Tinker 
How to use it
Choose the question you are asking, type value A and value B, and the answer appears instantly. Decimals and negative numbers are allowed. Results are rounded to four decimal places internally and shown to two.
The formulas
A% of B: multiply B by A and divide by 100.
A is what percent of B: divide A by B and multiply by 100.
Percentage change from A to B: subtract A from B, divide by A, and multiply by 100. A positive result is an increase; a negative result is a decrease.
Worked examples
15% of 200: 200 × 15 ÷ 100 = 30.
45 out of 60 on a test: 45 ÷ 60 × 100 = 75%.
A price rising from 80 to 100: (100 − 80) ÷ 80 × 100 = 25% increase. Falling from 100 back to 80 is (80 − 100) ÷ 100 × 100 = −20%, which shows why a rise and an equal fall are not the same percentage.
Practical tips
For discounts, use percentage-of to find the amount saved, then subtract it from the price. Percentage change is always measured against the starting value, so make sure A is the old figure. Percentage points are different from percent: a rate going from 4% to 5% is a 1 point rise but a 25% increase.
Frequently asked questions
How do I calculate a percentage of a number?
Multiply the number by the percentage and divide by 100. For example, 20% of 150 is 150 × 20 ÷ 100 = 30.
Why isn't a 20% drop undone by a 20% rise?
Each change is measured from a different starting value. 100 down 20% is 80, and 80 up 20% is only 96. You need a 25% rise to get back to 100.
Can a percentage be over 100?
Yes. If A is larger than B, A is more than 100% of B, and an increase can exceed 100% when a value more than doubles.
Why can't the starting value be zero?
Percentage change divides by the starting value, and dividing by zero is undefined, so no percentage change exists from zero.