Everyday Calculators

Some math problems are too small for a spreadsheet and too annoying to do in your head. This collection is for all of them: the little everyday calculations that show up while shopping, studying, planning and celebrating — each solved in seconds, with the working shown.

Shopping math is covered by the discount calculator (sale price, stacked discounts and tax) and the tip calculator (fair per-person splits at the table), while the percentage calculator handles the three percent problems behind almost everything else — X% of Y, X as a percent of Y, and percentage change. Students get the GPA calculator, which weights grades by credit hours properly and plans what you need next term to hit a target GPA.

Date math — the classic source of off-by-one errors — is handled two ways: the age calculator gives exact age in years, months and days from any birth date plus a birthday countdown, and the date duration calculator counts exact days, weeks and months between any two dates, with business days and inclusive counting. And when the group chat needs settling, the love compatibility calculator delivers a strictly-for-fun score from two names.

Every tool here is free, works on a phone one-handed, and explains its formula — because a calculator you cannot check is just a random number generator with confidence.