Daily Use Calculators

Four Kinds of Everyday Question

The tools on this page cover the arithmetic that comes up away from work and away from anything technical — dates, time, grades, shopping and splitting costs. They group into four kinds of question.

Five Traps That Account for Most Wrong Answers

Everyday arithmetic goes wrong in a small number of predictable ways, and each of these is covered in detail on the page it belongs to.

• Decimal hours read as minutes. A 7.5-hour shift is 7 hours 30 minutes, not 7 hours 50. Entering 8 hours 10 minutes as 8.10 rather than 8.17 understates every shift.
• Counting both ends of a date range. From 1 March to 8 March is seven days of difference and eight dates. Which one applies depends on whether you are counting hotel nights or days of leave.
• Averaging averages. Two class averages, two exam percentages or two semester GPAs cannot simply be averaged unless the groups are the same size. Add the raw amounts and divide once.
• Assuming discounts add. Thirty percent off then twenty percent off is forty-four percent off, not fifty. Multiply the multipliers rather than adding the rates.
• Reversing a percentage by adding it back. A price of 63 after 30% off was 90, not 81.90. Undoing a percentage is always a division.

Where the Answers Feed Into Each Other

Several of these tools are more useful in combination than alone.

Date of birth → Age calculator → exact age for a form or eligibility
Two dates → Days between → a deadline or a notice period
Course grades → GPA calculator → GPA to percentage → an application figure
Bill total → Bill split → each person's share including tip

The grade chain is the one worth following carefully. Work out the credit-weighted GPA first, then convert it — and convert against the scale your transcript actually uses, since a weighted average above 4.0 cannot be converted with the usual multiplier.

Why These Answers Are Exact

This section differs from the health and finance tools in a way worth stating. Those produce estimates: a calorie target depends on an activity level you chose, and a loan repayment depends on rates behaving as entered. The tools here mostly do not estimate at all.

A day count between two dates is exact. A percentage of a number is exact. A conversion between metres and feet uses a factor that is exact by international definition rather than measured. Where these tools return a wrong answer, it is because the wrong question was asked — the wrong base for a percentage, the wrong scale for a GPA, the wrong counting convention for a date range — rather than because the arithmetic is approximate.

The three genuine exceptions are worth naming. Converting days into months or years is approximate, because months and years vary in length. Currency conversion uses a rate that changes rather than a fixed factor. And any figure you enter carries its own precision, which no calculation can improve on. Everything else on this page is arithmetic you could check by hand, and each page shows the working so you can.