Count buildings before comparing numbers
One settlement is one production unit; one city is two. In the default example, the 6 blocks one opponent settlement: five expected cards per 36 rolls. The 5 blocks an opponent city plus settlement: twelve. It also blocks your own settlement for four expected cards. A bigger dice number weight alone does not identify the larger production loss.
Use the same blocking duration
These are rates per 36 independent fair rolls while the robber stays on the tile, not a claim that it will remain for 36 rolls. Halve them for an eighteen-roll comparison. The model assumes otherwise available bank supply. It excludes the immediate random theft because that requires separate assumptions about the chosen player's hand.
The strongest block is not always the best move
Preventing cards for the leader may matter more than preventing a larger total for trailing players. Scarce ore, a threatened award, friendly-robber rules and the possibility of retaliation also change the decision. The calculator totals opponent production without ranking players or validating the board. Check legal robber destinations and count only buildings adjacent to the selected hex.
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.