How to Calculate Percentages (With Worked Examples)

Type 1: What is X% of Y?

This is the most common percentage question. "What is 20% of 80?" The formula is: multiply the percentage by the number, then divide by 100.

Example: 20% of 80 = (20 × 80) ÷ 100 = 1,600 ÷ 100 = 16. A shortcut many people find easier: convert the percentage to a decimal first by moving the point two places left (20% becomes 0.20), then multiply: 0.20 × 80 = 16.

Type 2: X is what percent of Y?

Use this when you know two numbers and want the relationship between them. "You scored 42 out of 50 on a test — what percent is that?" The formula: divide the part by the whole, then multiply by 100.

Example: 42 ÷ 50 = 0.84, and 0.84 × 100 = 84%. This is also how you figure out what fraction of your monthly budget a purchase represents.

Type 3: Percentage increase or decrease

This tells you how much something changed relative to where it started. "A price went from $50 to $65 — what's the percentage increase?" The formula: subtract the old value from the new value, divide by the old value, multiply by 100.

Example: (65 − 50) ÷ 50 × 100 = 15 ÷ 50 × 100 = 30% increase. If the result is negative, it's a decrease. A common mistake is dividing by the new value instead of the old one — always divide by where you started.

Quick mental math tricks

A few shortcuts make everyday percentages instant:

• 10% of anything: move the decimal point one place left. 10% of 240 is 24.
• 5%: take 10% and halve it. 5% of 240 is 12.
• 15% (a common tip): add 10% and 5%. 15% of 240 is 24 + 12 = 36.
• 25%: divide by 4. 50%: divide by 2.
• X% of Y is always the same as Y% of X. 8% of 50 is the same as 50% of 8 — both are 4. Use whichever is easier.