Should you tip on the pre-tax or post-tax amount?
In the United States the usual convention is to tip on the pre-tax amount — the subtotal, before sales tax is added. Tax is money that goes to the state, not to your server, so it is not normally part of what you tip on.
This matters more than it sounds. On a $100 meal with 9% sales tax, tipping 20% on the post-tax total adds $1.80 you did not need to pay. Over a year of eating out, that adds up — and most calculators, including the one Google shows you, have no tax field at all, so they quietly assume the number you type is already what you want to tip on.
Enter your subtotal and your local sales tax rate above, and this calculator will tip on the pre-tax amount, add the tax back on top, and give you the real total.
What if my receipt only shows the total?
Most receipts print the subtotal, the tax, and the total as separate lines — use the subtotal as your bill amount. If yours only shows the tax-inclusive total, switch "Tip on" to Total including tax and enter that number as the bill with tax set to 0.
Rounding up
Handing over a clean number is easier for everyone, especially when you are splitting. Choose round total up to bring the whole bill to the next dollar, or round each person up so nobody has to find coins. Either way the rounding is added to the tip, never to the tax.
How much should you tip?
Customs vary by country. In the United States, 15–20% is typical for sit-down service; some places add a service charge automatically, so check the bill before you add anything. In many countries tipping is optional or already included in the price.
Tipping customs and sales tax rates vary by country, state, and situation — use your own judgment.