What is 90% of 20? 18 90% of 20 Copy 90% of 20 = 18. Formula UsedResult = (90 ÷ 100) × 20 = 18 Frequently Asked Questions What is 90% of 20?18. (90 ÷ 100) × 20 = 18.How to calculate 90%?Multiply by 0.9 (or multiply by 90 then divide by 100).