What is Killer Sudoku?
Killer Sudoku is Sudoku with an extra layer of maths. The grid is the same 9×9 board. But the cells are also grouped into small dotted shapes called cages. Each cage has a small number in its corner. The digits inside the cage must add up to that number. You can play Killer Sudoku online free on this page, on your phone or computer, with no download.
Killer Sudoku rules
- Put a digit from 1 to 9 in every cell.
- Each row, each column and each 3×3 box must contain 1 to 9 exactly once – just like normal Sudoku.
- The digits in a cage (dotted outline) must add up to the small number in its corner.
- A digit can not repeat inside the same cage.
- Every level here has exactly one correct solution. We checked each one with a computer solver.
How to use the buttons
- Tap a cell, then tap a number. Tap the same number again to remove it.
- ✎ Notes switches to pencil marks. Small numbers help you track options.
- ↶ Undo takes back your last move. ⌫ Erase clears a cell.
- 💡 Hint fills one correct cell (or fixes a wrong one). ✔ Check shows wrong digits in red.
- Under the board you see the cage of the selected cell, its sum and which digit sets still fit (for example “1+3”).
- Keyboard: arrow keys to move, 1–9 to write, Backspace to erase, N for notes, U or Ctrl+Z to undo.
- Your progress, times and solved levels are saved in your browser. Pick 📅 Daily for today's puzzle and build a streak.
What Killer Sudoku does for your mind
Killer Sudoku is a calm puzzle, not a test. But while you play, you practise some useful skills:
- Logic. Every step is a small “if this, then that” decision. You never need to guess.
- Mental arithmetic. You add and subtract small numbers all the time. “17 in two cells” must be 8+9.
- Working memory. You hold several options in your head and compare them.
- Focus. One cage, one row, one box at a time. Your attention stays in one place.
- Patience and planning. Hard levels reward slow, careful steps over fast guesses.
- A calm break. Many people use a puzzle as a short screen break from news and chats.
- A sense of achievement. The last digit clicking into place feels good – and you earned it.
What science says, honestly. In 2019, researchers from the University of Exeter and King's College London looked at more than 19,000 adults aged 50 and over in the online PROTECT study. People who did number puzzles like Sudoku more often tended to score better on tests of attention, reasoning and memory. But this is a link, not proof. The lead researcher said they can not say that puzzles reduce the risk of dementia. So play because you enjoy it – not as a treatment.
Who is it for?
- Beginners and older kids: start with Easy. Many digits are already given, and the cages are small.
- Daily players: the Daily pick changes every day and counts your streak.
- Sudoku experts: Hard levels start with only a few given digits. You must use cage maths.
- Older adults: big touch-friendly numbers, no timer pressure, and you can pause any time.
Killer Sudoku tips and strategy
- Start with tiny and extreme cages. A 2-cell cage of 3 is always 1+2. A 2-cell cage of 17 is always 8+9. A 3-cell cage of 6 is always 1+2+3, and 24 is always 7+8+9.
- Use the rule of 45. Every row, column and box adds up to 45. If a box is almost covered by full cages, the missing cell is 45 minus their total.
- Look for “innies” and “outies”. If some full cages cover a whole box plus one extra cell outside it, that extra cell is the cages' total minus 45.
- Read the help line. It lists the digit sets that still fit the selected cage. Fewer sets = an easier cage to crack.
- Use normal Sudoku logic too. Singles, pairs and the X-Wing still work here.
- Pencil in pairs. If a cage of 4 sits in a row, write 1 and 3 as notes. That row now can not use 1 or 3 elsewhere.
- Don't guess. If you are stuck, pick a cage and list its possible sets. One option usually breaks.
- Learn from hints. After a hint, ask yourself why that digit had to be there.
A short history of Killer Sudoku
Killer Sudoku is usually credited to the Tokyo puzzle setter Tetsuya Nishio, who built it from an idea by his student Miyuki Nisawa. In Japan it is called “samunamupure”, which means “sum number place”. It mixes Sudoku with ideas from Kakuro, the cross-sum puzzle.
Fun facts: Outside Japan, Killer Sudoku first appeared in The Times in 2005. It is also known by playful names like “sumdoku” and “addoku”.
Questions people ask
Is Killer Sudoku harder than normal Sudoku?
Usually yes, because Hard puzzles start with very few given digits and you must use cage sums. But Easy Killer Sudoku with many given digits can feel easier than a hard classic Sudoku.
Can a number repeat in a Killer Sudoku cage?
No. In standard Killer Sudoku, a digit can appear only once in each cage – even if the cage spans two boxes.
What is the rule of 45 in Killer Sudoku?
Every row, column and 3×3 box contains 1 to 9, and 1+2+…+9 = 45. So if you know the sum of most cells in a box, the missing cells must make up the rest of 45.
Does every puzzle here have one solution?
Yes. We generated every level with a program and checked with a solver that it has exactly one correct solution. No guessing needed.
Can I play Killer Sudoku on my phone?
Yes. The board and number pad are made for touch screens. It also works with a keyboard on a computer. Nothing to install.
Is Killer Sudoku good for your brain?
It practises logic, mental arithmetic, focus and working memory. Studies have found that people who do number puzzles often tend to score better on thinking tests, but that is a link, not proof that puzzles protect the brain.
Is my progress saved?
Yes, in your own browser. Solved levels, best times and the daily streak stay as long as you don't clear your browser data.
Where can I print Killer Sudoku puzzles?
Download our free printable PDF with 6 Killer Sudoku puzzles and answers: Killer Sudoku PDF.
More logic games
Read more: Sudoku tips for beginners · Puzzle game demos at Steam Next Fest (October 2026)
Killer Sudoku books on Amazon · Pencils & erasers
Als Amazon-Partner verdiene ich an qualifizierten Verkäufen.
Sources
- Wikipedia – Killer sudoku
- ScienceDaily – University of Exeter / King's College London (16 May 2019)
- 247 Sudoku – Killer Sudoku tips
This page is for fun and general information, not medical advice.