Percentage Tricks You'll Actually Use

The swap trick

X% of Y equals Y% of X. 18% of 50 is hard; 50% of 18 is 9. Same answer.

Tips in your head

10% is the decimal shift (18.20 → 1.82). 20% is double that. 15% is 10% + half of 10%.

Discount stacking

A 20% coupon on a 30% off item is not 50% off. It's 44% (0.7 × 0.8 = 0.56 remaining).

Percent change vs percent points

A rate going from 4% to 5% is a 1 percent-point rise but a 25% relative increase. News reports mix these constantly.