How to Calculate Percentages: The Complete Guide

For example, 15% of 240: convert 15% to 0.15, then 240 × 0.15 = 36. That single line answers "what is X% of Y?" forever.

Percentage Increase and Decrease

When a value changes, the percentage change compares the difference to the starting value:

A shirt goes from $40 to $50: (50 − 40) ÷ 40 × 100 = 25% increase. A salary drops from $60,000 to $54,000: (54,000 − 60,000) ÷ 60,000 × 100 = −10%, a 10% decrease. The sign tells you the direction; the magnitude tells you the size.

A common trap: a 10% rise followed by a 10% fall is not back to the start. $100 + 10% = $110, then −10% = $99. Percentages do not add — they multiply: ×1.10 then ×0.90 = ×0.99.

Percentage Difference

Percentage difference compares two values symmetrically, using their average as the base — the standard way to compare two measurements without favouring either as the "original":

Compare 50 and 60: |50 − 60| ÷ 55 × 100 ≈ 18.2%. The same answer comes out whether you compare 50 to 60 or 60 to 50, which is the point of the metric.

Quick Mental Tricks

• 10% is the number divided by 10: 10% of 480 = 48.
• 5% is half of 10%: 5% of 480 = 24.
• 1% is the number divided by 100: 1% of 480 = 4.8.
• Combine them: 15% = 10% + 5% = 48 + 24 = 72.

Check Your Work

If you are ever unsure, use the percentage calculator on this site — it handles all three modes (percent of a number, increase/decrease, and difference) and shows the decimal equivalent so you can verify the arithmetic.