How to Multiply by 15 in Your Head

Updated 4 October 2026

Multiply by 10, then add half of that result.

34 × 15 = 510

How do you multiply by 15 in your head?

To multiply by 15, multiply by 10 and add half of that. For 34 × 15, take 34 × 10 = 340, halve it to get 170, and add: 340 + 170 = 510. It works because fifteen of something is ten of them plus five more, and five is half of ten. For an odd number, the half ends in 5, which is no harder to add: 27 × 15 = 270 + 135 = 405. Another route is to multiply by 30 and halve, because 15 is half of 30. That route suits numbers that are easy to triple: 27 × 30 = 810, and half of 810 is 405. Bigger numbers take the same steps: 140 × 15 = 1400 + 700 = 2100. Check the answer two ways. A multiple of 15 always ends in 0 or 5, and its digit sum is a multiple of 3, as with 405, where 4 + 0 + 5 = 9.

1. Fifteen 34s is ten of them plus five of them.

2. Five 34s is half of ten 34s: 340 ÷ 2 = 170.

3. Add the parts: 340 + 170 = 510.

All 36 shortcuts: the Magic Tricks library.