How To Calculate a 15% Tip

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If you are tipping at a restaurant, 20% is considered normal for good service - 15% would reflect poor service, and of course you can go higher for excellent service. 10% is acceptable for delivery or carry-out food orders. (And no, I don't live in some fancy-smancy big city - I'm in a Mid-West town of ~20,000 people).

Know your local sales tax rate (the % may be printed on your bill). Rounding up and multiplying is usually very easy. My local rate is 8.75%, so tax on a $20 tab is $1.75 which is printed on the bill showing a total of $21.75. Round the tax up to $2.00, and it's easy to do 2+2=$4 in your head. This won't always be as perfect as the math dude's approach (10% of $20 is $2, 2+2=$4 which is 20%).