Budget for a family dinner at a restaurant.
A restaurant bill is the menu total plus roughly a quarter, and the two additions behave differently. Tax is a fixed statutory percentage of the subtotal. The tip is discretionary and, by convention in the US, calculated on the pre-tax subtotal rather than the taxed total — tipping on the post-tax figure quietly adds a tip on the tax. Drinks are the line families underestimate: four soft drinks at four dollars is sixteen, which on a 62 dollar food order is a quarter again. Ordering tap water for the children is usually the single largest saving available on a family meal out.
Family Dinner Out Budget
Subtotal = adults x main + children x child meal + party x drinks; tax = subtotal x tax%; tip = subtotal x tip%; total = subtotal + tax + tip
Pre-tax is the convention. Tipping on the taxed total means you are also tipping on the tax, which adds a couple of percent.
For two adults and two children with drinks, tax and an 18% tip, around 80-100 dollars at mid-range prices. Drinks and tip together are usually a third of it.