Not every percentage calculation needs a phone. A handful of mental shortcuts cover the situations that come up most often — tips, discounts, quick estimates, splitting bills. Once these are in your head, you can do most percentage arithmetic in a few seconds.
Trick 1: Use 10% as Your Starting Point
Finding 10% of any number is instant: move the decimal point one place to the left.
- 10% of 85 = 8.5
- 10% of 340 = 34
- 10% of 7,200 = 720
Once you have 10%, you can build almost any other common percentage from it:
- 5% = half of 10%
- 15% = 10% + 5%
- 20% = 10% × 2
- 25% = 10% × 2.5 (or just divide by 4)
- 30% = 10% × 3
10% of £48 = £4.80
5% of £48 = £2.40
15% tip = £4.80 + £2.40 = £7.20
Trick 2: The Commutative Property of Percentages
This one surprises most people: X% of Y equals Y% of X.
So 16% of 50 = 50% of 16 = 8. Half of 16 is a much easier calculation than 16% of 50, and it gives the same answer.
4% of 75 = 75% of 4 = 3 (75% of 4 = 3, obviously)
12% of 25 = 25% of 12 = 3 (a quarter of 12 = 3)
8% of 50 = 50% of 8 = 4 (half of 8 = 4)
When the percentage and the number swap to give you a rounder calculation, always swap. Look for opportunities where the percentage becomes 25%, 50% or a number that divides evenly.
Trick 3: Round, Then Adjust
If the numbers aren't convenient, round to the nearest easy figure and adjust.
Round to 17% of £60 = 10% of 60 + 7% of 60 = 6 + 4.20 = £10.20
The extra £3 at 17% = about £0.51
Estimate: £10.20 + £0.51 ≈ £10.70
Exact answer: £10.71 — close enough for most purposes
Trick 4: Quick Discounts at the Till
When shopping a sale:
- 10% off: subtract the last digit (roughly). £79 → roughly £71.
- 20% off: subtract twice the last digit. £79 → roughly £63.
- 25% off: divide by 4. £80 ÷ 4 = £20 off → £60.
- 50% off: divide by 2.
- 33% off: divide by 3.
Trick 5: The 1% Method for Awkward Percentages
For percentages that don't break down easily into 10%, find 1% first (move the decimal two places left), then multiply.
1% of £420 = £4.20
3% = £4.20 × 3 = £12.60
1% of £250 = £2.50
7% = £2.50 × 7 = £17.50
The 1% method works for any percentage and any number. It's slower than the 10% shortcuts for round percentages, but it handles anything.
Putting It Together: A Mental Arithmetic Workflow
Faced with a percentage calculation in your head:
- Can you swap the numbers (trick 2)? If the flipped version is easier, do that.
- Is the percentage a multiple of 10%? Use trick 1 and build up.
- Can you round to a convenient number and adjust?
- If all else fails, use 1% as a building block.
Most people who are "good with numbers" aren't doing harder maths — they're just pattern-matching to the easiest route. These tricks are the patterns.
For exact answers whenever you need them, all eight percentage calculations are available free on PercentageHQ.
Open the Calculator →Mental arithmetic shortcuts give good estimates, not guaranteed exact answers. For financial calculations, use a calculator to verify.