How to Calculate a Tip in Your Head — Quick Mental Math Tricks
Simple mental math tricks to calculate 15%, 18%, 20%, and 25% tips without pulling out your phone. Move the decimal, double it, and you're done.
7 min read · Updated
Why Mental Math Still Matters
Yes, you have a phone. Yes, we built a tip calculator specifically so you don't have to do math at the table. But there are moments when mental math is faster: your phone is dead, your hands are full, you're splitting a bar tab quickly, or you just don't want to be the person pulling out a calculator in front of your date.
These tricks are simple enough that anyone can learn them in five minutes. They don't require you to be "good at math." They require you to move a decimal point and do basic addition. That's it.
The Foundation: Finding 10%
Every tip calculation starts here. To find 10% of any number, move the decimal point one place to the left. That's the entire trick.
- Bill is $54.00 → 10% = $5.40
- Bill is $127.50 → 10% = $12.75
- Bill is $83.20 → 10% = $8.32
If you can do this, you can calculate any standard tip percentage. Everything else is just adding or halving that 10% number.
Calculating 15% (The Quick Shortcut)
Fifteen percent is 10% plus half of 10%. Two steps:
- Find 10% (move the decimal)
- Take half of that number
- Add them together
Example: $64.00 bill
- 10% = $6.40
- Half of $6.40 = $3.20
- 15% = $6.40 + $3.20 = $9.60
Example: $38.00 bill
- 10% = $3.80
- Half of $3.80 = $1.90
- 15% = $3.80 + $1.90 = $5.70
If the half-of-10% step gives you an awkward number, just round. Nobody is going to argue about 30 cents.
Calculating 18% (The Trickier One)
Eighteen percent is the most common auto-gratuity rate and a solid "good service" tip. It's a little harder to do in your head, but here's the approach:
Method: 20% minus a little bit.
- Find 10% (move the decimal)
- Double it to get 20%
- Subtract about one-tenth of your 20% number
Example: $50.00 bill
- 10% = $5.00
- 20% = $10.00
- A tenth of $10 is $1.00
- 18% = $10.00 - $1.00 = $9.00
Example: $72.00 bill
- 10% = $7.20
- 20% = $14.40
- Subtract roughly $1.40
- 18% ≈ $13.00
Honestly, 18% is the hardest common percentage to calculate mentally. If you're aiming for roughly 18%, just calculate 20% and round down a dollar or two. Nobody — including your server — is going to notice the difference between 18% and 19%.
Calculating 20% (The Easiest Good Tip)
Twenty percent is the simplest tip to calculate, which is one reason it's become the default for good service. It's just 10% doubled.
- Find 10% (move the decimal)
- Double it
Example: $45.00 bill
- 10% = $4.50
- 20% = $4.50 x 2 = $9.00
Example: $112.00 bill
- 10% = $11.20
- 20% = $11.20 x 2 = $22.40
That's it. Two steps. This is why, if you want to keep things simple, 20% is the easiest percentage to calculate in your head.
Calculating 25% (For Exceptional Service)
Twenty-five percent sounds tricky, but it's just a quarter of the bill. Here's the simplest approach:
Method: 20% plus half of 10%.
- Find 10%
- Double it for 20%
- Add half of 10%
Example: $80.00 bill
- 10% = $8.00
- 20% = $16.00
- Half of 10% = $4.00
- 25% = $16.00 + $4.00 = $20.00
Alternative method: Divide the bill by 4. For $80, that's $80 / 4 = $20. For round numbers, this can be even faster.
The Rounding Trick
Here's the real secret to fast mental tip math: round your bill to a friendlier number first, then calculate.
Your bill is $53.78? Round it to $54 — or even $55. Now calculate:
- 20% of $55 = $11.00
Your bill is $87.43? Call it $87 or $90.
- 20% of $90 = $18.00
- 15% of $90 = $13.50
The difference between calculating a tip on $87.43 and $90.00 is about 50 cents. Not worth the mental gymnastics. Round to the nearest $5 or $10 and your life gets much easier.
Round the Total, Not Just the Tip
Another approach: calculate a rough tip and then round the total (bill plus tip) to a clean number. This is especially useful when paying with a credit card and writing in the total.
- Bill: $43.60
- Rough 20% in your head: about $8.70
- Total would be about $52.30
- Round to $53.00 — clean, easy to write, and you tipped approximately 21.5%
This method is fast and eliminates the need for exact math. Your server won't know or care whether you tipped 20% or 21.5%.
Pre-Tax vs. Post-Tax: Does It Matter for Mental Math?
Etiquette traditionally says to tip on the pre-tax amount. But when you're doing mental math at the table, you're usually looking at the total on the bill, which includes tax.
Here's the practical truth: for most bills, the difference between tipping on the pre-tax and post-tax amount is $1-3. If you're already rounding and estimating in your head, this difference is already baked into your margin of error. Don't stress about it for mental math purposes.
If you want the full breakdown on this debate, read our guide on tipping on pre-tax vs. post-tax amounts.
Quick Reference: Mental Math Cheat Sheet
Here's the summary you can memorize:
- 10%: Move the decimal left one place
- 15%: 10% + half of 10%
- 20%: 10% x 2 (or move decimal and double)
- 25%: 20% + half of 10% (or divide bill by 4)
- Rounding: Round the bill to the nearest $5 or $10 first, then calculate
For any other percentage, start from one of these anchor points and adjust up or down.
When to Just Use the Calculator
Mental math is great for speed. But there are situations where pulling out the calculator makes more sense:
- Group dining with a split bill. Mental math falls apart when you're splitting a $247 tab five ways with different orders. See our guide on splitting the bill and tip for group dining.
- When you want to rate the service honestly. If the service was mediocre and you want to tip 12% instead of defaulting to 20%, a calculator gives you the exact number. Our tip calculator lets you rate the service and get a fair suggestion.
- Large bills. On a $400 dinner, the difference between 18% and 20% is $8. That's worth calculating precisely.
- When you've had a few drinks. Your mental math ability after three cocktails is not what you think it is. Use the calculator.
The Bottom Line
You only need to remember one thing: move the decimal point to find 10%, then build from there. That single trick unlocks 15%, 20%, and 25% with basic addition. Combine it with rounding, and you can calculate any reasonable tip in under five seconds.
But when the math gets complicated — or when you want to tip based on the actual quality of service you received — our tip calculator does the thinking for you. Rate the service, get a fair number, and get on with your evening.