How to Calculate Percentages Quickly

Percentages come up everywhere — from restaurant tips to sale discounts to understanding statistics. While reaching for a calculator is always an option, knowing a few mental math shortcuts can save you time and impress your friends.

The 10% Method

The easiest way to calculate any percentage is to start with 10%. To find 10% of any number, simply move the decimal point one place to the left. For example, 10% of 85 is 8.5, and 10% of 230 is 23.

From there, you can build any percentage: - 5% = half of 10% - 20% = double 10% - 15% = 10% + 5% - 25% = 10% + 10% + 5%

The 1% Method

For trickier percentages, find 1% first by moving the decimal two places left. Then multiply by whatever percentage you need. For instance, 7% of 400: 1% is 4, so 7% is 28.

Percentage of a Percentage

Sometimes you need to find "what percentage is X of Y?" The formula is simple: divide X by Y, then multiply by 100. For example, if you scored 42 out of 50 on a test: 42 ÷ 50 = 0.84, which is 84%.

Percentage Change

To find the percentage change between two numbers, subtract the old value from the new value, divide by the old value, and multiply by 100. A product going from $80 to $100 is a 25% increase: (100 - 80) ÷ 80 × 100 = 25%.

Quick Reference

Want to find	Do this
10%	Move decimal left 1 place
5%	Half of 10%
20%	Double 10%
25%	Quarter of the number
50%	Half the number
1%	Move decimal left 2 places

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