The slip has three different kinds of numbers
A shared restaurant check looks like one total at the bottom and a small argument at the top. The argument is usually about which number the tip multiplies. Some people tip on the grand total after tax. Some tip on the food. Some pick a round number that "feels right" and then discover the round number does not divide by four. The calm version uses the slip as written.
Find the pre-tax subtotal. That is the food, drinks, and any service charge that is already a price, before sales tax. Find the tax as a dollar amount, not as a percent you recompute, unless the slip failed to print the tax and you truly have to. Then choose the tip as a percent of the pre-tax subtotal. Add those three. That sum is what the table owes. Splitting comes after the sum exists, not before.
This is the rule the Bill Split calculator uses. Tip equals the percent times the pre-tax subtotal. Tax is added as dollars. People then divide the grand total. If you tip on the after-tax total instead, you will not match this calculator, and you will not match the worked example below. Pick the rule before anyone opens a payment app.
A full table, in cents
Four people. The pre-tax subtotal is $184.60. Tax printed on the slip is $15.23. The table agreed on 18 percent before the food arrived, which is the only time that agreement is cheap. Eighteen percent of $184.60 is $33.228, which rounds half-up to the cent as $33.23. Add the pieces: $184.60 plus $15.23 plus $33.23 is $233.06. That is the amount that has to leave the table. If your mental math produced a different tip, you either tipped on a different base or you rounded the percent in a different place. The calculator rounds the percent to a tenth of a percent and then rounds the tip half-up to the nearest cent. Eighteen is already a clean tenth, so the only rounding is the tip itself.
Now divide $233.06 by four. Two hundred thirty-three dollars and six cents is 23,306 cents. Divided by four, that is 5,826.5 cents each, and a half-cent cannot be paid. The fair split gives the extra cent to the earlier seats: $58.27, $58.27, $58.26, and $58.26. Add those four amounts. They return $233.06. A split that gives everyone $58.26 is short two cents. A split that gives everyone $58.27 is two cents heavy. The payment app may not care. The person who put the whole bill on one card does care, because the short cents come out of their pocket and the heavy cents become a group that overpaid.
Type it once to see the shape. Subtotal 184.60, people 4, tip 18, tax 15.23, even split. Read the tip line, the total, and the four shares. Then send the shares, not a screenshot of the food. The shares are the agreement.
What not to include in the tip base
A coupon that reduced the food should reduce the subtotal before you tip, if the slip already shows the reduced subtotal. Tip on what the restaurant is charging for the meal, which is the number above tax. If a coupon was applied after you did mental math, redo the math. Do not tip 18 percent on a subtotal that no longer exists.
A service charge already on the slip is not a second tip unless the menu said the charge is not a tip. Read the line. "Service 20%" that is already inside the subtotal means you are not also doing 18 percent on top unless the table explicitly wants to add more. Double tipping is a generous mistake. It is still a mistake if half the table did not agree. If the charge is outside the subtotal, decide whether it replaces the tip. Write the decision in one sentence before anyone calculates.
Tax is not a tip. Leaving 18 percent of the tax on top of 18 percent of the food is a different, larger tip. Some people like that habit. This tool does not. If your table prefers to tip on the after-tax amount, compute that percent yourself and type the result as a custom tip only if you are willing to leave the calculator's percent field. The cleaner path is to keep the percent on the pre-tax subtotal and let tax sit in its own field as dollars.
Even, by weight, and the friend who ordered the steak
Even split is the right default when the table shared plates or when the difference in orders is smaller than the social cost of itemizing. The example above is even. Four shares, two of them a cent higher, and dinner can end.
By weight is for a table that is not equal and can say so without a speech. One person brought a guest. One person does not drink. Weights of 1, 1, and 2 on a bill are a way to say the third share is double. A weight of zero is allowed for someone who is at the table and not paying, but not every weight can be zero. If you use weights, still compute tip and tax first. Weights apply to the grand total, not to the food alone, unless you have a reason to share tax and tip on a different pattern. Sharing them on the same pattern is the one you can explain at the curb.
Custom amounts are for the steak problem. Each person types what they owe for their own food. The calculator does not rescale those amounts to force a match. If the custom amounts add up short of the total, it shows the gap. If they run over, it shows the overage. Someone still has to cover tax, tip, and the shared fries. A custom list that ignores tip will always look short. Add each person's food, then decide how tip and tax ride along. A simple way: split tip and tax evenly, or by the same weights, and add that slice to each person's food. If you instead type only food into custom amounts, do not be surprised by the gap. The gap is the tip and the tax you left out.
Cash, cards, and the person who paid
One card on the check is the cleanest close if the restaurant allows it and one person is willing. That person is a short-term lender. Everyone else pays them the share, the same night. "I will get you tomorrow" is how $58.26 becomes a story. The share is small enough to send immediately and large enough to annoy if it lingers.
If you split tender at the table, the cents still have to match the restaurant's total, not your rounded version. Tell the server the amounts from the calculator. If the machine forces equal splits and drops a cent, the person with the card covers the cent and can ask for it or forget it on purpose. Forgetting one cent is fine. Forgetting the method is not, because the next dinner will restart the argument.
A large party that wants separate checks should ask before ordering, not after. The calculator is the backup when the restaurant says one check only. It is not a reason to itemize fourteen appetizers from memory. If you cannot reconstruct the plates, even split or a rough weight is more honest than a false itemized list. Honesty here means the shares add up. It does not mean each french fry has a price.
Write the habit down so the next dinner is boring
Agree the percent before you sit down, or at least before the bill lands. Eighteen is a common default in many cities and a low or high number in others. The right percent is the one the table said, not the one a guide prefers. If someone wants to tip more, they can add a personal extra on top of their share. They should not silently change the group's percent.
Keep a one-line note in the group thread for the houses that eat together every week. "Tip 18 percent of pre-tax, tax as printed, even split unless someone asks for weights before the order." That sentence prevents the monthly rediscovery of arithmetic. When the bill is unusual, open the Bill Split calculator, type the subtotal, the tax dollars, the percent, and the people, and send the shares. Copy the result. A photo of the slip plus four amounts is a complete record.
Check the addition once in your head at the scale of dollars, even though the tool is doing the cents. Subtotal about $185, tax about $15, tip about a fifth of $185 which is high thirties, so the tip at 18 percent should live in the low thirties. $33 is in the right neighborhood. A tip that comes out at $80 means you tipped on the wrong base or typed an extra digit. Neighborhood checks catch the typos that exact trust will not.
The dinner is finished when the shares sum to the total and each share has moved. Not when the app shows a request. Moved. The cents that differ by one are not an insult. They are the remainder of a number that was not divisible by the size of your table. Pay them as written, and leave the restaurant argument in the restaurant.