Hard Sudoku Puzzles

Hard sudoku for players who know the basics and want a proper workout. Only 27 numbers are given, so pencil notes and a few clever techniques will be your best friends.

Who Should Play Hard

Hard is for players who solve Medium grids comfortably and want to slow down and think. If you regularly finish Medium puzzles without hints, you are ready. If Hard feels like a wall at first, that is normal: moving from simple scanning to reasoning with notes is the biggest step in Sudoku, and everyone who plays it well has made that step at some point.

Only 27 Given Numbers

Each grid starts with 27 of its 81 numbers, five fewer than Medium. That may not sound like much, but it changes the game. Early on there may be only one or two squares you can fill by scanning, and after that you need to work out candidates to make progress. Every difficult sudoku here still has exactly one solution and is generated fresh for you, so careful logic will always get you to the end.

Techniques You Will Need

• Full pencil notes for the busy parts of the grid, so you can see every candidate at once.
• Hidden singles: a number that fits only one square in a row, column or box, even though that square has other candidates.
• Pointing: when a number's candidates in a box all lie in one row or column, it can be crossed out from the rest of that line.
• Naked and hidden pairs: two squares that share two numbers between them, which lets you clean up the notes around them.

All of these are explained with diagrams on our page about Sudoku strategies.

How Long a Game Takes

Expect 20 to 45 minutes. Some grids flow nicely, others make you stare at one box for ten minutes before the key move appears. Pause hides the board, so you can take a break without peeking, and your game stays saved in the browser if you close the tab.

Tips for Difficult Sudoku Online

• Fill in notes box by box rather than jumping around. A complete set of candidates is much easier to read.
• After every new number, check its row, column and box: placing one number often unlocks two or three more.
• Try switching Show mistakes off if you want the full challenge of checking your own work.
• When nothing seems to work, look for a pair: two squares in the same row, column or box with the same two candidates.

When You Want a Very Hard Sudoku

If you breeze through Hard, the next shelf up is Expert Sudoku, with only 24 numbers given: the closest thing on this site to a very hard sudoku. And on days when you would rather relax, there is always Medium Sudoku or the daily puzzle.