How To Calculate A Tip

A tip, or gratuity, is a voluntary payment on top of a bill, and the arithmetic is a single multiplication. Take the bill, multiply it by the tip rate written as a decimal, and add the result back to the bill. On 85.50 at 18 per cent that is 85.50 x 0.18 = 15.39 of tip and a total of 100.89. Almost nothing else in the calculation is difficult; the difficulty sits entirely in the decisions around it. The rate is the only free variable, and it is the part people argue about, while the bill is simply whatever the slip says it is.

The first decision is the base: the figure the percentage is actually applied to. In some places the convention is to tip on the pre-tax subtotal, on the reasoning that tax is not part of the service and should not itself be tipped. Elsewhere the custom is to tip on the full amount, including tax, because that is the number printed on the card slip. Both conventions exist and both are defensible, and the only genuine error is switching between them without noticing. The base matters because tax can be a meaningful fraction of a bill, and eighteen per cent of a tax is real money across a year of meals.

The size of that choice is worth seeing in numbers. Take a bill of 85.50 before tax with 9.50 of tax added on top, so the tax-inclusive total is 95. Tipping 18 per cent on the 85.50 gives 15.39, while tipping on the 95 total gives 17.10, which is 1.71 more. Run it the other way, with a bill of 85.50 that already contains 9.50 of tax, and the pre-tax base is 76, giving a tip of 13.68. Either way the swing is 18 per cent of the tax, not of the bill. It also shows why the base is worth settling before the arithmetic, since no amount of care with the multiplication can rescue a base chosen by accident.

Common rates cluster around a few round numbers. Fifteen per cent, eighteen per cent and twenty per cent cover most situations where tipping is expected, with eighteen sitting in the middle as a frequent default. These are conventions rather than calculations, and they differ by country, by city and by type of service. A rate that looks generous at a counter may look mean at a table with full service, so the same arithmetic can carry very different social readings. It helps to remember that the percentage is a social signal as much as a number, and that the same eighteen per cent reads differently in different rooms.

Mental arithmetic is where the method earns its keep, and the trick is to build everything from ten per cent. Moving the decimal point one place to the left gives ten per cent: 85.50 becomes 8.55. Twenty per cent is simply double that, 17.10. Eighteen per cent is twenty per cent minus two per cent, and two per cent is a fifth of ten per cent, so 8.55 / 5 = 1.71 and 17.10 - 1.71 = 15.39. Fifteen per cent, in the same style, is ten per cent plus half of ten per cent. The method works at any rate, which is the point: once ten per cent is in hand, every common tip is a small adjustment away.

Splitting a bill has one correct order: work out the tip on the whole bill, add it to get the total, and only then divide by the number of people. For 85.50 at 18 per cent across two people, the tip is 15.39, the total is 100.89, and each person owes 50.445, of which 7.695 is tip. Dividing first and then letting each person tip only their own share invites a different and usually smaller total, because the percentage ends up applied to a fraction of the bill. Each person's share then carries the same effective rate as the bill itself, which is the property that makes the method worth following.

Rounding is a normal part of paying and is harmless if it is done last. People often round the total up to the next convenient figure, which is fine, but the tip has to move with it. Rounding the bill while leaving the tip untouched quietly changes the effective rate, sometimes by a full percentage point on a small bill. Decide the rounded total first, then treat the tip and the rate as things derived from it. A tip of 15.39 rounded to 15.50 on a bill of 85.50 is an effective rate of 18.13 per cent, which is harmless; the same rounding applied to the tip alone would be the error.

A service charge is not the same thing as a tip, and stacking the two is a genuine error rather than a matter of taste. Many restaurants add a fixed service charge, often around ten or twelve per cent, and that charge may or may not be passed on to staff. If it is already on the bill, adding another eighteen per cent on top pays twice for the same service. Read the wording on the slip before deciding what the tip line means. When in doubt, ask what the charge is for, because a line described as a service charge is not the same as one described as gratuity.

Finally, context matters more than any formula. In some countries a tip is the main part of a server's wage and the difference between a good night and a bad one; in others it is unusual, already included, or even considered awkward. The calculation is the same everywhere — bill times rate, plus the bill, divided by the people — but what counts as a normal rate is a local question. The calculator here simply makes the arithmetic reliable so that the judgement can stay where it belongs. The arithmetic is the easy half, and getting it reliably right is all this page sets out to do.