How to Calculate a Tip in Your Head (Fast Mental Math)

You don’t need a calculator app to tip confidently. A handful of mental shortcuts get you to an accurate tip in seconds, and once you know the 10% trick, every other percentage builds from it.

The 10% shortcut: your foundation for everything else

To find 10% of any number, just move the decimal point one place to the left. A $64.00 bill becomes $6.40. That single move is the base every other percentage below builds on.

Building up to 15%, 18%, and 20%

• 15%: Take 10%, then add half of that amount again. $6.40 + $3.20 = $9.60.
• 20%: Just double the 10% figure. $6.40 × 2 = $12.80.
• 18%: Find 9% (10% minus 1%, or $6.40 − $0.64 = $5.76), then double it: $5.76 × 2 = $11.52.

A full worked example

On a $64.00 bill:

Tip %	Tip amount	Total with tip
10%	$6.40	$70.40
15%	$9.60	$73.60
18%	$11.52	$75.52
20%	$12.80	$76.80

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Splitting the tip across a group

Once you have the total with tip, divide by the number of people. A $76.80 total (bill plus a 20% tip) split four ways comes to $19.20 each, assuming an even split rather than itemized orders.

Should you tip on the pre-tax or post-tax amount?

Tipping etiquette generally calls for calculating the tip on the pre-tax subtotal, not the total after sales tax is added. On a $64.00 bill with 8% tax ($5.12), an 18% tip on the pre-tax amount is $11.52, while 18% of the post-tax total ($69.12) comes to $12.44, a difference of under a dollar, but the pre-tax method is the more standard convention.

Should I tip on the pre-tax or post-tax amount?

Standard etiquette is to calculate the tip on the pre-tax subtotal. Tipping on the post-tax total isn’t wrong, it’s just slightly more generous and less common.

What’s the fastest way to calculate a 20% tip in my head?

Move the decimal point one place left to get 10%, then double that number. A $50 bill becomes $5.00 at 10%, doubled to $10.00 for 20%.

Is 15% still an acceptable tip?

Tipping norms vary by region and type of service, but 18-20% has become the more common expectation for full-service dining in the US, with 15% now often seen as a minimum for adequate service rather than a strong tip.

How do I split a tip fairly when people ordered different amounts?

Calculate each person’s individual tip based on their own portion of the bill rather than splitting one combined tip evenly, this keeps the tip proportional to what each person actually ordered.