The fastest way to do a percentage in your head is the 10% trick: move the decimal point one place to the left to get 10% of any number, then build the percentage you want from there. To get 10% of $84.50, slide the decimal one spot and you have $8.45 - and almost every other percentage is just that number doubled, halved, or added together.
This guide shows you the handful of shortcuts that cover 95% of everyday percentage math: tips, discounts, test grades, and sales tax. For anything messy or high-stakes, do the head math to sanity-check, then confirm the exact figure with the Percentage Calculator.
First, know which of the three questions you are asking
Every percentage problem is one of three shapes. Naming the shape first is half the battle, because each one has a different mental shortcut.
- What is X% of Y? (find a part) - "What is 20% of $84.50?" This is the tip-and-discount question.
- A is what percent of B? (find the rate) - "27 out of 40 is what percent?" This is the grade-and-score question.
- X is P% of what number? (find the whole) - "$60 is 15% of what?" This is the work-backward question.
Most quick head math lives in question 1, so that is where the tricks pay off the most. Question 2 is a single division, and question 3 is a single division the other way. We cover all three below.
The 10% trick (your foundation)
To get 10% of any number, move the decimal point one place to the left. That is the whole trick, and it unlocks everything else.
| Number | 10% of it | How |
|---|---|---|
| $84.50 | $8.45 | Decimal moves one left |
| $250 | $25 | Drop one zero |
| $47.30 | $4.73 | Decimal moves one left |
Once you have 10%, you can build common percentages by combining:
- 5% = half of 10%. (10% of $84.50 is $8.45, so 5% is about $4.23.)
- 20% = 10% doubled. ($8.45 doubled is $16.90.)
- 15% = 10% plus 5%. ($8.45 + $4.23 = about $12.68.)
- 25% = 10% + 10% + 5%, or just a quarter of the number.
- 30% = 10% tripled.
The 1% trick (for awkward percentages)
To get 1% of any number, move the decimal point two places to the left. Use this when the percentage is not a clean multiple of 5 - like 7%, 8%, or a sales-tax rate.
Example: what is 7% of $250? Move the decimal two places to get 1% = $2.50. Then multiply by 7: $2.50 x 7 = $17.50. Done in your head, no calculator.
The 1% trick also rescues you on odd sales-tax rates. At an 8.25% rate on a $60 item, 1% is $0.60, so 8% is $4.80 and the extra 0.25% is about $0.15 - roughly $4.95 in tax. The Sales Tax Calculator nails the exact cent when it matters.
Restaurant tips, the fast way
Tipping is the most common "what is X% of Y" problem in daily life, and the 10% trick makes it trivial. Say the bill is $84.50:
| Tip you want | Mental steps | Tip amount |
|---|---|---|
| 15% | $8.45 + half of it ($4.23) | about $12.68 |
| 18% | 20% ($16.90) minus 2% ($1.69) | about $15.21 |
| 20% | $8.45 doubled | $16.90 |
The 18% trick - take 20% and back off 2% (which is 1% twice) - is the one people miss. For splitting the total check across friends, the Tip Calculator does the tip and the per-person split at once.
Grades and scores: A is what percent of B?
This is question 2: divide the part by the whole, then move the decimal two places to the right (the reverse of the 1% trick) to read it as a percent. Scored 27 out of 40? Divide 27 by 40 to get 0.675, then slide the decimal: 67.5%.
Clean denominators make this a head-math problem. 45 out of 60 is the same as 3 out of 4, which everyone knows is 75%. When the numbers are ugly, the Percentage Calculator handles the division for you.
Working backward: X is P% of what number?
This is question 3, and the shortcut is to divide by the percentage written as a decimal. If a $60 commission represents 15% of a sale, divide $60 by 0.15 to find the sale was $400.
A faster mental route: $60 is 15%, so 1% is $60 divided by 15 = $4, and 100% is $4 x 100 = $400. Same answer, no decimal division. This "find the whole" move is also how you reverse-engineer an original price from a sale price, which we cover in the FAQs.
Two shortcuts that feel like magic
The flip trick
X% of Y always equals Y% of X. So if 8% of 50 feels hard, flip it: 50% of 8 is just 4. Same answer, far easier. This single rule rescues you more often than you would expect.
Stacked discounts are not additive
20% off, then another 10% off, is not 30% off. You keep 80% of the price, then 90% of what is left: 0.80 x 0.90 = 0.72, so you keep 72% and the real discount is 28%. On a $100 item that is $72, not $70. Whenever you are stacking, multiply the "keep" factors instead of adding the discounts.
Don't confuse this with percentage change
All the tricks above answer "how much is this percentage?" They do not tell you how much something grew or shrank between two values. "My rent went from $1,500 to $1,650 - what percent increase?" is a different question that needs the Percentage Change Calculator. Use the percentage tool for a single slice; use the change tool to compare a before and an after.
When to stop doing it in your head
Head math is for speed and for catching obvious errors - if a tip "feels" like $40 on a $30 meal, you know something is wrong. But for money you actually pay or invoice, confirm the exact figure. Pop the numbers into the Percentage Calculator for any of the three questions, and reach for the Discount Calculator when a sale price is on the line.
For a clear primer on the underlying arithmetic, the U.S. Consumer.gov guides are a plain-language, ad-free reference.
Try it yourself
Run your own numbers in the free Percentage Calculator — instant, private, no sign-up.
Open the Percentage Calculator →Frequently asked questions
- How do I quickly calculate 15% in my head?
- Find 10% by moving the decimal one place left, then add half of that for the extra 5%. On an $84.50 bill, 10% is $8.45 and half of that is about $4.23, so 15% is roughly $12.68. For 20%, just double the 10% figure to get $16.90.
- What is the 1% trick for percentages?
- The 1% trick is moving the decimal point two places to the left to get 1% of a number, then multiplying up to the percentage you need. For 7% of $250, 1% is $2.50, so 7% is $2.50 x 7 = $17.50. It is the fastest way to handle odd percentages like 7% or 8.25%.
- Is 20% off then 10% off the same as 30% off?
- No. Stacked discounts multiply, they do not add. You keep 80% then 90% of the price: 0.80 x 0.90 = 0.72, so you pay 72% and save 28%, not 30%. On a $100 item the price is $72, which is $2 more than a flat 30% off would cost.
- How do I find what percent one number is of another?
- Divide the first number by the second, then move the decimal two places to the right to read it as a percent. Scored 27 out of 40? 27 divided by 40 is 0.675, which is 67.5%. A score of 45 out of 60 simplifies to 3 out of 4, which is 75%.
- How do I work backward from a percentage to the whole?
- Divide the known amount by the percentage written as a decimal. If $60 is 15% of a sale, $60 divided by 0.15 equals a $400 sale. In your head, find 1% first: $60 divided by 15 is $4, and 100% is $4 x 100 = $400.
- What is the flip trick for percentages?
- X% of Y always equals Y% of X, so you can swap the numbers to make the math easier. If 8% of 50 is awkward, flip it to 50% of 8, which is simply 4. The answer is identical, just easier to compute.
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