Group expense calculator

The example below is a shared flat over one month. Replace the names, the expenses and the amounts with your own — a dinner, an event, a house, a weekend, anything where several people paid for things everyone used.

Every expense contributes two things: it adds its full amount to whoever paid it, and it adds a share to each person who was part of it. Nothing else matters — not the order, not the dates, not who suggested it.

1. 1

   List everyone sharing the costs

   Add each person and give them a name. Two people is enough; the calculator handles up to twelve.

2. 2

   Add each expense with the person who paid it

   Enter what it was and how much, then choose who actually paid. Paying for something and owing for it are different facts, and keeping them apart is the whole reason a group balance is not obvious.

3. 3

   Untick anyone who did not share it

   By default an expense is split between everybody. Remove the people it did not apply to — the dinner two people missed, the taxi one person took alone. Only the remaining people are charged for it.

4. 4

   Read the balances

   Each person's balance is what they paid minus the total of their shares. Positive means they are owed money, negative means they owe it, and the balances always add up to zero.

5. 5

   Choose a settlement plan

   Either everyone pays back the person who paid for what they shared, or the debts are cancelled around the group so the balances clear in the fewest possible transfers. Both leave everyone at zero.

The one rule people get wrong

The person who paid for something is also usually one of the people sharing it. Their share does not disappear; it just cancels against part of what they paid. Someone who pays $120 for a dinner they shared with three others has paid $120 and owes $30, so they are owed $90 — not $120.

Dividing the group's total spend by the number of people is a different calculation and usually the wrong one. It only gives the right answer when every single expense was shared by everybody, which almost never survives contact with a real group.

Sam, Priya, Jon and Noor share a flat. Sam's account pays the rent, Priya holds the utilities, Jon does the shop, and one takeaway happened while Jon was away.

Once the balances are known there is more than one way to clear them, and groups genuinely disagree about which they want. The calculator gives both.

A group of N people can always be settled in at most N − 1 payments. The reason is simple: every payment in the fewest-payments plan zeroes at least one person, and once everyone but one is at zero, the last one must be too, because the balances sum to zero.

Expenddy's app shows the first of these — the pairwise breakdown of what you owe each specific person. It deliberately does no rerouting, so what the app tells you to pay always corresponds to expenses you actually shared.

This is the part a simple “divide by the number of people” calculator cannot do, and it is the difference between a number people accept and a number people argue with.

• Someone was away. Untick them for the groceries and meals during that week. Their share of everything else is unaffected.
• Only some people went. The concert, the boat trip, the taxi home for two — tick only those people.
• Someone paid for a guest. Enter the whole amount, shared by whoever was actually covering it.
• One person has already been paid back. Do not delete the expense. Enter the repayment as its own line — the amount, paid by the person who repaid, shared only by the person who received it — and the balances adjust correctly.

For splits that are not equal between the people sharing — rent by room size, a bill split by income, an itemised restaurant bill — see exact and percentage splitting, which the app supports directly.

• It splits each expense equally between the people you tick. For unequal shares within one expense, split it into separate lines, or use exact and percentage splits in the app.
• One currency at a time. Expenddy applies no exchange rate anywhere. Run a separate calculation per currency and settle each one.
• It saves nothing and shares nothing. The figures live in this browser tab and vanish when you close it. Copy the result out as text if you want to keep it.
• It does not move money. Expenddy is a record-keeping app, not a bank or a payment service.

For costs that keep arriving — a flat, a couple, a group that shares things every week — the calculator is the wrong shape of tool, because you would be retyping the whole month every time. That is what groups in the Expenddy app are for: everyone adds their own expenses, the balances update as they go, and settlements are recorded when the money actually moves.