Key takeaway
X percent of Y is the slice you get by turning X% into a decimal and multiplying by Y: value = (X ÷ 100) × Y. So 20% of 50 = 0.20 × 50 = 10. The only way to get it wrong is to skip the ÷100 and multiply X directly by Y.
Scope and assumptions
General arithmetic method. Applied rates (compound tax, tiered tips) may differ. This site is informational and not official.
Short answer: X percent of Y is the slice you get by turning X% into a decimal and multiplying by Y: value = (X ÷ 100) × Y. So 20% of 50 = 0.20 × 50 = 10. The only way to get it wrong is to skip the ÷100 and multiply X directly by Y. This page explains the meaning; for the fast calculation method and mental shortcuts, see X Percent of Y.
Quick navigation
- The formula → percentage-formula
- % of a number → percentage-of-a-number
- % to decimal → percent-to-decimal
- The calculator → Percentage Calculator
1. The meaning
"X percent of Y" asks: if Y were split into 100 equal parts, how much are X of them?
value = (X ÷ 100) × Y
The percent becomes a decimal first, then scales Y.
2. Worked example
What is 20% of 50?
- 20% → 0.20
- 0.20 × 50 = 10
So 20% of 50 is 10. The same idea gives a tip, a discount, or a tax amount.
3. Everyday uses
| Ask | X | Y | value = (X÷100)×Y |
|---|---|---|---|
| 15% tip on $40 | 15 | 40 | 0.15 × 40 = 6 |
| 30% off $80 | 30 | 80 | 0.30 × 80 = 24 |
| 8% tax on $25 | 8 | 25 | 0.08 × 25 = 2 |
| 20% of 50 | 20 | 50 | 0.20 × 50 = 10 |
4. A multi-step case
A $120 item is 25% off, then 10% tax is added to the reduced price.
- Discount: 0.25 × 120 = 30 → price $90
- Tax: 0.10 × 90 = 9 → total $99
Apply each percent to the current amount, in order.
5. Common mistakes
- Multiplying X by Y directly — 20 × 50 = 1000, not 10; convert the % first.
- Applying tax to the pre-discount price — percent of a number always uses the amount in front of you.
- Rounding the percent too early — keep 0.20 exact until the final step.
- Confusing with part-of-whole — "20% of 50" (10) is not "20 out of 50" (40%).
6. Picking the rate
When a question says "what is X% of Y", the rate is X and the number is Y. Say them out loud: "20% of 50 — rate 20, number 50". Convert the rate, multiply the number. If you are ever unsure which is which, remember the percent sign always travels with the rate, so the other number is the base you scale. Stating the two roles first prevents the 100× error.
7. Reading the rate in context
Once you have the amount, sanity-check it against the rate. If the rate is under 100%, the amount must be smaller than the number — 20% of 50 is 10, clearly less than 50. If the rate is over 100%, the amount is larger, which happens when the question asks for more than the whole. This simple size check catches the most common slip, where the percent was never turned into a decimal and the answer landed 100 times too big. Read the rate, predict the size, then confirm the calculation agrees with that prediction before you write it down.
FAQ
Q: What does 'X percent of Y' mean? The part you get when you take X% of the amount Y — computed as (X ÷ 100) × Y.
Q: What is 20% of 50? (20 ÷ 100) × 50 = 0.20 × 50 = 10.
Q: How do I calculate any X% of Y? Convert X% to a decimal (÷100) and multiply by Y. That is the whole method.
Q: Why not just multiply X by Y? Because X% is not X — 20% is 0.20, so 20 × 50 would be 100× too big.
Q: Is 'percent of' the same as 'percent change'? No. 'Percent of' finds a slice of one number; 'percent change' compares two numbers. See how-to-calculate-percentage.
Sources: Khan Academy percentage basics; Maths is Fun — Introduction to Percentages.
Disclaimer: General arithmetic method. Applied rates (compound tax, tiered tips) may differ. This site is informational and not official.
Sources and references
- Khan Academy percentage basicshttps://www.khanacademy.org/math/pre-algebra/x31947de81e3f6453:percentages
- Maths is Fun — Introduction to Percentageshttps://www.mathsisfun.com/percentage.html