The three percentage sums
To find X% of Y, divide X by 100 and multiply by Y.
To find what percentage X is of Y, divide X by Y and multiply by 100.
To find the percentage change from X to Y, subtract X from Y, divide by X and multiply by 100. A positive answer is an increase and a negative answer is a decrease.
Worked examples
20% of 150 is 20 ÷ 100 × 150 = 30.
30 as a percentage of 150 is 30 ÷ 150 × 100 = 20%.
Going from 150 to 180 is a difference of 30, and 30 ÷ 150 × 100 = a 20% increase. Going back from 180 to 150 is a 16.6667% decrease, because the change of 30 is now compared with 180.
Common mistakes
A percentage increase followed by the same percentage decrease doesn't take you back to where you started. A 20% rise and then a 20% fall leaves you 4% lower.
A change from 4% to 5% is a rise of 1 percentage point, but a 25% increase. Be clear which one you mean.
Inputs for the percentage calculator
What do you want to work out?
Pick the type of sum. The input boxes change to match.
Percentage (X)
The percentage you want to take, for example 20 for 20%. Only shown for 'What is X% of Y?'.
Of (Y)
The number you're taking the percentage of. Only shown for 'What is X% of Y?'.
Value (X)
The part you want to express as a percentage. Only shown for 'X is what percentage of Y?'.
Total (Y)
The whole amount you're comparing against. It can't be zero. Only shown for 'X is what percentage of Y?'.
Original value (X)
The starting value. It can't be zero. Only shown for percentage increase or decrease.
New value (Y)
The value after the change. Only shown for percentage increase or decrease. The result also shows the difference between the two values.
CALCULATOR QUESTIONS
Percentage calculator questions
How do I work out a percentage of a number?
Divide the percentage by 100 and multiply by the number. For example, 15% of 80 is 0.15 × 80 = 12.
How do I work out a percentage increase?
Subtract the old value from the new one, divide by the old value and multiply by 100. From 150 to 180 is 30 ÷ 150 × 100 = 20%.
Why can't the original value be zero?
A percentage change compares the difference with the original value. You can't divide by zero, so a change from zero has no percentage.
Can I use negative numbers?
Yes. For percentage change, the difference is divided by the size of the original value, so a change from −50 to −25 shows as a 50% increase.