Percentage Calculator — % of, Increase, Decrease
Four percentage calculations in one tool: % of a number, X is what % of Y, percentage increase, percentage decrease. Live answer with the formula shown. No signup, no ads.
About this tool
A percentage is just a fraction expressed out of 100, but the four common variations — “what is 15% of 200”, “30 out of 120 is what %”, “what's the % change from 80 to 100”, and “a 25% drop from 100 is what number” — trip up most people because the formulas look almost identical but the base number shifts.
WRRK's percentage calculator separates those four cases into tabs, runs the math instantly, and shows the formula it used so you can verify the answer or paste it into a spreadsheet. Everything runs in your browser — no values are sent to a server. Useful for tips and tax, discount comparisons, statistics homework, year-over-year revenue change, exam scores, salary hike percentages, and conversion rates.
A common gotcha: percentage increase and percentage decrease are not symmetric. Going from 80 to 100 is a 25% increase, but going back from 100 to 80 is only a 20% decrease — the denominator changes. The calculator handles this correctly for both directions.
How to calculate a percentage (5 steps)
- Pick the mode. Tap the tab for the calculation you want — 'What is X% of Y', 'X is what % of Y', '% increase', or '% decrease'.
- Enter the first number. For 'X% of Y' it's the percentage. For the rest it's the original or part value.
- Enter the second number. The total, the new value, or the value you want compared against.
- Read the answer. The result updates live. The formula used is shown below so you can double-check or copy it into a spreadsheet.
- Try another. Click 'Try another' to clear and start a new calculation.
Use cases
- Tip calculation at restaurants (15%, 18%, 20% of the bill)
- Sales tax and VAT addition or removal
- Salary hike or appraisal percentage
- Year-over-year revenue or growth rate
- Discount on a marked price (combine with the discount calculator)
- Exam scores and grade conversion
- Conversion rate, click-through rate, churn rate
- Stock price gain or loss percentage
- Body weight change, fitness progress
Frequently asked questions
+−How do I calculate X% of a number?
Divide the percentage by 100, then multiply by the number. Example: 15% of 200 = (15 ÷ 100) × 200 = 30. The first tab on this calculator does it for you instantly.
+−How do I figure out what percentage one number is of another?
Divide the part by the whole and multiply by 100. Example: 45 is what % of 360? (45 ÷ 360) × 100 = 12.5%. Use the second tab — 'X is what % of Y'.
+−What's the formula for percentage increase?
((New − Original) ÷ Original) × 100. Example: from 80 to 100 is ((100 − 80) ÷ 80) × 100 = 25% increase. Negative results indicate the value actually decreased.
+−What's the formula for percentage decrease?
((Original − New) ÷ Original) × 100. Example: a price drop from 100 to 75 is ((100 − 75) ÷ 100) × 100 = 25% decrease.
+−Why do I get a different number for % increase vs % decrease for the same two values?
The denominator is different. Going from 80 to 100 is a 25% increase (gain ÷ original 80). Going back from 100 to 80 is a 20% decrease (loss ÷ original 100). Same absolute change, different base.
+−Can I use this for tip and tax calculations?
Yes. For an 18% tip on a $50 bill, use 'What is X% of Y' with X=18, Y=50. The answer is $9. Add it to the bill for the total. For sales tax it's the same pattern.
+−Does the calculator handle decimals and large numbers?
Yes. Type any decimal value (e.g. 7.5, 0.025) and any size number. Results round to 4 decimal places to keep them readable.