How to calculate a bill split, step by step
To calculate a bill split, add the sales tax to the bill, add the tip, and divide the total by the number of people. The formula is per person = (bill + tax + tip) ÷ people, and the tax is the bill times the tax rate while the tip is the bill times the tip percentage. On the worked example below, a $120 bill with 8% tax and a 20% tip on the pre-tax bill is $153.60 in all, or $38.40 each for 4 people. The whole method is four steps, and the only decision in it is the tip base — whether the tip is a percentage of the bill before tax or of the bill after tax.
The worked example: $120, 4 people, 8% tax, 20% tip
The worked example, step by step. Step 1 — write down the bill: $120 before tax. Step 2 — work out the tax: $120 × 8% is $9.60. Step 3 — work out the tip: $120 × 20% is $24.00, on the pre-tax bill. Step 4 — add them up: $120 + $9.60 + $24.00 is $153.60. Step 5 — divide by the people: $153.60 ÷ 4 is $38.40 each. That is the whole calculation, and the bill-split-calculator does it for any bill: a $60 bill for 2 people is $38.40 each too, because the bill and the people halve together, and a $200 bill for 5 people is $51.20 each.
What a bill split is
A bill split is the per-person share of a restaurant bill after tax and tip. It has three pieces: the bill itself, which is the food and drink before tax; the sales tax, which the restaurant charges on the bill and passes to the state; and the tip, which is a percentage you choose, usually 15% to 25% in the United States. The tax and the tip are both percentages of the bill, so the total is the bill times one plus the tax rate plus the tip rate — on the worked example, $120 × 1.28 is $153.60. The split is the total divided by the people, and it only works as an even split when everyone ordered roughly the same amount.
The tip base: pre-tax or post-tax
The tip base is the number the tip percentage applies to, and it is the one decision that changes the answer. The usual convention is the pre-tax bill: a 20% tip on a $120 bill is $24.00, and each of 4 people pays $38.40. The post-tax tip applies the same 20% to the bill plus the tax, $129.60, which is $25.92 — $1.92 more, and the per-person share rises to $38.88. Neither is wrong, but the table has to agree on one of them, because two people tipping on different bases will get two different totals. If the group has not said which, use the pre-tax bill: it is the standard, and it is the default on the bill-split-calculator. If the receipt already includes a service charge, the tip is already in the total — check before you add another one.
How to work backwards from a per-person budget
To find the biggest bill a budget allows, work the formula backwards — or use the reverses in the bill-split-calculator. For 4 people to pay $40 each at 8% tax and a 20% tip, the bill can be $125.00: $125.00 × 1.28 is $160.00, and $160.00 ÷ 4 is $40.00. The people count works the other way: on a $120 bill the most people who can split it and still pay $30 or more each is 5, at $30.72 each — a sixth person drops the share to $25.60. And the tip works backwards too: a tip of about 25.3% takes the $120 bill to a $160.00 total. Working backwards is how a group sets its budget before the ordering starts, instead of arguing about the total after the food arrives.
How to split a bill unevenly
When one person had the lobster and another had the salad, an even split is unfair, and the fix is to split the food first and the tax and tip in the same proportions. Work out what each person ordered, then give each person the same share of the tax and the tip as their share of the food: if you ordered $60 of a $120 bill, you pay half the $9.60 tax ($4.80) and half the $24.00 tip ($12.00), so $76.80 in all. Do the same for everyone else, and the shares add up to the $153.60 total. The even split is the special case where everyone ordered the same amount — the uneven split is the same arithmetic, done per person instead of once for the table. If the group is large, one person should collect the shares and pay the card, so the restaurant gets one payment instead of six.
Common mistakes
The first mistake is tipping on the total after tax without saying so: the post-tax tip is $1.92 more on a $120 bill, and a table that mixes the two bases will not agree on the total. The second is forgetting the tax: $120 ÷ 4 is $30.00, but the real share is $38.40, because the tax and the tip add $33.60 to the bill. The third is splitting the bill before the tip: tipping on the per-person share instead of the whole bill gives the same answer only if the arithmetic is done right, and it is easy to round the share and lose a dollar. The fourth is rounding each share down: with 3 people on a $100 bill the share is $33.33, and three shares add to $99.99 — one cent short, which the person paying the card usually covers. And the fifth is tipping twice on a bill that already includes a service charge: check the receipt, because a 20% service charge is the tip.
A quick check you can do in your head
You can sanity-check any bill split without a calculator. First, the pieces must add up: $9.60 + $24.00 is $33.60, and $120 + $33.60 is $153.60 — if the pieces do not add to the total, one of them is wrong. Second, the per-person share times the people must give the total back: $38.40 × 4 is $153.60. Third, the tip must be the bill times the tip percentage: 20% of $120 is $24.00, and 20% of $129.60 is $25.92 — if the tip matches neither, the base is wrong. Fourth, the share must rise with the bill and fall with the people: a $200 bill for 5 people is $51.20 each, and a $60 bill for 2 people is $38.40 each. If a calculator breaks any of these checks, the number is wrong — run your numbers through the bill-split-calculator and compare them with the worked example above.