and UTC date boundaries. Time duration uses minutes after midnight. The take-home pay calculators apply published income tax and payroll deduction rates for the tax year shown on each page — Income Tax, National Insurance and student loan in the UK; federal, state and FICA in the US; federal, provincial, CPP and EI in Canada; income tax, the Medicare levy and HELP repayments in Australia; and PAYE, ACC and KiwiSaver in New Zealand — and are an estimate rather than a substitute for a payslip or tax advice. The results are calculated from fixed formulas and tax tables.
Rounding and precision
Most results display up to 12 significant digits. Discounts display two decimal places. Calculations use JavaScript floating-point numbers, so they are approximate and are not a substitute for an exact-decimal accounting system. The displayed steps may be rounded separately from the underlying calculation.
Input limits
The arithmetic tools accept finite numeric values between −1 trillion and 1 trillion, subject to each tool’s limits. Salary tools have separate limits. The average calculator accepts up to 1,000 entries. Percentage change requires a positive starting value, and discounts must lie between 0% and 100%.
What happens to your inputs
Calculations run on your device. We do not put input values into URLs, save them to browser storage, or send them to an API. The Copy result button writes the result to your clipboard only when you select it.
Checking the results
The source includes automated tests for the formulas, invalid inputs, leap days, overnight periods, and generated pages. The arithmetic tools also include worked examples you can check by hand.