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Quick calculators for everyday arithmetic — percentages, discounts, tips, dates and unit conversions.
Calculate exact age in years, months, days.
Standard arithmetic operations.
Convert length, weight, area & volume.
Calculate percentages and changes.
Calculate durations between dates.
Calculate tips and split bills.
Convert hourly, daily, annual wages.
Split your income into needs, wants, and savings.
Live exchange rates for world currencies.
Percentage change is the most commonly reversed calculation in everyday use. A price that rises 20% and then falls 20% does not return to where it started — it ends about 4% lower, because the second percentage applies to a larger base. The same asymmetry explains why a 50% investment loss requires a 100% gain to recover.
Stacked discounts work the same way. "30% off, then an extra 20% off" is not 50% off. It is 44% off, because the second reduction applies to the already-discounted price. Retailers rely on the intuition being wrong in their favour.
Date arithmetic is less obvious than it looks once leap years and varying month lengths are involved. The age and date calculators here handle both correctly rather than approximating a year as 365 days.
Each of these runs instantly in your browser with no signup and works on mobile, which is usually where you need them.