How to play
Ultra hard grids are stripped until removing any more givens would allow a second solution, usually leaving about 23.
- Fill every row, column and 3×3 box with the digits 1 to 9, each used once.
- Tap a cell, then tap a number. On a computer you can type digits and move with the arrow keys.
- Switch on Notes (or press N) to pencil in candidates. Placing a digit clears it from the notes in the same row, column and box.
- Repeated digits turn red straight away. Check marks any cell that differs from the solution; Hint fills one cell for you.
Tips for ultra hard grids
- X-Wing: if a digit fits in only two cells in each of two rows, and those cells share two columns, remove it from the rest of those columns.
- Write full notes before you start. Ultra grids reward a complete candidate map.
About sudoku
Sudoku is a logic puzzle played on a 9×9 grid divided into nine 3×3 boxes. The goal is to fill every row, every column and every box with the digits 1 to 9, each exactly once. Some digits, called givens, are already placed, and the fewer givens a grid has, the harder it usually is.
The modern puzzle first appeared as “Number Place” in Dell Magazines in 1979 and is credited to the American puzzle maker Howard Garns. The Japanese publisher Nikoli popularised it in the 1980s and gave it the name Sudoku.
Frequently asked questions
- How many clues does an ultra hard sudoku have?
- Usually about 23. The generator keeps removing givens for as long as the grid still has exactly one solution.
- What is the hardest sudoku technique?
- Expert grids can need chains and fish patterns such as X-Wing and Swordfish, which track one digit across several rows and columns.