What this does
Three questions, stacked as sentences you fill in rather than fields you have to decode: what is X% of Y, X is what percent of Y, and percent change from X to Y. All three recalculate on every keystroke and stay independent, so you can leave one set up while you work in another. Nothing is submitted anywhere — the arithmetic is plain JavaScript in the page, and it keeps working offline.
How do I use the three rows?
- Pick the row that matches your question — they are unrelated calculations.
- Type over the two numbers in that sentence.
- Read the answer on the right of the same row.
Rows two and three already print the % sign, so their answer is a
percentage. Row one's answer is a plain amount in whatever you were counting.
What is 20% of 150?
30. The formula is (X ÷ 100) × Y — the percentage is turned into a
decimal, then multiplied by the value. Nothing stops X going above 100:
150% of 40 is 60. This is the row for discounts, tax lines, and commission —
take 8.25% of 240 and you get 19.8.
How do I find what percent one number is of another?
Divide the part by the whole and multiply by 100: (X ÷ Y) × 100.
30 out of 150 is 20%. 1 out of 4 is 25%. Test scores, completion rates, and
"how much of the budget is this" all live here. The part can exceed the whole,
which just gives an answer above 100%.
How do I calculate a percentage increase or decrease?
((new − old) ÷ old) × 100, always relative to the
old value. A rise is shown with an explicit
+ sign; a fall is shown with a minus sign and no plus; an
unchanged pair reads 0. Note that increases and decreases are not
symmetrical, because the denominator changes:
| From | To | Change |
|---|---|---|
| 150 | 180 | +20% |
| 200 | 150 | −25% |
| 80 | 100 | +25% |
| 100 | 80 | −20% |
| 50 | 50 | 0% |
Going up 25% and back down 20% returns you to where you started — the classic trap when reading price or traffic changes.
Why do I see a dash instead of an answer?
Because the calculation is undefined. Row two cannot divide by a whole of zero, and row three cannot measure change from a starting value of zero — any move away from nothing is infinite in percentage terms. Fix the offending number and the answer reappears; the other rows keep working meanwhile.
How much does it round?
Results are rounded to four decimal places, with trailing zeros dropped, so a
third of something shows as 33.3333 and a clean answer shows as
20 rather than 20.0000. That is more precision than most uses
need, but it stops repeating decimals from being quietly truncated. For a
bill share with the gratuity built in, the tip calculator
handles the rounding to cents; for growth compounded over years rather than a
single step, see compound interest.