How do you calculate a tip quickly?

Move the decimal point one place left to get 10%, then add half of it again for 15% or double it for 20%.

Should you tip on the pre-tax or post-tax total?

Convention is to tip on the pre-tax amount, though many people tip on the total. The difference is usually small.

How much should you tip?

It depends entirely on where you are. In the United States 15–20% is expected. Across much of Europe, Asia and Australia tipping is optional and 5–10% is generous. In Japan it can cause offence.

How to Split a Bill and Work Out a Tip

Move the decimal one place left for 10%. Everything else builds from there.

An 84 bill: 10% is 8.40. Want 15%? Add half again → 12.60. Want 20%? Double it → 16.80.

Splitting evenly

Add the tip, then divide.

84 + 12.60 = 96.60
96.60 ÷ 4 = 24.15 each

Tip first, divide second. Dividing first and having each person tip separately gives the same total, but invites rounding gaps — four people each rounding 21 up to 22 leaves the tip 1.60 heavier than intended, which is fine, whereas four people rounding down leaves the server short.

Uneven orders

Two people had 12 meals, two had 30. Splitting evenly means the cheap orders subsidise the expensive ones by 9 each.

The fair method is proportional. Everyone pays their own item total plus the same percentage on top:

Bill 84, tip 15% → multiplier 1.15
12 order → 12 × 1.15 = 13.80
30 order → 30 × 1.15 = 34.50

Check: (13.80 × 2) + (34.50 × 2) = 96.60 ✓

One multiplier, applied to each person’s own total. This scales to any number of people and any mix of orders.

Tipping on tax

Convention is to tip on the pre-tax subtotal, since the tax is not part of the service.

In practice the difference is small. On a 100 meal with 10% tax, tipping 20% pre-tax is 20 and post-tax is 22. Two units. Worth knowing, not worth arguing about.

Where it does matter is on large bills — a 400 dinner makes that gap 8 or 9, which is real money.

The rounding question

Rounding up to something tidy is common and usually kind, but round the total, not each share.

Rounding a 96.60 bill to 100 adds 3.40 and gives a clean 25 each. Rounding each 24.15 share to 25 individually gets you to the same place — but only if everyone actually rounds the same way, which they will not.

Decide the total first, then divide.

Where tipping norms differ

This is the part no calculator can answer, and it varies more than most people expect.

United States — 15–20% is the standard and effectively expected. Service staff are often paid on the assumption that tips will arrive.
Canada — similar, typically 15–18%.
United Kingdom — 10–12.5%, and frequently added to the bill already as a service charge. Check before adding more.
Much of Europe — service is usually included. Rounding up or leaving 5–10% is normal.
Australia and New Zealand — optional. 10% for genuinely good service is generous.
Japan — not customary, and can be taken as rude.

When in doubt, look at the bill for a service charge before adding anything. Paying it twice is the more common error.