Killer Sudoku

Killer Sudoku

Daily Puzzle

Killer Sudoku - Cage Sums Instead of Starting Numbers, Solo or With Friends

Variants:Sudoku OnlineSudoku Online

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Killer Sudoku - Cage Sums Instead of Starting Numbers, Solo or With Friends

Killer Sudoku

Killer sudoku is a free online sudoku variant where the grid gives you sums instead of starting numbers. The board is carved into cages, small groups of neighbouring cells drawn with a dotted outline, and each cage prints the total its digits must reach. No digit repeats inside a cage, and the rest is normal sudoku: every row, every column, and every 3x3 box holds 1 to 9 once each. You can play it on this page right now in your browser, alone or with friends on a shared board, at four difficulty levels, with no download and no signup.

Because the cages carry the information, killer grids start almost bare. An Easy puzzle here opens with four given digits and Expert usually opens with none at all, so your first move comes from arithmetic rather than from scanning for a number that is already on the board.

Killer Sudoku Rules

There are two rules on top of ordinary sudoku, and they are both about cages. A cage's digits have to add up to the small number in its top-left corner, and a digit can only appear once inside a cage. The second rule matters more than it looks, because it is what makes a total useful. A three-cell cage adding to 7 could be 1+2+4 or 1+3+3 if repeats were allowed, but they are not, so 1+2+4 is the only option and all three cells are settled before you have placed anything else.

Extreme totals are the way in. Low totals have to use the small digits and high totals have to use the large ones, so a two-cell cage adding to 3 is 1 and 2, a two-cell cage adding to 17 is 8 and 9, and a three-cell cage adding to 24 is 7, 8, and 9. Middling totals like 15 are the vaguest and are usually the last cages you fill.

The other technique worth learning early is the 45 rule. Every row, column, and 3x3 box contains each digit once, so each one adds up to 45. Find a row, column, or box where every cage sits fully inside it except for a cell or two, add up the cage totals, and the difference from 45 is what is left over.

One last detail: a cage can be a single cell. When it is, its total is simply the digit that goes there, which is the same thing as a given number in classic sudoku. Easy puzzles here include eight of those and Expert includes three, and that gap is a large part of why the difficulties feel so different.

Play Killer Sudoku Online Free

The board at the top of this page is already set to killer, so pressing play is the fastest way in. From anywhere else on Foony, open Sudoku, choose Create Room, and pick the Killer Sudoku card from the presets, or flip the Variant setting from Classic to Killer on any room you are setting up.

Four difficulty levels change both the cage shapes and how much of the grid you get for free:

Easy and Medium

Cages hold up to four cells, and plenty of them hold just one. Easy starts with four given digits and Medium with one, on top of the single-cell cages.

Hard and Expert

Cages stretch to five cells and single-cell cages get rare. Both normally open with no given digits at all, so the sums are all you have.

Every puzzle has exactly one solution, and it is checked before you ever see it. The generator builds cages from a finished grid and then solves the result under killer rules to confirm the answer is forced, so you never have to guess.

The rest of the room settings work the same as classic sudoku. Mistake tolerance is Casual (unlimited), Normal (three mistakes), or Perfectionist (one mistake and the puzzle ends). The host can put a clock on it, anywhere from 5 to 60 minutes, or leave it off. Killer solves are given more time for the same reward than classic ones, because they take longer, and they are scored on their own scale so a long solve is not penalised.

While you play, the board does the arithmetic with you. Each cage total sits in the corner of its first cell, turns green once the cage is full and correct, and turns red the moment your digits pass the total, so a slip shows up on the spot instead of forty cells later. Selecting a cell tints the rest of its cage, which is the fastest way to see the shape of a cage that wraps around a corner. Pencil marks work exactly as they do in classic sudoku.

Multiplayer Killer Sudoku

Killer is the sudoku variant that benefits most from a second pair of eyes, and every Foony room supports one. Turn Co-op on when you create the room and everyone joins a single team sharing one grid, with live cursors, so you can split the board by region, have someone work the cage arithmetic while another person scans rows, or just talk through a corner that will not open. It is also the easiest way to teach the variant to someone, since they can watch a cage get cracked instead of reading about it.

Leave Co-op off and everyone gets the same puzzle on their own grid, with first to finish legally taking the win. That covers a 1v1 race, a friends-only match, and a tournament room, which holds up to 1,000 players on one seed. Share the room link and people join from any modern browser on desktop, tablet, or phone. No account is needed to play, only to keep levels, ranks, and unlocks across devices.

