Splitting a restaurant bill sounds like simple arithmetic until you're actually doing it at the table, and the reason it gets confusing isn't the math itself -- it's that there are a few different ways to sequence the calculation, and each one gives a slightly different answer.
Tip on the subtotal or the total?
The tip is conventionally calculated on the pre-tax subtotal, not on the total after tax is added. Tipping on the post-tax total means you're effectively tipping extra on money that went to the government, not the server -- a small difference on a modest bill, but it adds up on a large one. Some people do it anyway out of habit or because it's easier to eyeball 20% of the number printed at the bottom of the receipt, but if you're aiming for precision, the subtotal is the correct base.
Splitting evenly vs. splitting by what you ordered
There are two legitimate ways to split a group bill, and they're not interchangeable. Splitting evenly means the total (bill plus tax plus tip) is divided by the number of people, regardless of who ordered what. Splitting by item means each person pays for what they actually ordered, plus their share of tax and tip proportional to their order.
Even splits are simpler and fine when everyone ordered roughly the same amount. They get contentious fast when one person had a salad and water and another had two cocktails and an appetizer -- the even split quietly subsidizes the bigger order.
The rounding problem nobody mentions
When you split a tip evenly among an odd number of people, the per-person amount almost never comes out to a clean number of cents. Someone has to round up, someone rounds down, or the group agrees to round to the nearest dollar and eat the small discrepancy. This isn't a flaw in anyone's math -- it's just what happens when you divide a number that doesn't divide evenly, and it's worth deciding upfront how the group wants to handle it rather than arguing over a few cents after the fact.
Why the total changes depending on what you tip on
A useful gut check: 20% of the pre-tax subtotal is always a smaller dollar amount than 20% of the post-tax total, since tax is added on top of the subtotal. If your mental math for a tip is based on the number at the very bottom of the receipt, you're tipping on a slightly inflated base without realizing it. It's not a huge difference on most bills, but it's the kind of small, consistent overpayment that's easy to fix once you know where it's coming from.
Do the math instead of estimating at the table
None of this is complicated once you separate the steps: calculate the tip on the subtotal, add tax separately, then decide whether to split evenly or by what each person ordered. The hard part is doing all three steps correctly in your head while everyone's waiting.
The Tip Calculator handles the tip percentage and even split for you -- pick a preset percentage or enter a custom one, enter the number of people, and it does the division so nobody's stuck doing fraction math on a napkin.