Hard Sudoku: A Free Hard Sudoku Every Day

What changes on a hard board

On a hard sudoku you will often scan all nine digits and find nothing to place. That is expected. The way forward is to write down candidates, the digits still possible in each empty square, and then remove them one by one with logic until singles appear. A hard board takes 20 to 45 minutes for most experienced players, and longer on a tough day.

Techniques for hard sudoku

Full candidates

List every possible digit for every empty square. It feels slow, but it turns the puzzle into a set of small, checkable eliminations. Keep the list on paper next to your screen and update it every time you place a digit.

Naked pairs and triples

If two squares in one row, column or box share exactly the same two candidates, those two digits are locked into them. Remove both digits from every other square in that unit. Three squares sharing three candidates work the same way.

Pointing and claiming

If all the possible spots for a digit within a box sit on one row or column, the digit must be in that box on that line, so it can be removed from the rest of the line. The reverse also works: if a digit's spots in a row all fall inside one box, remove it from the rest of that box.

X-wing

Find a digit with exactly two possible spots in each of two rows, where those spots line up in the same two columns. The digit must sit at two opposite corners of that rectangle, so it can be removed from every other square in those two columns. The same pattern works with columns and rows swapped.

When you get stuck

• Recheck your candidate list against the board; a single missed elimination is the most common reason a hard sudoku stalls.
• Look at the units with the fewest empty squares first.
• Search each digit separately for pointing patterns.
• Use Hint once, then work out why that square had to hold that digit. It is the fastest way to learn the move you missed.

Our boards are generated fresh every day, and on the hardest of them you may occasionally reach a point where two digits both seem to fit. Any complete, valid fill counts as a solve here, so if every row, column and box checks out, the board is yours.

More to play

Need a breather? The medium sudoku and easy sudoku change daily too, and the daily crossword gives your vocabulary a turn. Curious about where it all started? Read who invented sudoku.