Sudoku Made and maintained by Brian Hamilton and Karan Hamilton.
Brian builds the playable puzzle tools, while Karan helps test clarity, difficulty feel, and the player experience.
Playable puzzles and controls are tested in the browser for rule accuracy, usability, and a clear solving experience. See how we make and test puzzles, our editorial standards, or report a problem.
What is Hard Sudoku?
Hard Sudoku is classic sudoku with 26 of the 81 squares filled at the start, and exactly one answer, on every board this page deals. It is the first level where looking alone does not always finish the board: most of the time singles still take you to the end, but on the rest you need the next step up, locked candidates, which comes in two directions. Nothing beyond that is ever needed.
What does a Hard board ask of you?
We dealt 200 Hard boards from this page’s own generator and tried each with one step at a time. Naked and hidden singles finished 171 of them. On the other 29 they stall partway, and locked candidates got every one of those to the end. Fifteen could not be finished without locked candidates even with naked pairs allowed in their place, and no Hard board needed pairs at all.
| Level | Given digits | Singles alone finish | What the rest need |
|---|---|---|---|
| Easy | 40 | 200 of 200 | Nothing more |
| Medium | 32 | 200 of 200 | Nothing more |
| Hard | 26 | 171 of 200 | Locked candidates finish all 29 |
| Expert | 22–25 (median 23) | 135 of 200 | Locked candidates finish 47 more; naked pairs the last 18 |
200 boards a level, dealt by this page’s own generator and measured on 10 October 2026; every one had exactly one answer, counted by a separate solver. “Singles” means naked and hidden singles together. Nothing past naked pairs was needed on any board.
Locked candidates are about where a digit can still go, so on Hard you will usually want notes — either your own or the full set from Start Notes — once the singles run out.
What is a pointing pair?
A pointing pair starts from a box. If a digit’s only places in a box all lie along one row or column, the box’s copy of that digit is on that line, so the digit cannot go anywhere else on the line outside the box. In the board above the 4 is locked into column 5 by the centre box, and comes out of R3C5 and R7C5. With three places it is a pointing triple; the logic is the same. More in the pointing pairs guide.
What is box/line reduction?
Box/line reduction is the same idea from the other side. If a digit’s only places in a row or column all fall inside one box, the line’s copy of that digit is in that box, so it cannot go in the box’s other squares. Here row 8 can only put its 5 in the bottom-right box, which clears the 5 from R7C7 and R7C8. The box/line reduction guide works through more. Either move rarely places a digit by itself; what it does is take a candidate away, and that is often what turns the next square into a single.
How should you work a Hard board?
- Play it like Medium first: hidden singles digit by digit, naked singles wherever a house is nearly full. On most Hard boards that is enough to finish.
- When a full round finds nothing, put the notes in, and keep them up to date — a stale candidate hides every locked pattern behind it.
- Go box by box: for each digit a box is missing, do its places lie in one row or one column? If so, strike the digit from the rest of that line.
- Then line by line: does a digit’s every place in a row or column fall in one box? If so, strike it from the rest of the box.
- After every strike look for singles again. One locked candidate usually opens several squares at once.
Hard boards are dug so that singles and locked candidates finish them, and the generator checks every one. Naked pairs are Expert’s extra step. They are still a sound move on a Hard board if you see one, but you never have to find one.
Has this page always dealt boards like these?
No. From 28 March 2026, when the page went up, until 13 April, its generator blanked squares at random until 26 were left and checked nothing. Run again, every one of 200 such Hard boards had more than one answer, and the game marks digits against one stored answer, so a digit that broke no rule could be marked wrong.
From 13 April every board had one answer, but nothing checked how it could be solved. Measured then, 12 of 40 Hard boards needed more than singles, locked candidates, pairs and the fish patterns our solver tries, under a page that said “No guessing, no trial and error — just deduction” and listed naked pairs, hidden pairs and claiming pairs as what Hard required. Since 10 October 2026 the generator only keeps a Hard board that singles and locked candidates finish, and the numbers here are measured from it.
What do the buttons do?
- Notes switches the keypad to pencil marks, and Start Notes fills every empty square with the digits it can still hold, in one go.
- Hint picks a square at random from the ones that are empty or wrong and puts its answer in. It does not explain the step and it is an ordinary move: Undo takes it back.
- Undo and Redo step back and forward through your moves (Ctrl+Z and Ctrl+Y, or Cmd on a Mac); Erase clears the selected square.
- A wrong digit turns red and adds one to the mistake count. There is no limit: the count never ends the game.
- Solve asks before it fills the grid. New Game deals another Hard board, and the level buttons above the grid switch level without leaving the page.
- The timer starts when the board is dealt and runs until it is finished. This page keeps its own saved board, so a long game can wait while you play elsewhere.
Where next?
Expert adds one more step, the naked pair, on boards that start with 22 to 25 digits. Medium is there when you want singles only. For one new Hard board a day, the same for everyone, try Daily Hard Sudoku. And if you want to see each step named as you go, the sudoku helper finds the next one on any board you give it.