SudokuHard
Hard sudoku
Two 9 × 9 sudoku per page with very few starting numbers, still built to have one solution. It is for experienced solvers who want a longer puzzle.
- Fresh problems every time
- Answer key on its own page
- Print or PDF, Letter or A4
- Free, no sign-up
On this page
With about 26 numbers printed, a hard grid hands you little to start on. The puzzle is still fair, because the generator never leaves two possible answers.
What prints
The sheet reads "Hard Sudoku", then the rule in one sentence: every row, every column and every 3 × 3 box takes the numbers 1 to 9 once. Two big grids sit side by side, titled Puzzle 1 and Puzzle 2, with bold clue numbers and thick lines around the boxes. A separate answer key page fills every square, and the numbers you solved appear in color.
The settings
- Grid: 9 × 9 is selected; the 4 × 4 and 6 × 6 boards are smaller options.
- Difficulty: Hard is selected, so the clue count is lowest.
- Puzzles per page: starts at 2 and can go to 1, 4 or 6.
Why "about 26"
The generator begins from a full grid and removes numbers in random order. Each time, a solver counts solutions and stops at two. If removing a number would give the puzzle a second solution, the number is put back. The program stops early in that case, so the clue count can finish a little above 26. The tradeoff is intended: a hard puzzle that has two answers is not a puzzle, so uniqueness wins over the clue count.
On the smaller grids the Hard target is 5 clues on 4 × 4 and 12 on 6 × 6.
Strategies for fewer clues
Basic scanning gets you the first few squares and then usually stops. The next step is to keep candidate notes for every empty square in one band and look for naked pairs: two squares in a row that can only hold the same two numbers, which rules those numbers out of the rest of the row. Pointing pairs help too. If a number can only go in one row of a box, it cannot appear elsewhere in that row outside the box.
Give yourself time, take a break when stuck, and come back to a fresh eye. A pencil with a good eraser is more useful here than at lower levels.
Worked example: a long flight
Dev prints the Hard page on two copies before a trip, two puzzles each. On the plane he fills in about a dozen numbers in the first ten minutes, then stalls. He writes candidates in the bottom-middle box, sees that two squares can only be 2 or 8, and rules out those numbers from the rest of the row. The chain of answers that follows takes the grid to the end. He leaves the key page in his bag until he lands and compares his notes with it.
Easing off
If the level is a stretch, drop back to medium sudoku for a few sessions. Printable sudoku opens with four grids on Medium if you want volume.
Sources
- NRICH, University of Cambridge: Sudoku-style puzzles nrich.maths.org