Everyday Calculators

The arithmetic here is simple enough to do by hand, which is precisely why it goes wrong so often. Percentages in particular have three distinct meanings that people routinely swap: a percentage *of* a number, a number *as* a percentage, and a percentage *change* between two. The percentage calculator answers all three at once rather than making you pick first.

Two errors show up repeatedly and both cost real money. Stacked discounts do not add — 30% off then 20% off is 44% off, not 50%. And removing tax from an inclusive total is a division, not a subtraction; taking 18% off a GST-inclusive price understates the value by more than 3%.

The date calculators handle the awkward parts properly: real month lengths rather than an assumed 30 days, leap years, and the choice of whether to count the end date — which differs between measuring a gap and counting days of entitlement.