Percent mental math tricks

Most percents can be found in your head. Start from 10% or 1%, build up, swap when it helps, and estimate to check.

The 10% trick

To find 10% of a number, divide it by 10. Every digit moves one place to the right: 10% of 340 is 34, and 10% of 7 is 0.7. This one step unlocks almost every other percent.

See it: pictures to draw with your child

Copy these onto paper or a whiteboard. A quick sketch makes the idea concrete before any written method.

percentamount0%010%3420%6850%170100%340
From 10% of 340 (34), double for 20% and multiply by 5 for 50%.
25%100%2080percentamount
25% is half of half: 80 → 40 → 20.
15% = 10% + 5%
15% splits into 10% and 5%.

The 1% trick

To find 1%, divide by 100. Every digit moves two places right: 1% of 340 is 3.4. Use 1% for small or awkward percents.

For example, 3% of 340 is 3 × 3.4 = 10.2. And 9% is 10% − 1%: 34 − 3.4 = 30.6. Taking away can be quicker than adding.

The swap trick

x% of y is always the same as y% of x. So 8% of 50 is the same as 50% of 8, which is 4. Swapping turns a hard question into an easy one.

It works because “percent of” means multiply. 8/100 × 50 and 50/100 × 8 are the same product. For example, 4% of 75 = 75% of 4 = 3.

Check your mental math

Work it out in your head first, then type it in to check each step.

Halving and doubling

50% is half, and 25% is half of a half. 12.5% is half again. So 12.5% of 80 is 10, because 80 → 40 → 20 → 10.

Doubling works upwards too. If 10% of a bill is $4.50, then 20% is $9. Children who are quick at halving and doubling find percents easy.

Estimating percents

An estimate is a quick check. Round the percent, the number or both to friendly values. For example, 48% of 203 is about 50% of 200, which is 100.

Estimate before calculating, then compare. If a calculation says 19% of 60 is 114, the estimate (about 20% of 60, so 12) shows a slip with the decimal point.

How to practice mental percents

Short daily drills work best. Ask three quick questions at breakfast: 10% of 80, 25% of 60, 5% of 120. Then ask how your child worked each one out.

Use real prices whenever you can. In a shop, ask “what is 20% off this $35 game?” Real money makes the tricks stick, and children enjoy catching a good deal.

Quiz: percents in your head

Use the 10%, 1%, halving and swap tricks. Type the number only.

Where this fits: percentages step by step

  1. What is a percent
  2. Percent to fraction
  3. Fraction to percent
  4. Percent to decimal
  5. Decimal to percent
  6. Comparing fractions, decimals and percents
  7. Benchmark percents
  8. Percent mental math (this page)
  9. Over 100% and under 1%
  10. Percent of a number
  11. Finding the percent
  12. Finding the whole
  13. Percent change
  14. Percent increase and decrease
  15. Discounts and sale price
  16. Sales tax
  17. Tips
  18. Simple interest
  19. Markup, commission and percent error
  20. Percent word problems

What to say to your child

A short script for teaching it the first time. Pause after each line and let your child do the next step.

  1. “What is 10% of 60?” (6.)
  2. “Then what is 5%?” (3, half of 6.)
  3. “So what is 15% of 60?” (9.)
  4. “What is 8% of 25? Try swapping.” (25% of 8 = 2.)
  5. “Is 9% of 200 more or less than 20?” (Less: 10% is 20.)

More worked examples

Example 1: 15% of 60

10% is 6 and 5% is 3, so 15% is 9.

Example 2: 35% of 80

10% is 8, so 30% is 24. 5% is 4. Total: 28.

Example 3: 4% of 25

Swap: 25% of 4 is 1.

Example 4: 99% of 300

100% is 300, and 1% is 3. So 99% is 297.

Practice questions (answers below)

Ask these out loud or write them down. Check each answer, and ask “how did you get that?” for one or two.

  1. 10% of 90
  2. 20% of 45
  3. 5% of 80
  4. 1% of 650
  5. 15% of 40
  6. 25% of 120
  7. 12% of 50 (swap)
  8. 90% of 40
  9. Estimate 49% of 81
  10. Stretch: 17.5% of 200
Show the answers
  1. 9
  2. 9
  3. 4
  4. 6.5
  5. 6
  6. 30
  7. 6
  8. 36
  9. about 40
  10. 35

Common mistakes

10% of 50 is 10Uses the percent as the answer.Fix: divide by 10: 5.
1% of 300 is 30Divides by 10 instead of 100.Fix: 1% means ÷ 100: 3.
Adding percents of different numbersAdds 10% of 40 and 5% of 60.Fix: build a percent from pieces of the same number.

How to teach it at home

Breakfast drill

Three quick percent questions each morning.

Shop sale hunt

Find sale signs and work out the savings in your head.

Restaurant tip

Let your child work out a 10% or 15% tip.

By grade

More for each grade: Grade 6 math and the CBSE class 6 maths guide; Grade 7 math and the CBSE class 7 maths guide.

Worksheets and practice

Percent of a number worksheet, page 1

Percent of a number worksheet: printable PDF with an answer key. · Make a fresh test

Percent practice · percent calculator · fraction, decimal and percent converter · percentage quiz · percent levels · percent games · percentages for kids

Last reviewed

Questions parents ask

How do you find a percent in your head?

Start from 10% (divide by 10) or 1% (divide by 100), then build up the percent you need.

How do you find 15% of a number quickly?

Find 10%, halve it for 5%, and add the two. 15% of 60 is 6 + 3 = 9.

Why is 8% of 50 the same as 50% of 8?

Because percent of means multiply, and multiplying works in any order.

How do you estimate a percent?

Round to friendly numbers. 48% of 203 is about 50% of 200, which is 100.

What grade learns percent mental math?

Grades 6 and 7, under Common Core 6.RP.A.3c and 7.EE.B.3.