Overview
Percentages are the quiet arithmetic of everyday money: the discount on the jacket, the tip on the bill, the raise on the salary, the interest on the loan. Most people can do the easy ones and quietly guess the rest. This calculator covers the three questions that actually come up — what is X% of Y, what percent X is of Y, and the percent change between two numbers — with live answers as you type.
Each mode is deliberately separate, because mixing them up is the classic error. “20% off 150” and “what percent is 30 of 150” are different questions with different boxes, and keeping them apart is half the battle.
What is a percentage, precisely?
A percentage is a fraction with a denominator of 100 — “per cent” literally means “per hundred”. So 20% is the fraction 20/100, and “20% of 150” is just (20/100) × 150 = 30. The reverse question, “30 is what percent of 150”, divides instead: 30/150 = 0.2, times 100 to express it per hundred: 20%.
Percentage change is the one people get wrong most. The change from 80 to 100 is (100−80)/80 = 25% — note the denominator is the old value, the starting point. Going back down from 100 to 80 is a 20% decrease, not 25%, because the starting point changed. Asymmetry like this is why the calculator shows the direction explicitly.
Features
- Three dedicated modes: X% of Y, X as a % of Y, and % change — each with its own inputs.
- Live computation with every keystroke; no buttons to press.
- Direction labels on change mode: increase, decrease or no change.
- Sensible formatting that trims floating-point noise (no 22.49999999%).
- Worked examples built into the defaults so each mode is self-explanatory.
- Private: all math happens in your browser.
Benefits
- Kill mental-math anxiety. Tips, discounts and raises in seconds.
- No mode confusion. Separate boxes stop the classic mix-ups.
- Exact, not approximate. Clean decimal handling you can trust.
- Free and offline-capable once the page loads.
Who can use it
Shoppers checking discounts, diners splitting bills, freelancers quoting with tax or commission, students doing homework, and anyone who has ever nodded along to a percentage in a meeting while quietly unsure. Managers presenting growth figures will appreciate the change mode's explicit direction labels.
How to use
- Choose the mode: X% of Y, X is what % of Y, or % change.
- Type the two numbers into the boxes.
- Read the live answer underneath — it updates as you type.
- For change mode, note the increase/decrease label to describe it correctly.
- Switch modes with the tabs any time; your numbers stay put.
Frequently asked questions
How do I calculate 20% off a $150 item?
Use the first mode: 20% of 150 = 30, so the discount is $30 and you pay $120. Equivalently, you pay 80% of 150, which is the same $120 — the calculator's first mode handles either framing.
Why is going from 80 to 100 a 25% increase but back down only 20%?
Because the reference point differs. Up: (100−80)/80 = 25%. Down: (80−100)/100 = −20%. Percentages are always relative to where you started, which is why “up 50% then down 50%” does not return you to the start.
What is a percentage point vs percent?
A rise from 20% to 25% is a 5-percentage-point increase, but a 25% relative increase (5/20). News about interest rates and polls uses percentage points precisely to avoid this confusion — worth copying in your own writing.
Can percentages exceed 100%?
Absolutely. 150% of 80 is 120, and a 200% increase triples the original. “Percent” just means “per hundred” — there is no rule capping it at one hundred.
Is my input stored anywhere?
No. The arithmetic runs entirely in your browser; the numbers vanish when you leave the page.
Conclusion
Percentages reward the two minutes it takes to learn their three shapes: of, is-what-percent-of, and change. With those shapes in separate boxes and live answers, the everyday money math stops being guesswork.