Killer Sudoku rule reference
The cage sum, no-repeat and rule-of-45 explanations follow the standard killer Sudoku rules published by Sudoku.com. The worked examples on this page are original.
How killer Sudoku differs from classic Sudoku
In a standard 9 × 9 killer Sudoku, each row, column and 3 × 3 box contains the digits 1 to 9 once. Outlined groups of cells are cages. The small number in a cage is the total of its digits, and a digit cannot repeat within that cage. Cages may cross box boundaries, so their outlines matter as much as the thicker box borders. Read the puzzle’s own instructions for any additional restrictions.
Start with cage combinations
A two-cell cage totalling 3 must contain 1 and 2. A two-cell cage totalling 4 must contain 1 and 3; 2 plus 2 is excluded by the no-repeat cage rule. A two-cell cage totalling 16 must contain 7 and 9. These combinations identify possible digits without necessarily determining their order.
For a three-cell cage totalling 6, the digits must be 1, 2 and 3. If one of those cells shares a row with an existing 1, that cell cannot hold the cage’s 1. Combine the sum with all the row, column and box restrictions before entering a digit.
Use the rule of 45
The digits 1 to 9 add up to 45, so every completed row, column and 3 × 3 box has that total. Suppose cages entirely contained inside one box have totals adding to 38 and leave exactly one cell in that box uncovered. That remaining cell must be 7, because 45 minus 38 is 7.
This subtraction works only when you account for each cell exactly once. Do not subtract the entire total of a cage that crosses the box boundary as though it lay fully inside. Keep track of which cells are inside the unit and which lie outside it.
Keep combinations separate from cell candidates
A cage combination is a set of digits that could make the total. A cell candidate is a digit that can fit a particular position. Record combinations first, then remove orders that break a row, column or box rule. After a placement, update both the cell candidates and the remaining cage total. If no combination survives, recheck the arithmetic and earlier placements.
Easy and hard killer Sudoku
Difficulty depends on how the cage sums interact with the ordinary Sudoku constraints. An easy puzzle may have several small cages with forced combinations. Harder puzzles can require subtracting groups of cages, tracking several combinations and making eliminations before a number can be placed. Expert classic Sudoku and expert killer Sudoku are different experiences: the classic game here does not check cage totals.
Playing and printing on this site
The playable grids and printable sheets here are classic Sudoku and small grids, without killer cages. The 9 × 9 solver checks classic constraints only and cannot validate a killer puzzle from cage totals. For standard puzzle practice, play classic Sudoku, use printable Sudoku with answers, or compare other types of Sudoku.