Hard Sudoku: Play Hard and Expert Puzzles Online

Hard sudoku is the same game with less help. The rules do not change: every row, column and 3×3 box must hold the digits 1 to 9 once. What changes is how many numbers you start with and how far you have to think before the next one is certain. Press Play above and a Hard puzzle starts right away. For Expert, tap New Game and pick Expert. If you have an unfinished game, the menu opens first so you can continue it.

Hard or Expert?

• Hard starts with 30 to 34 given numbers, and every box has at least two. Most cells still fall to scanning, but you will need notes to finish.
• Expert starts with 25 to 28 given numbers, and a box can have just one. Long stretches of the grid open up only after you remove candidates with the techniques below.

Both levels give you three hints per game, and three mistakes end the game. Each level keeps its own best score on your device, so you can track Hard and Expert separately.

Techniques for hard puzzles

If you are new to the game, start with how to play sudoku first. These are the next steps once singles stop being enough.

1. Hidden singles. Pick a digit and a box. If that digit can only go in one cell of the box, it goes there, even if the cell has other candidates. Do the same for rows and columns.
2. Naked pairs. If two cells in the same row, column or box can only hold the same two digits, those digits are used up. Remove them from every other cell in that row, column or box.
3. Pointing pairs. If all the places for a digit inside a box sit in one row, that digit must go in that row inside the box. Remove it from the rest of that row outside the box. The same works for columns.
4. Box and line. The reverse: if all the places for a digit in a row are inside one box, remove that digit from the other cells of the box.
5. X-Wing. If a digit has exactly two possible cells in each of two rows, and those cells line up in the same two columns, the digit must take one corner of that rectangle in each column. Remove it from the rest of both columns.

How to practise

Turn notes on from the first move and fill them in for one box at a time instead of the whole grid. After every number you place, update the notes in its row, column and box. Most mistakes on hard grids come from stale notes, not from bad logic. If you prefer paper, the printable sudoku page has hard grids with answer keys.