Calculate
12 tools
Arithmetic you would otherwise do twice to be sure: percentages, unit conversion, dates and the money maths that comes up at work. Every answer shows its working.
- Percentage CalculatorFive percentage questions, with the working shown.
- Unit ConverterSeven measurements, exact factors, both directions.
- Number Base ConverterBinary, octal, decimal and hex, at any width.
- Roman Numeral ConverterNumbers to numerals and back, both ways.
- Days Between DatesDays apart, with working days alongside.
- Add or Subtract DatesDate arithmetic that tells you when it clamped.
- Age CalculatorYears, months and days, counted properly.
- Compound Interest CalculatorCompound growth, with the effective rate shown.
- Loan Repayment CalculatorMonthly payment, total repaid, and the interest.
- Sales Tax and VAT CalculatorTax on or off, without the subtraction mistake.
- Margin and Markup CalculatorBoth numbers, because they are not the same one.
- Discount CalculatorSale price, savings, and stacked discounts done right.
About these tools
This is the category the site did not originally have, and the reason it exists is that four groups of tools had nowhere sensible to live. Percentages, unit conversion, date arithmetic and the small financial calculations that come up in ordinary work are not generators, not text tools and not file conversion. Filing them under one of those would have made that category mean less, so they got their own.
The thing these tools share is that the arithmetic is easy and getting it right is not. Percentage change and percentage difference are different calculations that people reach for interchangeably. Margin and markup are not the same number and quoting one as the other is how a job comes in under cost. Adding a month to the 31st of January has three defensible answers and every library picks one. Counting days between two dates is off by one depending on whether you meant to include both ends.
So every tool here shows its working. Not just the answer, but the expression that produced it, with the numbers substituted in. That is the difference between a calculator you check against and one you have to check.
Units are exact where exactness exists. A kilometre is 1000 metres and an inch is 25.4 millimetres by definition, so those conversions carry no rounding beyond what the display asks for. Temperature is a scale conversion rather than a ratio, which is why doubling twenty degrees Celsius does not double anything, and the tools that treat it as a ratio are wrong.
Dates use the browser own time zone database through Intl, which the browser vendor maintains, so daylight saving and zone changes stay correct without anyone here updating a table.
Nothing here is advice. The money tools do arithmetic on numbers you supply and stop there. They will tell you what a repayment schedule adds up to and they will not tell you whether to take the loan.