The three percentage questions
Almost every percentage problem is one of three: what is a% of b (multiply b by a/100), X is what % of Y (divide X by Y and multiply by 100), and percent change (the difference divided by the starting value, times 100).
Percent change explained
Percent change is always measured against the starting value. Going from 80 to 100 is a +25% change, but going from 100 to 80 is only −20%, because the base you divide by is different.
Frequently asked questions
How do I calculate 15% of 200?
Multiply 200 by 0.15 — the answer is 30. Enter 15 and 200 in the first row above for the same result.
How do I work out a percentage increase?
Subtract the old value from the new value, divide by the old value, then multiply by 100. From 80 to 100 that is (100−80)÷80×100 = 25%.
Why is percent change different in each direction?
Because the starting base differs. A rise from 80→100 is +25%, while a fall from 100→80 is −20%.