"Percent" means five different operations depending on the sentence, and the hard part is never the division — it is working out which one you are actually asking. So this calculator is organised by the question in plain words: what is X% of Y, X is what percent of Y, the change from one number to another, applying an increase or decrease, and finding the total from a part. Every answer comes with the arithmetic written out underneath, because a bare number does not tell you whether you asked the right question. It also spells out the two things that catch everyone: percentage change is not symmetric — 100 to 150 is up 50%, but 150 to 100 is down 33% — and adding 10% then taking 10% off does not get you back where you started. Everything runs in your browser.
This tool does not use any data storage or network traffic.
What are you working out?
—
Two things that catch everyone out
Percentage change is not symmetric. Going from 100 to 150 is a 50% increase, but going back from 150 to 100 is a 33.3% decrease, not 50%. The percentage is always relative to where you started, and you started somewhere different each time.
Adding a percentage and then taking it off again does not undo it. Add 10% to 100 and you get 110. Take 10% off 110 and you get 99, not 100 — because the second 10% is 10% of the larger number. This is why a 20% discount on a price that was raised 20% is not the original price.
Nothing leaves your browser
Everything here is arithmetic done on this page. Nothing is uploaded and nothing is stored.