How to calculate a tip

Move the decimal point one place to the left. That is 10%. Everything else is arithmetic on top of it.

18% and 22% are the two that stall people. 18% is 20% minus a tenth of itself: $16.84 less $1.68 is $15.16. 22% is 20% plus a tenth of itself: $16.84 plus $1.68 is $18.52. If that is more mental arithmetic than you want at the table, that is what this site is for.

Before or after tax?

Both are common and neither is wrong. Tipping on the pre-tax subtotal is the stricter reading; tipping on the total is simpler and slightly more generous. On a $84.20 check with $7 of tax, the difference at 20% is about $1.40. Pick one and stop thinking about it.

When there is already a service charge

Large parties, some bars and a growing number of restaurants add a service charge or an automatic gratuity to the check. Read the line before you add anything: if a 20% service charge is already there, an additional 20% is a 40% tip. Some houses pass the service charge to staff, some do not, and the check rarely says which — asking is fair game.

Rounding

Rounding to a whole dollar is normal and nobody minds. There are two ways to do it: round the tip so the tip is a clean number, or round the total so the amount you sign for is clean. TYPTAPP does either, and when you round the total it adjusts the tip rather than the check — the check is the check.

There is no universally correct percentage. In US sit-down restaurants most people land between 15% and 25%. Norms differ by country, by service and by situation, and a calculator is not the right thing to be taking etiquette instruction from.