Percentage Calculator

What this calculator solves

Percentage problems come in a few distinct shapes that people often mix up: finding what X% of a number is (what is 15% of 80?), finding what percentage one number is of another (what percentage is 12 of 80?), and finding the percentage change between two values (a price that went from $80 to $92 — what's the percentage increase?). This calculator handles all three modes so you can pick the one that matches the question you're actually trying to answer, rather than rearranging a formula by hand.

Each mode uses straightforward arithmetic under the hood — there's no hidden complexity, just the correct formula applied automatically so you don't have to remember which one to use or risk a sign error when a value decreases rather than increases.

The formulas behind each calculation

"X% of Y" is calculated as (X ÷ 100) × Y. "What percentage is X of Y" is calculated as (X ÷ Y) × 100. Percentage change — the one people most often get wrong by hand — is ((new value − old value) ÷ old value) × 100; a positive result is a percentage increase, and a negative result is a percentage decrease. The key detail people miss is that percentage change is always calculated relative to the original (old) value, not the new one, which is why going from 100 to 150 is a 50% increase, but going back from 150 to 100 is a 33.3% decrease, not 50%.

Common use cases

Shoppers use it to calculate a discount amount or the final price after a percentage-off sale. Students use it for math homework involving percentages, ratios, and grade calculations. Professionals use the percentage change mode to calculate revenue growth, price increases, or year-over-year metrics for a report. Tippers use the "X% of Y" mode to quickly calculate a restaurant tip, and anyone comparing two numbers — like a survey response rate or a test score — uses the "what percentage is X of Y" mode to express the relationship as a percentage.

A common mistake to avoid

A frequent error is assuming that a percentage increase and the matching percentage decrease are the same size — they're not, because each is calculated relative to a different starting point. If a stock drops 50%, it needs to rise 100% (not 50%) from its new, lower value just to get back to where it started. Keeping in mind that percentage change is always relative to the original value avoids this trap.

Percentages in everyday financial decisions

Percentage calculations show up constantly in situations with real financial stakes, which is exactly why getting the formula right matters. Comparing loan interest rates, calculating how much a raise actually increases take-home pay, figuring out the real savings on a "20% off, then an extra 10% off" sale (hint: that's not a 30% total discount, since the second discount applies to the already-reduced price), and estimating investment returns all rely on the same underlying percentage math this calculator handles. Because stacked percentage changes don't simply add together, working through the actual numbers with a calculator rather than estimating in your head avoids a surprisingly common source of costly miscalculation.