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A percentage is a way of expressing a number as a fraction of 100 — "18%" means 18 out of every 100. It is a ratio, so it always compares one amount against another, whether that other amount is a total, a starting value, or the number 100 itself.
To find a percentage of a number, multiply the number by the percentage and divide by 100. For example, 18% of 10,000 is 10,000 × 18 ÷ 100 = 1,800. Use this when you already know the rate and need the amount it represents — a discount, a tip, or a share of a total.
To find what percentage a value is of a total, divide the value by the total and multiply by 100. For example, 18 is what percentage of 100? 18 ÷ 100 × 100 = 18%. This answers questions like "what share of the whole is this part?"
A percentage increase compares a rise from one value to a larger one. Subtract the starting value from the new value, divide by the starting value, and multiply by 100. Going from 100 to 120 is a rise of 20, and 20 ÷ 100 × 100 = 20% increase.
A percentage decrease works the same way, in reverse: subtract the new, smaller value from the starting value, divide by the starting value, and multiply by 100. Going from 100 to 80 is a fall of 20, and 20 ÷ 100 × 100 = 20% decrease.
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Multiply the number by the percentage, then divide by 100. For example, 18% of 10,000 is (10,000 × 18) ÷ 100 = 1,800.
Divide the smaller number by the larger one, then multiply by 100. For example, 18 is what percentage of 100? (18 ÷ 100) × 100 = 18%.
Subtract the original value from the new value, divide the result by the original value, then multiply by 100. Going from 100 to 120 is a 20% increase, because (120 − 100) ÷ 100 × 100 = 20.
Subtract the new, smaller value from the original value, divide by the original value, then multiply by 100. Going from 100 to 80 is a 20% decrease, because (100 − 80) ÷ 100 × 100 = 20.
A percentage change is measured relative to the starting value, so the calculation divides by it. Dividing by zero has no defined answer, so a starting value of zero cannot produce a percentage change.