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Three calculators in one, pick what you need below.
A percentage is just a fraction out of 100, and three questions cover almost every everyday case. What is X% of Y? Multiply Y by X and divide by 100 (20% of 150 = 150 × 0.20 = 30). X is what percent of Y? Divide X by Y and multiply by 100 (30 out of 150 = 30 ÷ 150 × 100 = 20%). Percentage change? Take the difference, divide by the original, and multiply by 100. The calculator above handles all three.
Increase and decrease. To add a percentage, multiply by (1 + the rate as a decimal); to subtract one, multiply by (1 − the rate). This is quicker and less error-prone than working out the amount and adding it separately, and it is the basis of tax, tips, discounts and interest.
The reversal trap. A 50% rise followed by a 50% fall does not return you to the start: 100 rises to 150, then falls to 75. Percentage changes are relative to whatever the current value is, so an increase and an equal-percentage decrease never cancel out. Keep track of what each percentage is being taken of.