Finding Percentages Quickly Without a Calculator

There are mental math shortcuts for percentages that are faster than reaching for a calculator. Knowing them makes you quicker at estimation and more confident in checking computed results. The 10% anchor Almost any percentage can be calculated quickly from 10%. 10% of any number: move the decimal point one place left. 10% of 85 = 8.5 10% of 230 = 23 From there: 5% = half of 10%: 5% of 230 = 11.5 20% = double 10%: 20% of 230 = 46 15% = 10% + 5%: 15% of 230 = 23 + 11.5 = 34.5 1% = 10% divided by 10: 1% of 230 = 2.3 25% = divide by 4: 25% of 230 = 57.5 The tip calculation trick For a 20% tip: move the decimal left (10%), then double it. Bill: $47.50 10%: $4.75 20%: $9.50 For 15%: 10% plus half of 10%. 10%: $4.75 5%: $2.38 15%: $7.13 The flip trick "What is 8% of 25?" is the same as "What is 25% of 8." 25% of 8 = 2.0. Much easier to compute mentally. This works because X% of Y = Y% of X. The proof is trivial: (X/100)*Y = (Y/100)*X. But it is remarkably useful for mental math. Common refer