How to Split a Bill (With Tax and Tip)

Splitting a bill is a small exercise in fair arithmetic: you take a shared subtotal, add the tax and tip the way your group agreed, then divide the result across the people at the table. The complications — what the tip is calculated on, how to handle rounding, and how to weight uneven orders — are where most disagreements live.

The basic equal split

Start with the simplest version. A $120 subtotal, an 8% sales tax, and an 18% tip, split evenly among 4 people. Work top-down:

tax = subtotal × taxRate          → 120 × 0.08 = 9.60
tip = subtotal × tipRate         → 120 × 0.18 = 21.60
total = subtotal + tax + tip     → 150.20
per person = total / people       → 150.20 / 4 = 37.55

Each person owes $37.55. The key decision baked in here is that the tip is computed on the pre-tax subtotal, which is the common convention in the US. You can just as well tip on the taxed amount — see the next section — but you must pick one and apply it consistently.

Tip on the subtotal, or on the taxed amount?

This is the most common source of "wait, I got a different number." Tipping on the pre-tax subtotal means tax is not tipped on. Tipping on the post-tax total means you also tip on the sales tax, which raises the tip slightly. With the numbers above, tipping on the taxed amount gives 18% of $129.60 = $23.33 instead of $21.60 — about $1.73 more, or $0.43 per person.

Neither is wrong. Tipping on the pre-tax amount is the more precise convention (you reward service on the food, not the government's cut), but tipping on the total is faster at the table because it's the number printed at the bottom of the receipt. The important thing is that everyone at the table agrees before the math starts.

Weighted splits: when people ordered differently

An equal split is unfair when one person had a salad and another had a steak and three drinks. A weighted split assigns each person a share proportional to what they ordered. Give each person a weight (1 for the salad, 2 for the steak-and-drinks), and the per-person share becomes:

fraction = personWeight / sum(weights)
share = total × fraction

With weights 1, 1, 2, 2 the fractions are 1/6, 1/6, 2/6, 2/6 — the two heavier orders pay twice what the light orders pay. The subtotal, tax, and tip all split in the same proportions, so the books still balance to the cent. This is exactly what the enhanced tip splitter does when you pass it a list of weights instead of leaving it as an equal split.

Rounding and the overage

When paying cash it's annoying to owe $37.55. A round-up mode rounds each person's share up to the nearest whole dollar — $38 each — and reports the overage, the extra cents that rounding produced. Four people paying $38 instead of $37.55 generates $1.80 of overage, which effectively becomes extra tip for the server. Round-up keeps the math friendly and the tip generous, and it always balances because the overage is tracked explicitly.

Without tracking the overage, rounding silently inflates the total and you can't reconcile it. The rule for any rounding scheme: round the shares, then report (rounded total − exact total) so the difference is visible, not hidden.

Why a calculator beats mental math

The number of moving parts — subtotal, tax rate, tip rate, tip base, people, optional weights, optional rounding — is exactly where mental math breaks down. Someone forgets to tip on tax, or rounds wrong, or mis-splits the weights, and the group is short. A tool that takes every input and returns a per-person breakdown of subtotal, tax, and tip removes the argument. The Bill Splitter handles the equal-split case with tax and tip, the Enhanced Tip Splitter adds weights and cash round-up, and the Tip Calculator computes the tip alone for a single payer.