How do I calculate a percentage of a number?

Convert the percent to a decimal and multiply: 15% of 80 = 0.15 × 80 = 12. The calculator's first mode does this for any pair of numbers.

Why is a 50% increase undone by a 50% decrease?

Percentages apply to different bases. 100 up 50% is 150; 150 down 50% is 75, because the 50% now acts on 150. To return to the start after a 50% gain you need a 33.3% drop.

Percentage Calculator

Q: How do I calculate percentage change?

Subtract the old value from the new one, divide by the old value, and multiply by 100: ((new − old) / old) × 100. A negative result is a decrease.

Three percentage problems, one tool: percent-of, percent-of-total, and percent change between two values.

1. What is X% of Y?

X (%)

Y (number)

Answer

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2. X is what percent of Y?

X (part)

Y (whole)

Answer

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3. Percent change from old to new

Old value

New value

Change

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The three percentage formulas

Nearly every percentage question falls into one of three patterns, each with a one-line formula:

Part = Whole × (Percent ÷ 100)

Percent = (Part ÷ Whole) × 100

Change % = ((New − Old) ÷ Old) × 100

The third is the one people get wrong most often. Percent change is always measured against the original value. A price moving from $250 to $310 rose by $60, and $60 ÷ $250 = 24% — not 19.4% (which would wrongly divide by the new price).

Where percentages trip people up

Percent vs. percentage points

If a savings rate rises from 3% to 4%, that's a one percentage-point increase — but a 33% relative increase. News headlines mix these constantly; knowing which one is being quoted changes how significant the move really is.

Gains and losses aren't symmetric

A 50% gain followed by a 50% loss does not return you to the start: 100 → 150 → 75. After any gain of G%, you need a smaller loss to get back to even — after +50% you only need −33.3%. The same asymmetry is why investment recovery after a crash feels slower than the crash itself.

Successive percentages don't add

A 10% discount on top of a 20% discount is not 30% off. 20% off $100 is $80; a further 10% comes off the $80, giving $72 — an effective 28% total discount. Stack discounts in the calculator above to see the real number.

Frequently asked questions

What is 15% of 80?

12. Multiply 80 × 0.15.

How do I reverse a percentage?

If a price includes 20% tax and you want the pre-tax amount, divide by 1.20 — don't subtract 20% of the total, which over-corrects.

How do I calculate percentage change?

((New − Old) ÷ Old) × 100, measured against the original value. Negative results are decreases.