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Hard Sudoku
Around 30 givens. Hard Sudoku is where the grid stops handing you cells and starts asking for eliminations — the move that fills nothing in but rules something out.
What makes a Sudoku hard
The count of givens matters less than what the givens leave behind. A hard board is built so that the obvious deductions run out early, and the next move is not a digit at all: it is knowing that a digit cannot go somewhere.
That is a real shift. On easy and medium boards you make progress by writing numbers. On hard boards long stretches of the solve consist of crossing candidates out, and the numbers arrive in bursts afterwards.
Expect a hard grid to take fifteen to thirty minutes and to include at least one point where you are convinced it is unsolvable. It is not — every board here is carved to a single solution, so the deduction exists whether or not you have spotted it yet.
How to solve a hard board
Mark candidates properly and keep them accurate. On a hard board a stale pencil mark does not just slow you down, it actively misleads you into an elimination that is not there.
Work the intersections. If a digit's only homes inside a box all sit in one row, that digit is gone from the rest of the row even though you have not placed it — that single idea, pointing pairs, solves a large share of hard boards on its own.
When intersections dry up, look for an X-Wing: the same digit confined to the same two columns in two different rows locks a rectangle and clears that digit from the rest of both columns. It is the first technique that reasons about the whole grid at once, and it is usually what a hard board is waiting for.
Every board has exactly one solution
Boards here are carved a cell at a time, and any cell whose removal would leave the grid with two valid answers is put straight back. That is what makes the 30-ish clue count on this page a difficulty rather than a gamble: there is always a deduction available, so you never have to guess and back out.
If you cannot find the next move, the honest fix is a technique rather than a hint. The techniques index covers them in the order they start paying off — or paste the grid into the solver and ask for one digit rather than the whole answer.
Frequently asked questions
What makes a Sudoku hard?
Fewer givens, around 30, but more importantly an arrangement where the obvious deductions run out early. On a hard board long stretches of the solve are spent ruling digits out of cells rather than placing them, and the numbers arrive in bursts afterwards.
How long should a hard Sudoku take?
Fifteen to thirty minutes is normal, including at least one stretch where the board looks stuck. It is not — every puzzle here has exactly one solution, so the deduction exists whether or not it has been spotted.
Which techniques do I need for hard Sudoku?
Pointing pairs first: if a digit's only homes inside a box all sit in one row, it is eliminated from the rest of that row. After that, the X-Wing, which is the first technique that reasons about the whole grid rather than one unit.
Do hard Sudoku puzzles ever need guessing?
No. A puzzle with a single solution is always solvable by pure deduction. If a board seems to need a guess, a technique is missing rather than the puzzle being unfair.
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Take it to a real opponent
Action Sudoku puts a second person on the same grid, racing you cell by cell. Free on iOS and Android, with a daily puzzle and a solo journey when you would rather play alone.