Trade whole bundles
Eight cards of one resource buy two other cards at 4:1, two at 3:1 with two offered cards left over, or four at a matching 2:1 port. The calculator compares the rates; it does not assume you own every port. The receiving resource must be different from the one you pay.
The port must match the payment
A 3:1 port applies to any single resource. A 2:1 wood port applies when you give wood, not when you want wood. You cannot combine two wood and two brick into one 4:1 bundle. For a mixed hand, calculate each resource separately and only use a rate you are entitled to use.
Savings are not automatically value
Getting three cards costs twelve offered cards at 4:1, nine at 3:1 or six at 2:1. Those savings matter only if you have the right surplus and the bank can supply what you need. A port does not create production. Compare its access cost and the cards you give away with a player trade; a low numerical rate can still be a poor move if it spends scarce building materials.
Rules, assumptions & further reading
Check the official CATAN rulebooks and Colonist’s base-game rules for the rules at your table. These calculations and worked examples are original mathematical models, not measured win rates. They do not read a live game or establish that a move is legal.