Learn the three rules, then use cage combinations and the 45 rule to turn small clues into a complete grid.
The three rules
- Classic Sudoku still applies. Fill every row, column and bold 3×3 box with digits 1–9, using each digit once.
- Every cage has a target sum. The number in the top-left cell of a dashed outline is the total for all cells inside that outline.
- No repeats within a cage. Each digit can appear at most once in a cage, even when the cage crosses row, column or box boundaries.
A two-cell cage totalling 3 has only one digit combination: 1 + 2.
Its order is not fixed. Use the surrounding row, column and box to determine which digit belongs in which cell.
A practical solving routine
1. Start with the narrow combinations
Look for very small or large totals. Two cells summing to 17 must contain 8 and 9; three cells summing to 6 must contain 1, 2 and 3. Add notes rather than guessing an order.
2. Cross-check Sudoku units
Remove candidates already present in the same row, column or box. If a cage must contain a digit, that fact can also rule it out elsewhere in a shared unit.
3. Keep each cage distinct
A cage may stretch across multiple units, but the no-repeat rule applies to the whole cage. If a 7 is already inside it, none of its remaining cells can be 7.
4. Use the 45 rule
Every completed row, column and 3×3 box adds up to 45. If complete cages cover all but one cell of a unit and total 39, the remaining cell is 6.
5. Understand innies and outies
If cages cover a whole 45-total unit and exactly one extra cell outside it, subtract 45 from their sums to find that outside cell. If one inside cell is missing, subtract the fully contained cage totals from 45. Count only the cells actually covered; do not double-count a crossing cage.
6. Revisit after every placement
A new digit can tighten several cages at once. Scan the affected row, column, box and cage before taking a hint.
Controls and difficulty
Select a square, then use the number pad or keyboard digits 1–9. Press N to toggle notes and Backspace, Delete or 0 to erase. Arrow keys move the selected square. Undo restores the last value or notes; hints remain counted after Undo.
Easy includes eight given digits and smaller cages. Medium and Hard have no pre-filled digits; Hard permits larger cages and a higher solver-search allowance. Single-cell cages may still reveal a digit through their sum. These labels are relative settings, not a formal rating.
Your timer starts with your first value, note or hint. It pauses when the page is hidden and resumes with your next move. Progress stays in this browser. Daily boards reset at midnight UTC; archive and unlimited puzzles do not add extra streak days.
For every valid cage total, use the complete cage combinations reference. Ready to try the rules? Play daily Killer Sudoku online free.
Frequently asked questions
What makes Killer Sudoku different from classic Sudoku?
Both use the same row, column and 3×3 box rules. Killer Sudoku adds cages with target sums and a no-repeat rule within each cage.
Do I need to calculate difficult arithmetic?
No. Most steps involve small sums, simple subtraction and checking distinct-digit combinations. The full combinations reference can help.
Can I repeat a number in a cage if it lies in another box?
No. Digits cannot repeat anywhere inside the same cage, regardless of box boundaries.
What if a cage contains just one cell?
That cell must equal the cage’s printed sum. The generator can retain a few single-cell cages to preserve a unique solution.
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