What is the fastest way to work out 15% or 20% in my head?

Find 10% by moving the decimal one place left. Half of that is 5%, so 10% plus 5% gives 15%; doubling 10% gives 20%. On a 68.40 bill: 6.84, then 3.42, so 15% is 10.26 and 20% is 13.68.

Should I tip when a service charge is already included?

Generally no — that is what the charge is for. Some people add a small amount for exceptional service, but adding a full percentage tip on top of an included charge means paying twice for the same thing.

Tip Calculator

Tip calculator

Calculate tip amount and split the bill per person.

Enter bill amount to see results.

Tipping: the arithmetic and the conventions

The maths is a single multiplication. The harder part is knowing what is expected where, because tipping norms vary enormously between countries and getting it wrong is embarrassing in both directions.

How it works

Multiplies the bill by the tip percentage and adds it to give the total.
Splits the total across a group, so nobody has to do the division at the table.
Lets you round the final figure up, which is what most people actually do.

tip   = bill × rate
total = bill × (1 + rate)
per person = total ÷ people

mental method
  10%  move the decimal one place left
  15%  10% plus half of it
  20%  10% doubled

Worked example

A 68.40 bill for four people, tipping 18%.

10% of 68.40 = 6.84
5% = 3.42, so 15% = 10.26
18% ≈ 15% + 3% → 10.26 + 2.05 = 12.31
total = 68.40 + 12.31 = 80.71
per person = 80.71 ÷ 4 = 20.18

Just over 20 each. In practice most groups round to 21 a head, which lands the tip nearer 23% — rounding up is where the real generosity happens.

Reading the result

Norms differ sharply by country. In the United States 18–20% is effectively mandatory because tipped staff are paid below minimum wage. Across most of Europe service is included and rounding up or leaving 5–10% is generous. In Japan tipping can cause genuine offence.
Check whether service is already on the bill before adding anything. A 12.5% service charge plus a 15% tip is a 27.5% total that you probably did not intend.
Tip on the pre-tax amount if you want to be precise about it, though most people do not bother and the difference is small. Never tip on a total that already includes a service charge.
Card machines increasingly suggest percentages calculated on the post-tax, post-service total, and the suggested options often start at 18%. The figure is a default, not a rule.

Common questions

What is the fastest way to work out 15% or 20% in my head?: Find 10% by moving the decimal one place left. Half of that is 5%, so 10% plus 5% gives 15%; doubling 10% gives 20%. On a 68.40 bill: 6.84, then 3.42, so 15% is 10.26 and 20% is 13.68.
Should I tip when a service charge is already included?: Generally no — that is what the charge is for. Some people add a small amount for exceptional service, but adding a full percentage tip on top of an included charge means paying twice for the same thing.