Killer Sudoku

Killer sudoku is a 9×9 sudoku with addition. Dashed cages tell you what a group of cells must add up to, and those digits cannot repeat. You play it in the browser on this page. Easy, medium, hard, and expert are separate boards, there is one daily puzzle for everyone, and the combination table lists every legal way to make a sum.

How to play Killer Sudoku

Three rules cover the whole game. A row, a column, and a 3×3 box each contain the digits 1 through 9 once. A cage adds up to the small number in its corner. A digit is not repeated inside its cage. The board on this page is a medium puzzle: select a cell, enter a digit, and the cage panel lists the combinations still possible for that sum. Notes mode stores the candidates you are not ready to place.

A two-cell cage summing to 3 can only be 1 and 2. That is the smallest useful example. Write both digits as notes in both cells. Whichever cell later loses the 1 must be the 2, and the partner is the 1. You did not guess. The sum had only one combination, and the row did the rest. The same pattern is 4 as 1+3, 16 as 7+9, and 17 as 8+9. Three-cell cages have the same kind of ends: 6 is 1+2+3 and 24 is 7+8+9. Those cages are where an easy puzzle opens, and they are listed on the combination table.

Why our puzzles are fair

Each published puzzle has a fixed number, a stored solution, and a technique score. Before that number is assigned, two solvers have to count exactly one solution, and a logical solver has to finish the grid using only the techniques allowed for that difficulty. Hard and expert boards are published with no given digits. Easy boards may show a few digits where the cages alone would not offer an opening.

Difficulty is the hardest technique the logical solve needs, and how often that technique appears. Easy stays on singles: the last cell of a cage, a cell with one candidate, or a digit with one home in a house. Medium adds a one-cell 45-rule step or a pair. Hard and expert repeat the heavier cage ideas instead of hiding a single advanced step at the end of an otherwise easy puzzle. The hint button names the next step and links to the page that teaches it. A second press is what fills the digit.

Choose your level

Easy is the place to learn the unique cages and to trust a single. The sums are the ones a player can memorize, and the first moves are visible without a long candidate list. Stay here until the last-cell rule and the hidden single feel automatic.

Medium is the default board on this page. Unique cages still open the puzzle, then a combination has to be deleted because a digit is already used, or a single house falls short of 45 and names one cell. If easy has started to feel like bookkeeping, medium is the next board.

Hard gives you no digits. The opening is usually a 45-rule step across more than one house, or an innie or outie on a box border. Pairs and locked digits show up more than once. The combination list for a cage gets shorter as the grid fills, rather than starting unique.

Expert keeps the same promise: the puzzle can be finished without guessing. The steps that repeat are the ones on the advanced strategy pages, including cages that cross a box and have to be split, and sums you calculate because no dashed line printed them. It is the same grid and the same three rules.

Tips for your first puzzle

Mark the unique cages first. The cage combination page lists which sums have only one set of digits. Pencil both digits into both cells of a two-cell unique cage, then look along the row before you hunt for something clever.

Fill a cage that has one empty cell. Its digit is the printed sum minus the digits you already wrote, provided that digit is still legal in the row, column, and box. That is the move described on singles.

When the unique cages are marked and no last cell is left, add a house. Every row, column, and box totals 45. The 45 rule turns a cage that spills out of that house into a digit or a small sum. Do the addition twice before you place anything.

Keep the combination list short. If a cage still has more than four ways to make its sum, leave it and clear something else. Deleted digits come back as fewer combinations. The panel beside the board strikes through the combinations the current grid has ruled out.

Stop when you cannot name the reason for a digit. The hint will name a technique and highlight the cells. Read it, then try the next step yourself. The rules page is the longer version of the three cards above if the cages themselves are still new.

Daily puzzle and streaks

The daily puzzle is one board for a calendar date, the same board for everyone who opens that date. It is taken from the scheduled set, not generated in your browser, so a friend who says they are on today’s puzzle is on the same grid. Your streak is the number of consecutive local days you finish. It lives in this browser. There is no account, and a missed day resets the count you see. The difficulty through the week is mixed on purpose: the set is not seven easy boards.

Your times and the puzzles you have started stay in local storage on this device. Refreshing the page restores the digits, the notes, and the timer. Clearing site data clears that record. Nothing on the board is sent to an account, because there is not one.