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

Find 10% by moving the decimal point one place left, then double it. On a $46 bill, 10% is $4.60, so 20% is $9.20. No multiplication by 0.2 required.

How do I calculate an 18% tip without a calculator?

Take 10% (move the decimal one place left), then add half of that 10% (which is 5%) to get 15%, then add roughly a fifth more to approximate 18% — or more simply, take 20% and subtract 10% of that 20% figure. On a $50 bill: 10% is $5, 20% is $10, and 18% is 20% minus 10% of $10, i.e., $10 − $1 = $9.

What's 15% of a bill, quickly?

Take 10% of the bill, then add half of that 10% amount. On a $40 bill, 10% is $4, half of that is $2, so 15% is $4 + $2 = $6.

Is there a shortcut that works for any percentage?

Yes — find 1% by moving the decimal two places left, then multiply by the percentage you want. It's more steps than the 10%-based shortcuts above, but it works for any number, including odd ones like 22% or 17%, where the doubling/halving tricks don't line up as cleanly.

Guide

Tip math without a calculator: 15%, 18%, 20% fast facts

Every common tip percentage is a cheap operation on 10% of the bill — you never actually need to multiply by 0.18 or 0.20 directly.

By Buğra SözeriPublished September 25, 2026

You don’t need to multiply by 0.15, 0.18, or 0.20 to find a tip — every one of those common percentages is a cheap operation on 10% of the bill, which itself is just moving the decimal point one place to the left. Once you have 10%, the rest is addition and doubling.

Start with 10%, build everything else from it

Target tip	How to get there from 10%
10%	Move the decimal one place left
15%	10% + half of 10% (5%)
20%	Double 10%
18%	20% − 10% of that 20% figure

Worked example: a $46.00 bill

10% = $4.60 (move decimal one place left)
15% = $4.60 + $2.30 (half of $4.60) = $6.90
20% = $4.60 × 2 = $9.20
18% = $9.20 − $0.92 (10% of $9.20) = $8.28

Check against exact math: $46.00 × 0.18 = $8.28 — matches. The 18% shortcut (take 20%, then subtract a tenth of that number) works because 18% is exactly 90% of 20%, and subtracting 10% of a number is the same one-decimal-place move you already used to find the original 10%.

Reference table: common bill amounts

Bill	15%	18%	20%
$20.00	$3.00	$3.60	$4.00
$35.00	$5.25	$6.30	$7.00
$50.00	$7.50	$9.00	$10.00
$75.00	$11.25	$13.50	$15.00
$100.00	$15.00	$18.00	$20.00

These three percentages (15%, 18%, 20%) span the range most commonly cited for US restaurant service, though what counts as standard varies by region and service type — see how tipping works around the world for how that range shifts outside the US, and whether tax should factor into the base amount at all in should you tip on pre-tax or post-tax.

Frequently asked questions

What's the fastest way to calculate a 20% tip in my head?: Find 10% by moving the decimal point one place left, then double it. On a $46 bill, 10% is $4.60, so 20% is $9.20. No multiplication by 0.2 required.
How do I calculate an 18% tip without a calculator?: Take 10% (move the decimal one place left), then add half of that 10% (which is 5%) to get 15%, then add roughly a fifth more to approximate 18% — or more simply, take 20% and subtract 10% of that 20% figure. On a $50 bill: 10% is $5, 20% is $10, and 18% is 20% minus 10% of $10, i.e., $10 − $1 = $9.
What's 15% of a bill, quickly?: Take 10% of the bill, then add half of that 10% amount. On a $40 bill, 10% is $4, half of that is $2, so 15% is $4 + $2 = $6.
Is there a shortcut that works for any percentage?: Yes — find 1% by moving the decimal two places left, then multiply by the percentage you want. It's more steps than the 10%-based shortcuts above, but it works for any number, including odd ones like 22% or 17%, where the doubling/halving tricks don't line up as cleanly.

Sources & references

Authoritative references cited by this piece. Verified by Buğra Sözeri on the dates shown and re-checked at every deploy.

CFPB — Understanding Your Restaurant Bill and Tipping — Background on typical US restaurant tipping ranges referenced here(as of 2026-09-25)