How to Multiply by 9 in Your Head

Updated 4 October 2026

Multiply by 10, then subtract the original.

48 × 9 = 432

How do you multiply by 9 in your head?

To multiply by 9, multiply by 10 and subtract the original number. For 48 × 9, take 48 × 10 = 480, then subtract 48: 480 − 48 = 432. It works because nine of something is ten of them with one taken back. Subtracting in two steps keeps it easy: take off the tens first, then the units, so 480 − 40 = 440 and 440 − 8 = 432. The same move works for any size of number: 213 × 9 = 2130 − 213 = 1917. For single digits, the answer from 1 × 9 = 9 up to 10 × 9 = 90 always has digits that add to 9: 7 × 9 = 63, and 6 + 3 = 9. That is the check for larger answers too, because the digit sum of any multiple of 9 is itself a multiple of 9. So 1917 passes, since 1 + 9 + 1 + 7 = 18.

1. Nine 48s is ten 48s with one taken back: 480 − 48.

2. Take the 48 in two bites: 480 − 40 = 440, then 440 − 8 = 432.

How do you multiply bigger numbers by 9?

The same move, and the subtraction gets easier when the number is round. 125 × 9 is 1250 − 125 = 1125. 76 × 9 is 760 − 76: take 70 first, which leaves 690, then 6 more, which leaves 684.

How do you check a × 9 answer?

Every multiple of 9 has a digit sum that is itself a multiple of 9. 432 gives 4 + 3 + 2 = 9, 684 gives 6 + 8 + 4 = 18, and 1125 gives 1 + 1 + 2 + 5 = 9. If the digits of your answer do not add to a multiple of 9, the answer is wrong. It works because 10, 100 and 1000 each leave remainder 1 when divided by 9, so a number and its digit sum leave the same remainder.

All 36 shortcuts: the Magic Tricks library.