Guides to settling up after a trip

The calculator does the arithmetic. These guides are about the decisions around it: which exchange rate is fair, what to do when one friend covers another, what to agree before you leave, and what "fewer transfers" really changes. Each one ends with how the calculator handles it, and the exact rules are on the methodology page.

• Which exchange rate to use when you settle a trip
  The day it was paid, the day you settle, or the day the card statement shows it. Why the three give different answers, and which one to pick.
• When one friend pays for another on a trip
  A card that will not work, a ticket bought for two, cash lent at the market. Two ways to handle these small loans, and the mistake that makes someone pay twice.
• What to agree before a group trip, so settling up stays friendly
  Drinks, rooms, people who join late, card fees, exchange rates. Where the arguments start, and a message you can paste into the group chat before the trip.
• What “fewer transfers” settling actually does
  How a group's debts shrink to a few payments, why everyone's total stays the same, and the case where the simple method takes one transfer more than it needs.