Everyday Calculators

The arithmetic here is easy enough to do by hand, which is exactly why it goes wrong so often. Percentages alone have three meanings people swap without noticing: a percentage of a number, a number as a percentage of another, and a percentage change between two. The percentage calculator answers all three at once so you do not have to pick first.

Two mistakes cost real money every festival season. Stacked discounts do not add: 30 percent off and then a further 20 percent off is 44 percent off, not 50. And on a restaurant bill the 5 percent GST is charged on the service charge too, so declining a 10 percent service charge saves you 10.5 percent, not 10.

The date and age calculators handle the parts that trip people up: real month lengths, leap years, whether the last day counts, and age as on a cut-off date the way UPSC, SSC and school admissions actually compute it.