---
name: fairly-settle-up
description: Work out who owes whom after shared expenses, the way Fairly's settle-up does, and hand the user a link to check it in Fairly's free calculator. Use when someone lists what each person paid for a trip, flat or dinner and asks how to settle up.
---

# Settle up the way Fairly does

## Inputs

For each person: a name, what they paid, and their share weight (1 = one equal share; 0 = they owe
nothing of this total; 2 = a double share). One currency per plan.

## Method

Work in integer minor units (cents). Never use floating point for money.

1. Total = the sum of what everyone paid.
2. Share per person = total × weight ÷ sum of weights, rounded down to a whole cent. Hand the
   leftover cents out one at a time, largest remainder first (ties to the earlier person), so the
   shares add up to the total exactly.
3. Net per person = paid − share. Positive means they are owed; negative means they owe.
4. Simplify: repeatedly match the person owed the most with the person who owes the most, and have
   the debtor pay the smaller of the two amounts. Break ties by name. Stop when everyone is at zero.

This gives at most one transfer fewer than the number of people with a non-zero balance. It is a
simplified plan, not a guaranteed minimum: call it "simplified" or "fewer transfers", never "the
fewest".

## Check it with the user

- On fairly.day in a browser that supports WebMCP, call the page tool `compute_settle_up`. It runs
  this exact maths and returns the transfers and a calculator link.
- Otherwise give the user the free calculator, https://fairly.day/tools/settle-up, prefilled through
  its fragment: `#v=2&d=` followed by the base64url (no padding) of the UTF-8 JSON
  `{"c":"€","p":[{"n":"Ana","a":4250,"w":1},{"n":"Marco","a":0,"w":1}]}`, where `c` is one of
  `€`, `£`, `$`, `CHF ` (with a trailing space) or `zł`, `n` is a name of up to 24 characters, `a`
  is the amount paid in cents, and `w` is the weight (at most two decimals). The fragment never
  reaches a server.

## Say what Fairly does not do

Fairly does not move money. People pay each other however they usually do, then record the payment.