The Math Behind Splitting a Bill and Calculating a Tip
How tip and bill-splitting calculations work, and a quick mental-math shortcut for common tip percentages.
Tipping math is simple percentage arithmetic, but a couple of shortcuts make it fast enough to do without a calculator at the table.
The formula
Tip = Bill × (Tip % / 100). Total = Bill × (1 + Tip% / 100). Per Person = Total / Number of People.
Worked example
An $85 bill with an 18% tip, split between 2 people: Tip = 85 × 0.18 = $15.30. Total = $100.30. Per person: $100.30 / 2 = $50.15.
A mental-math shortcut for 18-20%
Since 10% of any bill is just moving the decimal point one place left, and 20% is double that, most tip percentages can be estimated from those two anchors. For an $85 bill: 10% = $8.50, so 20% = $17.00. An 18% tip sits close to but slightly under that 20% mark — roughly $15.30, matching the exact calculation above without needing a calculator at all.
Splitting unevenly
The formula above assumes an even split, but real group dinners often split by what each person ordered instead. In that case, calculate the tip on the total bill first, then either divide the tip proportionally to each person's share of the pre-tip bill, or simply add the same tip percentage to each individual's own subtotal — both approaches produce a fair result, just via a different order of operations.
Common mistakes to avoid
- Calculating the tip on the post-tax total rather than the pre-tax subtotal — standard etiquette in most places is to tip on the pre-tax amount, though this varies by local custom
- Rounding the per-person split down in a way that leaves the total tip short of the intended percentage
- Forgetting that some venues already add an automatic gratuity for large groups, in which case an additional tip may not be expected
Calculate your own split with the tip calculator, and figure sales tax separately with the sales tax calculator.