Three percentage questions, one tool
Most everyday percentage problems are one of three types, and this calculator handles all of them. 'What is X% of Y' finds a part of a whole — a tip, a discount, a tax. 'X is what percent of Y' works the other way, turning two raw numbers into a percentage, useful for scores, completion rates or shares. 'Percentage change from X to Y' measures growth or decline between two values, the figure behind price rises, statistics and performance comparisons. Choose the mode that matches your question and the calculator applies the right formula automatically.
How each calculation works
Finding X% of Y multiplies Y by X divided by 100 — 20% of 150 is 150 × 0.20 = 30. Working out what percent X is of Y divides X by Y and multiplies by 100 — 30 out of 150 is 20%. Percentage change subtracts the old value from the new, divides by the old value, and multiplies by 100, so a move from 150 to 180 is a 20% increase. The same subtraction the other way (180 to 150) is a 16.7% decrease — note that the base you divide by changes the answer, which is why increases and decreases are not symmetric.
Common percentage mistakes
The most frequent error is the asymmetry of increases and decreases. A 50% rise followed by a 50% fall does not return you to the start: 100 becomes 150, then 75. Another trap is confusing percentage points with percent — moving from 10% to 12% is a two percentage-point rise but a 20% relative increase. And reverse-engineering an original price from a post-discount figure requires dividing, not adding the percentage back. Picking the correct mode here avoids these slips by always dividing by the right base.
Everyday uses
Percentages run through daily life — sales tax and tips, store discounts, exam marks, interest rates, nutrition labels, opinion polls and business growth. Being able to switch quickly between 'how much is this percentage' and 'what percentage is this' saves time and prevents costly misreadings, especially when money is involved. Bookmark this calculator for the next time a receipt, a report or a headline statistic needs checking.
Frequently asked questions
How do I calculate a percentage of a number?
Multiply the number by the percentage divided by 100. For example, 20% of 150 is 150 × 0.20 = 30. Use the 'What is X% of Y' mode for this.
How do I find what percent one number is of another?
Divide the first number by the second and multiply by 100. 30 out of 150 is (30 ÷ 150) × 100 = 20%. Use the 'X is what % of Y' mode.
How is percentage change calculated?
Subtract the old value from the new, divide by the old value, and multiply by 100. From 150 to 180 is a 20% increase.
Why isn't a 50% rise cancelled by a 50% fall?
Because each percentage is taken from a different base. 100 rises 50% to 150, then a 50% fall is taken from 150, dropping it to 75 — not back to 100.