Killer Sudoku Tips and Cage Combinations

Sweep for extreme cages first. Before you place anything, look for two-cell cages totalling 3, 4, 16, or 17, and three-cell cages totalling 6, 7, 23, or 24. Each of those has exactly one possible set of digits, and one settled cage usually gives you a second through the ordinary row and column rules.

Use the 45 rule on the edges of the grid, where boxes are most likely to have a single cell poking out of a cage. It also works across several rows or columns at once. Two full rows add up to 90, three add up to 135, so a cage that spills a single cell out of a three-row band gives that cell away.

Watch for cages inside one row, column, or box. Their digits cannot repeat anywhere in that unit either, so a two-cell cage totalling 16 inside a single box removes 7 and 9 from every other cell in that box, which is often more useful than knowing where they go.

Play Hard and Expert in Casual mode until the arithmetic is second nature. A wrong digit in killer usually comes from a miscount rather than a broken deduction, and there is little to learn from a puzzle that ends on one. Switch to Perfectionist once you stop miscounting, and chase Flawless Victory then.

Sudoku Leaderboards, Achievements, and Boards

Killer solves count towards everything classic solves do. Wins feed the sudoku leaderboards, which filter by day, week, month, year, or all-time.

There are eight sudoku achievements and killer is a fair route to most of them. Flawless Victory asks for zero mistakes on any puzzle, Zen Master for a solve with no hints, and Hard as Nails and Puzzle Master for Hard and Expert solves with three mistakes or fewer, all of which count on a killer board. Quick Solve is the one to leave for classic, since it wants an Easy puzzle finished inside five minutes.

Playing also drops sudoku boards and number styles. There are 12 boards, including Wooden, Zen Garden, Blueprint, and Neon, and 12 number styles such as Ink Brush, Fountain Pen, and Gilded, with the Calligraphy style handed out at level 50. They change how the grid looks and nothing else. The dotted cage outlines and their totals draw over whichever board you have equipped.

Classic Sudoku and Other Puzzles

If the cage arithmetic is new to you, the sudoku page covers the base rules the variant is built on, along with the same co-op rooms, races, and difficulty levels. Working through a Hard classic puzzle first makes killer much easier to pick up, because once the cages have given you a dozen digits the rest of the solve is ordinary sudoku.

