How to calculate a tip in your head: 15%, 18%, 20%

To calculate a tip in your head, find 10% of the bill by moving the decimal point one place to the left, then build from it: 20% is double that, 15% is that plus half of it, and 18% is the 20% figure minus a small adjustment. Once you can find 10%, every common tip is one or two easy steps away.

This is general education about arithmetic. Tipping customs vary a great deal by country, by region and by type of service, so the percentages here are examples for practising the arithmetic, not a rule about what to leave.

Step 1: find 10%

Move the decimal point one place to the left. Three hypothetical bills, which we will use all the way through:

• A $40.00 bill gives $4.00.
• A $64.00 bill gives $6.40.
• An $83.50 bill gives $8.35.

Everything else is built from this number, so practise it until it feels automatic.

A 20% tip: double the 10%

Find 10%, then double it.

• $40.00 bill: 10% is $4.00, so 20% is $8.00.
• $64.00 bill: 10% is $6.40, so 20% is $12.80.
• $83.50 bill: 10% is $8.35, so 20% is $16.70.

This is the simplest of the three, and it is the stepping stone for 18%.

A 15% tip: 10% plus half of 10%

Find 10%, then halve it to get 5%. Add the two together.

• $40.00 bill: 10% is $4.00, half of that is $2.00, so 15% is $6.00.
• $64.00 bill: 10% is $6.40, half of that is $3.20, so 15% is $9.60.
• $83.50 bill: 10% is $8.35, half of that is about $4.18, so 15% is about $12.53.

Another way to see it: 15% is three-quarters of 20%. If you already know that 20% of $40.00 is $8.00, three-quarters of $8.00 is $6.00.

An 18% tip: 20% minus 2%

Eighteen percent looks awkward head-on, but it is easy as a subtraction. Find 20%, then take away 2%. To find 2%, find 1% by moving the decimal point two places to the left, then double it.

• $40.00 bill: 20% is $8.00. 1% is $0.40, so 2% is $0.80. 18% is $8.00 minus $0.80, which is $7.20.
• $64.00 bill: 20% is $12.80. 1% is $0.64, so 2% is $1.28. 18% is $12.80 minus $1.28, which is $11.52.
• $83.50 bill: 20% is $16.70. 1% is about $0.84, so 2% is $1.67. 18% is $16.70 minus $1.67, which is $15.03.

If that is too much subtraction for a crowded table, an estimate is fine: 18% is a little less than 20%, so take the 20% figure and shave a bit off. On the $64.00 bill, $11.50 is within a few cents of the exact $11.52.

Round the bill first

Real bills rarely land on round numbers, so round to a convenient amount before you start.

• A hypothetical $47.80 bill rounds up to $48.00.
• 10% of $48.00 is $4.80, and 20% is $9.60.
• Rounding up costs very little: 20% of the extra 20 cents is just 4 cents.

Before tax or after tax

Some people tip on the subtotal before tax, others on the final total. The arithmetic is identical; only the starting number changes. On a hypothetical $50.00 subtotal with $4.00 of tax, 20% of the subtotal is $10.00, and 20% of the $54.00 total is $10.80. Whichever you choose, the method above works the same way.

Splitting the bill

• Add the tip to the bill to get the total.
• Divide the total by the number of people.
• Example: a hypothetical $90.00 bill with a 20% tip of $18.00 comes to $108.00. Split four ways, that is $27.00 each.

Dividing by four is just halving twice: $108.00, then $54.00, then $27.00.

Check the bill before you calculate

• Look for a service charge already on the bill. Some restaurants add one, often for larger groups, and a full tip on top of it would pay for the same thing twice.
• Customs differ around the world. In some places service is included in the price or tipping is not expected, so when you travel, look up local practice before you go.

A quick reference

• 10%: move the decimal point one place left.
• 20%: double the 10%.
• 15%: the 10% plus half of it.
• 18%: the 20% minus 2%, where 2% is 1% doubled.

That is the whole toolkit. With a little practice, each of these takes a few seconds, and you can check the card machine's suggested amounts before you tap.

This post covers the arithmetic only; for budgeting or other decisions about your own money, a qualified financial professional can look at your full situation.