How do you calculate a tip in your head?

Anchor everything on 10 percent, which is just the bill with the decimal moved one place left. On a $46 check, 10 percent is $4.60. Halve that for 5 percent, $2.30, so 15 percent is $6.90. Double the anchor for 20 percent, $9.20. Every common tip is one or two easy operations from that starting point, and rounding to convenient numbers is entirely acceptable, since nobody is auditing whether you left $9 or $9.20.

Two shortcuts speed it further. For 20 percent, double the bill and move the decimal one place: $46 doubles to $92, which becomes $9.20 directly. And on receipts showing tax, in many jurisdictions the tax is a known percentage you can double or scale to approximate a tip, though rates vary enough that this is a rough tool rather than a rule. When splitting a check, calculate the tip on the full total first and then divide, which avoids the rounding errors that accumulate when each person tips separately. Worth knowing on the etiquette side: tip on the pre-tax subtotal is technically standard, tipping on the total is common and generous by a small margin, and for takeout, delivery, and counter service the norms differ enough that they are their own conversation.