5 Mental Math Tricks That Actually Speed You Up
Most “mental math tricks” lists are really the same handful of shortcuts repeated with different examples. These five are the ones that hold up under actual time pressure — the kind you hit in our math games when a timer is running out.
1. Multiplying by 9: use 10, then subtract
To multiply any number by 9, multiply it by 10 and subtract the original number once.
47 × 9 = 47 × 10 − 47 = 470 − 47 = 423
This works because 9 is just 10 − 1, so n × 9 is always n × 10 − n. Once this becomes automatic, multiplying by 9 stops being a lookup and turns into two easy steps.
2. Multiplying by 11 (two-digit numbers)
For any two-digit number, add its two digits together and drop the sum in the middle.
52 × 11: add 5 + 2 = 7, insert it between → 572
If the digits add up to 10 or more, carry the extra 1 into the left digit: 68 × 11 → 6 + 8 = 14 → write 4, carry 1 → 6+1 and 4 → 748.
3. Squaring numbers that end in 5
Any number ending in 5, squared, follows a fixed pattern: take the digit(s) before the 5, multiply by the next number up, then append 25.
35² : 3 × 4 = 12 → append 25 → 1225
85²: 8 × 9 = 72 → append 25 → 7225
No estimation, no rounding — this one is exact every time.
4. Subtracting by adding up
Instead of borrowing across columns, count up from the smaller number to the larger one in convenient jumps.
1000 − 437: 437 → 440 (+3) → 500 (+60) → 1000 (+500) → total added: 3 + 60 + 500 = 563
This is the trick cashiers use for making change, and it’s almost always faster than column subtraction once you’re comfortable chunking the jumps.
5. Breaking multiplication into friendly pieces
For anything that isn’t a clean round number, split one factor into parts you already know.
23 × 6 = (20 × 6) + (3 × 6) = 120 + 18 = 138
The skill isn’t memorizing more facts — it’s recognizing which split turns an unfamiliar problem into two familiar ones.
Where to actually practice these
Reading a trick and being able to pull it out under a countdown timer are two different skills. A few games on the site are built specifically to force the second one:
- Math is easy! — straightforward arithmetic against the clock, the most direct way to drill trick #1 and #5 until they’re automatic.
- Number Maze Puzzle — forces you to plan a sequence of operations ahead of time rather than solving one sum at a time, closer to how multi-step mental math actually works.
- Mind Games: Math Crosswords — crossword-style number grids where each answer constrains the next, good for catching careless arithmetic slips.
If arithmetic speed is what you’re after specifically, our math games category has the full list; browsing by logic games is a better fit if it’s reasoning rather than raw calculation you want to train next.
