How to Calculate Percentages

7 min read · Updated September 26, 2026 · by the SolveCalcPro editorial team

A percentage is a number out of 100. “Per cent” literally means “per hundred”. You use percentages for discounts, grades, interest, statistics and tips, so a few reliable methods are worth knowing.

The three basic formulas

Percent of a number: part = percent ÷ 100 × whole. 15% of 80 = 0.15 × 80 = 12.
What percent is one number of another: percent = part ÷ whole × 100. 30 is 30 ÷ 120 × 100 = 25% of 120.
Find the whole: whole = part ÷ (percent ÷ 100). If 12 is 15% of a number, the number is 12 ÷ 0.15 = 80.

Try any of these in the percentage calculator.

Percentage increase and decrease

To increase a number by a percentage, multiply by (1 + rate). To decrease, multiply by (1 − rate). A $200 item with 15% tax costs 200 × 1.15 = $230. A $200 item at 15% off costs 200 × 0.85 = $170.

Percent change is (new − old) ÷ old × 100. Going from 50 to 65 is +30%. Going from 65 back to 50 is −23.08%, because the base is now 65.

Successive percentages do not add

A 20% discount followed by an extra 10% off is not 30% off. The price becomes 100 × 0.8 × 0.9 = 72, a 28% total discount. Always multiply the factors.

Mental math shortcuts

To find	Do this	Example (on 240)
10%	Move the decimal one place left	24
5%	Half of 10%	12
20%	Double 10%	48
25%	Divide by 4	60
15%	10% + 5%	36

For a tip of 18% on $60: 10% is 6, 5% is 3, and 3% is 1.80, so the tip is $10.80.

Percent versus percentage points

If an interest rate moves from 4% to 5%, it rose by 1 percentage point but by 25% in relative terms (1 ÷ 4). News headlines often blur the two, so check which one is meant.

Practice questions

Test yourself, then tap to check. Answers are calculated by the tools linked below.

What is 35% of 240?
Show answer
35% of 240: 84
Write 0.85 as a fraction.
Show answer
Fraction (lowest terms): 17/20
A recipe uses 3 eggs for 5 cups of flour. How many eggs for 20 cups?
Show answer
5 : 3 = 20 : x, so x =: 12