Figuring out how much to leave for a restaurant tip was a little more difficult for some of us before there were apps to calculate it. There are several ways to calculate a tip in your head, though, and they aren't difficult -- this method will work no matter what (some calculations are based on the tax, but some states don't charge tax so those methods aren't foolproof).
Here's what you're aiming for: Tip your waiter or waitress 15 percent of the pre-tax bill (and as much as 20 percent for excellent service) [source: The Emily Post Institute]. Despite how many of us have trouble with this, one of the best ways to figure out that 15 percent in your head is no more than a two-step process.
First, calculate one percent of the bill. Let's use a $24.00 bill as our example: One percent of a $24.00 bill is $0.24 (just move the decimal point two places to the left).
Next decide how much you'd like to tip. For a 15 percent tip, multiply $0.24 by 15, which equals a $3.60 tip. Twenty percent for great service? Multiply $0.24 by 20 for a $4.80 tip -- and so on for calculating 16, 17 or any other percent you'd like.