I've taken to tipping 18%, and the process I use to calculate that mentally starts with what you mentioned, but with one more step: Divide by 10 for 10%; doubl

I've taken to tipping 18%, and the process I use to calculate that mentally starts with what you mentioned, but with one more step: Divide by 10 for 10%; double that number for 20%. Take 10% away from that number to get 18%. E.g. 100 check move the decimal => 10 double that => 20 move the decimal => 2 subtract previous value from the one before (thus 90% of 20%, or 18%) => 18 This feels like a good mental math exercise to me on a 4- or 5-digit number since it requires a couple of values to be remembered while you're calculating. Gotta stay sharp!