Percentage Calculator
Find what a given percentage of a number is — for example, 18% of a $42 restaurant bill, or 7.5% sales tax on a $129 purchase. This calculator handles the most common percentage question that comes up in everyday math: "what is P% of N?" Enter the percentage and the base number, and you get the exact partial amount. It is the building block behind tip calculators, discount calculators, tax calculators, and most quick financial estimates you do on the fly.
About this calculator
The formula used here is: result = (percentage ÷ 100) × number. The word "percent" literally means "per hundred" (from the Latin per centum), so a percentage is just a fraction with a denominator of 100. Dividing the percentage by 100 converts it into its decimal form — 25% becomes 0.25, 7.5% becomes 0.075 — and multiplying that decimal by the base number gives the corresponding part. The formula is symmetrical and works for any real numbers: a negative percentage produces a negative result (useful for losses), and a percentage greater than 100 produces a result larger than the base (useful for markups or growth). Edge cases worth noting: if the base number is 0, the result is always 0 regardless of percentage; if the percentage is 0, the result is also 0. The calculator does not handle "percentage of what" or "X is what percent of Y" questions directly — for those, swap the inputs algebraically or use a dedicated percentage-change calculator. Behind the simplicity, percentages dominate finance, statistics, and science precisely because they normalize quantities onto a common 0–100 scale, making heterogeneous things directly comparable.
How to use
Example 1 — Tip on a restaurant bill. You want to leave an 18% tip on a $42 bill. Enter 18 in the Percentage field and 42 in the Number field. Result: 7.56, meaning you should add $7.56 to the bill for a total of $49.56. Verify: 18 ÷ 100 = 0.18, and 0.18 × 42 = 7.56. ✓ Example 2 — Sales tax on a purchase. Your state charges 7.5% sales tax and you are buying a $129 pair of shoes. Enter 7.5 as the Percentage and 129 as the Number. Result: 9.675, which rounds to $9.68 in tax — bringing the total to $138.68. Verify: 7.5 ÷ 100 = 0.075, and 0.075 × 129 = 9.675. ✓
Frequently asked questions
How do I calculate a percentage of a number by hand?
Convert the percentage to a decimal by dividing by 100, then multiply by the number. For 20% of 80, divide 20 by 100 to get 0.2, then multiply by 80 to get 16. A useful mental shortcut for round percentages: 10% of any number is that number with the decimal point moved one place to the left, so 10% of 80 is 8, and 20% is just double that. For 5%, take 10% and halve it. For 15%, take 10% and add half of 10% (so 15% of 80 is 8 + 4 = 12). These tricks let you check the calculator output instantly.
What is the difference between percentage and percentage points?
A percentage measures a part of a whole, while a percentage point measures a difference between two percentages. If a poll moves from 40% to 45% support, that is an increase of 5 percentage points — but in relative terms it is a 12.5% increase (because 5 is 12.5% of 40). Mixing these up is one of the most common mistakes in news reporting and political analysis. Always ask whether a quoted "5% rise" means five percentage points or a 5% relative change; the two can differ by an order of magnitude depending on the base. Use this calculator for absolute percentage amounts and a dedicated percentage-change calculator when you mean relative change.
Why does multiplying by a percentage less than 100 make the number smaller?
Because percentages below 100% are equivalent to decimal fractions less than 1, and multiplying by a fraction always shrinks a positive number. 50% of 200 is 100 because 0.50 × 200 = 100; 25% of 200 is 50 because 0.25 × 200 = 50. Conversely, percentages above 100% are equivalent to numbers greater than 1, so 150% of 200 = 300 (a 50% markup). This is the same arithmetic intuition that explains why a $50 item discounted 30% costs less ($35) while a $50 item marked up 30% costs more ($65). Keep the decimal form in mind — it removes most of the confusion percentages cause in mental math.
What are the most common mistakes people make with percentages?
The single most frequent error is reversing the base — applying a percentage to the wrong number. For example, if a $100 item is discounted 20%, the new price is $80; but adding the 20% back to $80 gives $96, not $100, because the 20% now applies to a smaller base. A second common mistake is stacking percentages additively when they actually compound: a 10% raise followed by another 10% raise is not a 20% total raise, it is 21% (1.10 × 1.10 = 1.21). A third is treating a percentage and a percentage point as the same unit, as discussed above. Finally, people often forget that percentages can exceed 100% — a stock that triples in value has gone up 200%, not 300%.
When should I not use this calculator?
This tool answers only the question "what is P% of N?" It does not answer the inverse questions like "X is what percent of Y?" or "X is P% of what?" — for those, use a dedicated percentage calculator that supports all three forms, or compute manually by rearranging the formula. It is also the wrong tool for percentage change (how much something grew or shrank as a fraction of its starting value) — that requires comparing two numbers, not multiplying a percentage by one. For statistical work involving margins of error, confidence intervals, or weighted percentages, use a statistics calculator. And for compound percentage growth over many periods, use a compound interest calculator instead of repeatedly applying this formula by hand.