Sudoku Online: Frequently Asked Questions

How do I play sudoku online with friends?
Open Foony Sudoku, click "Play with Friends" to spin up a private room, then share the invite link. Friends join from any modern browser on desktop, tablet, or phone, with no accounts to create and nothing to install. Pick the difficulty, pick the mode (Casual / Normal / Perfectionist), pick whether the room is co-op or race, and you are at the board.
Can I play co-op sudoku on the same board with a friend?
Yes. Enable Co-op (or "coop", same thing) when you create the room on Foony Sudoku, and everyone in the lobby joins one team that shares a single 9x9 board with live cursors in real time. You can divide sections among teammates, double-check each other's placements before they lock in, or just chat through the trickier deductions on Hard and Expert puzzles. It is the easiest way to teach a beginner without leaning over their shoulder.
Can I race friends, play 2 player sudoku, or run a sudoku tournament?
Yes, all three on a single mode. Leave Co-op off when you create the room on Foony Sudoku and every player in the lobby gets the same starting puzzle on their own grid; first solver to fill the board legally wins. That covers 1v1 / 2 player sudoku, a friends-only race, and a competitive sudoku tournament with up to 1,000 entrants on a single seed. Pair it with Perfectionist mistake tolerance for a brutal sprint where one wrong placement eliminates you, or Casual for a relaxed "who finishes first" round.
Can I play sudoku without signing up?
Yes. Open Foony Sudoku and you are at the board, no account required. Sign up only if you want cross-device level sync, persistent leaderboard ranks, and shop unlocks. The full ruleset, all four difficulty levels, all three game modes, and the multiplayer rooms are available without an account.
Is Foony Sudoku free to play?
Yes. Foony Sudoku is fully free in the browser. There is no paywall on the puzzles, the multiplayer rooms, the tournaments, or the achievements. The shop sells optional cosmetic boards and number styles; nothing in there changes the puzzle generator or the rules.
Can I play Foony Sudoku unblocked at school or work?
Foony Sudoku runs entirely in the browser, so it works on most school and office networks that block standalone game installs. There is no executable to run and no app store account, and the page itself loads from foony.com over HTTPS. If your network blocks foony.com directly, ask whoever runs IT to allow it; we are not on the standard "blocked games" lists.
How do you play sudoku?
Sudoku is a logic puzzle on a 9x9 grid divided into nine 3x3 boxes. The grid starts partially filled with numbers from 1 to 9. Your goal is to fill every empty cell so that each row, each column, and each 3x3 box contains every digit 1 through 9 exactly once. No arithmetic is involved, only logical deduction. Foony Sudoku highlights legal placements, surfaces conflicts in red, and ships pencil marks for tracking candidate numbers in cells you have not committed yet.
What sudoku difficulty levels does Foony offer?
Foony Sudoku has four difficulty levels, picked once per room. Easy only needs the two moves everybody starts with: the one number a square can still take, and the one square a number can still go in. Medium adds pairs, where two squares share the same two numbers and shut everything else out of both, and the X-wing, where a number is pinned to the same two columns on two rows. Hard adds triples and larger versions of that pattern, and this is where writing pencil marks starts to pay off. Expert adds chains, where you follow one number through several squares to rule it out somewhere far away. Higher difficulties start you with fewer numbers, and they reward more XP and item drops on completion.
What is the difference between Casual, Normal, and Perfectionist mode?
The three modes on Foony Sudoku change how strict the mistake tolerance is. Casual allows unlimited mistakes; the puzzle never ends in failure, which is ideal for learning. Normal allows up to three mistakes before the game ends. Perfectionist ends the game on a single wrong placement, which is the right setting if you are chasing the Flawless Victory achievement or pushing your skill ceiling. Higher-stakes modes pay out more XP and more item drops.
Can I set a time limit on a sudoku puzzle?
Yes. The host on Foony Sudoku can set a time limit anywhere from a tight 5 minutes up to 60 minutes per puzzle, or disable the clock entirely for a relaxed untimed session. Time limits are independent of the difficulty and mistake-tolerance settings, so you can run an Expert puzzle on Casual mistake tolerance with a 60-minute clock, or an Easy puzzle on Perfectionist mode with a 10-minute sprint.
What is killer sudoku?
Killer sudoku is sudoku with one extra rule. The grid is divided into cages, small groups of cells marked by a dotted outline, and a small number in a cage's top-left corner is the total its digits have to add up to. No digit repeats inside a cage, on top of the usual one-of-each rule for every row, column, and 3x3 box. Because the sums carry the information, killer grids start with very few given numbers, and Hard and Expert start with none at all. Foony Sudoku ships killer as a room option with the cage sums shown live and colored green once a cage is complete and correct.
How do you play killer sudoku, and what is the 45 rule?
Start from the cages with only one possible combination. A two-cell cage summing to 3 can only be 1 and 2, and a two-cell cage summing to 17 can only be 8 and 9, so those cells are settled before you place anything. From there use the 45 rule: every row, column, and 3x3 box holds 1 through 9 exactly once, so it always totals 45. Find a box where all the cages sit fully inside except one cell, add the cage sums, and 45 minus that total is the cell. On Foony Sudoku the cage sum turns red the moment your filled digits pass the target, so an arithmetic slip shows up straight away.
Is killer sudoku harder than normal sudoku?
It is different rather than strictly harder, though it takes most players longer. Classic sudoku gives you a set of starting digits to work outward from. Killer gives you almost none, so the first few placements come from arithmetic on the cages instead of scanning for a number that is already on the board. Once the grid opens up it plays like normal sudoku again. Foony scores killer on its own scale and allows more time for the same reward, so a killer solve on Foony Sudoku is not penalised for taking longer.
Are Foony sudoku puzzles guaranteed to have a unique solution?
Yes. Every puzzle generated on Foony Sudoku has exactly one solution that is reachable through pure logic, no guessing required. If you find yourself guessing, you have missed a constraint somewhere or you need a more advanced technique (Hard leans on patterns spread across two or three rows at once, and Expert on chains you follow from square to square). A puzzle with multiple solutions is by definition an invalid sudoku, and our generator never produces one. Every shipped puzzle is re-checked by Sudoku Explainer, the reference grader, and thrown away if it disagrees with the grade we gave it.
Does Foony Sudoku have leaderboards, achievements, and cosmetic items?
Yes, all three. Wins on Foony Sudoku feed the public leaderboards, filterable by day, week, month, year, or all-time. There are 12 in-game achievements, including Flawless Victory (zero mistakes on any puzzle), Quick Solve (an Easy puzzle in under 5 minutes), Hard as Nails and Puzzle Master (Hard / Expert with three or fewer mistakes), Zen Master (a puzzle solved with no hints), and four for the daily puzzles, up to Hardest of the Day for 100 expert dailies. There are 13 cosmetic boards (Wooden, Zen Garden, Blueprint, Stained Glass, Koi Pond, Ancient Scroll, and more) and 13 number styles (Ink Pen, Fountain Pen, Ink Brush, Gilded, Calligraphy, Gemstone, and more), earned from the shop, from drops, or from the daily puzzles. Skins are purely visual and never affect the puzzle generator.
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