Play hard sudoku on the web on your phone or computer for free and without ads. No downloads and no sign-up.
OK Sudoku has four different grades of board: 'easy', 'medium', 'hard', 'advanced'. If you've been playing sudoku for a while, you'll want to try 'hard sudoku'. Hard sudoku requires use of more sophisticated techniques than for easy or medium.
Play nowPress to load the hard sudoku board.
Once the board is loaded, you can press to try a different board. You will see a screen where you can choose hard sudoku, or another difficulty grade.
Press a cell, and use the on-screen keyboard to enter a value, or use your device's physical keyboard.
Press to see if you've made any mistakes. Try to avoid relying on the check functionality. It's good to use it every now and then to make sure you're on the right track, but if you use it to guess and check if your guess was right, you're depriving yourself of the satisfaction of solving the puzzle logically.
Only one possible number can go in a cell.
A number can only go in one cell within a row, column, or box, even if other candidates exist in that cell.
When two (or three, or four) cells in a row, column, or box contain the same set of candidates, those numbers can be eliminated from other cells in that same row, column, or box.
When two (or more) numbers appear in only two (or more) cells within in row, column, or box, those cells must contain those numbers.
If a number is confined to a row or column within a box, it can be eliminated from the rest of that row or column outside the box.