Percentage Calculator
Pick a question, type the numbers, and get the answer as you type. Same maths as Anvil's free percentage API.
How it works
Choose what you want to know: What is X% of Y?, X is what % of Y?, percent change from A to B, increase or decrease a value by a percent, or undo a percent change to recover the original. Fields update live; nothing is uploaded.
Percent of is the everyday school form: 20% of 50 is 10. Is what % flips it: 15 is 25% of 60.
Percent change is relative to the starting value: going from 80 to 100 is a +25% change ((100 − 80) ÷ 80), not +20%. The absolute gap and whether it was an increase or decrease are shown too.
Apply multiplies by (1 + percent/100): +15% turns 200 into 230; −15% turns it into 170. Reverse undoes that — useful for “£115 after a 15% markup; what was the pre-markup price?” (answer: 100). A −100% change cannot be reversed because the original information is gone.
Need the same maths from a script or CI job? Anvil's free percentage API returns JSON with no key. Sibling tool: the unit converter.
Questions
Is my number uploaded?
No. Every mode is pure JavaScript in your browser.
Why is 80→100 +25% and not +20%?
Percent change divides by the starting value. The 20-point gap is 20% of 100, which answers a different question.
Can I reverse a 100% discount?
No. Multiplying by zero leaves no clue what the original was.
Is there an API?
Yes. Anvil's keyless /v1/percent endpoint at api.oddlathe.com does the same five modes and returns JSON.