Killer Sudoku Online

Killer sudoku strategy: the 45 rule and cage combinations

Every row, column, and 3×3 box in killer sudoku contains 1 to 9, so each one totals 45. That single fact — the 45 rule — is your sharpest tool: add up the cages inside a unit, and the difference from 45 reveals a hidden cell (an "innie") or a cell spilling out of the unit (an "outie"), often pinning a digit outright.

Lean on cage-combination logic too. A two-cell cage summing to 3 can only be 1+2; a three-cell cage summing to 7 must be {1,2,4}; a three-cell summing to 24 has to be {7,8,9}. Pencil in those candidates first, then let the standard sudoku rules eliminate the rest. Start gentle on Easy, then push into Hard and Expert as the cages grow.

Killer sudoku vs classic sudoku

Classic sudoku hands you starting numbers; killer sudoku gives you none — only dashed cages with target sums. The row, column, and box rules are identical, but killer adds two: no digit repeats inside a cage, and a cage's digits must add up to its sum. That blend of light arithmetic and pure logic is why many solvers find killer the most rewarding variant once it clicks.

Killer sudoku questions

How do you play killer sudoku online?

Pick a difficulty above and press Start Playing. Fill the 9×9 grid so every row, column, and 3×3 box holds 1 to 9, while each dashed cage adds up to its target sum and never repeats a digit. Tap a cell, choose a number, and use Show Candidates if you get stuck — no download or sign-up required.

What is the 45 rule in killer sudoku?

Because every row, column, and 3×3 box contains 1 to 9, each one totals 45. Comparing 45 to the cage sums inside a unit exposes a missing "innie" cell or an extra "outie", which often reveals a digit immediately. It is the core killer-solving shortcut.

Is killer sudoku harder than regular sudoku?

It is different rather than strictly harder. Killer starts with no given numbers, so the cage sums replace them — that feels tougher at first, but the arithmetic also hands you extra clues. Easy killer puzzles are very approachable, while Expert killer is among the hardest sudoku you can play.

Do you need to be good at maths for killer sudoku?

Only simple addition. The sums rarely involve more than adding a few small numbers, and the real work is logical deduction, not arithmetic. If you can add up to 45, you have all the maths you need.

Are these killer sudoku puzzles free?

Yes — every puzzle on Sudoku247Online is free, freshly generated each time you play, and guaranteed solvable by pure logic. Sign up free if you would like to play ad-free and save your progress.