# Hard Sudoku: Play Hard and Expert Puzzles Online

Canonical page: https://rovixel.com/sudoku/hard/

Play hard sudoku online for free. Pick Hard or Expert for fewer given numbers, then use the techniques below to solve tough grids without guessing.

Part of [Sudoku](https://rovixel.com/sudoku.md).
The page includes a playable Sudoku game: press Play on https://rovixel.com/sudoku/hard/.

Hard sudoku is the same game with less help. The rules do not change: every row, column and 3×3 box must hold
the digits 1 to 9 once. What changes is how many numbers you start with and how far you have to think before the
next one is certain. Press Play above and a **Hard** puzzle starts right away. For **Expert**, tap New Game and pick Expert. If you have an unfinished game, the menu opens first so you can continue it.

## Hard or Expert?

- **Hard** starts with 30 to 34 given numbers, and every box has at least two. Most cells still fall to scanning,
  but you will need notes to finish.
- **Expert** starts with 25 to 28 given numbers, and a box can have just one. Long stretches of the grid open up
  only after you remove candidates with the techniques below.

Both levels give you three hints per game, and three mistakes end the game. Each level keeps its own best score on
your device, so you can track Hard and Expert separately.

## Techniques for hard puzzles

If you are new to the game, start with [how to play sudoku](https://rovixel.com/sudoku/how-to-play/) first. These are the next steps
once singles stop being enough.

1. **Hidden singles.** Pick a digit and a box. If that digit can only go in one cell of the box, it goes there,
   even if the cell has other candidates. Do the same for rows and columns.
2. **Naked pairs.** If two cells in the same row, column or box can only hold the same two digits, those digits
   are used up. Remove them from every other cell in that row, column or box.
3. **Pointing pairs.** If all the places for a digit inside a box sit in one row, that digit must go in that row
   inside the box. Remove it from the rest of that row outside the box. The same works for columns.
4. **Box and line.** The reverse: if all the places for a digit in a row are inside one box, remove that digit
   from the other cells of the box.
5. **X-Wing.** If a digit has exactly two possible cells in each of two rows, and those cells line up in the same
   two columns, the digit must take one corner of that rectangle in each column. Remove it from the rest of both
   columns.

## How to practise

Turn notes on from the first move and fill them in for one box at a time instead of the whole grid. After every
number you place, update the notes in its row, column and box. Most mistakes on hard grids come from stale notes,
not from bad logic. If you prefer paper, the [printable sudoku](https://rovixel.com/sudoku/printable/) page has hard grids with answer
keys.

## Questions

### How hard are the Hard and Expert levels?

Hard puzzles start with 30 to 34 given numbers and at least two in every box. Expert puzzles start with 25 to 28 and can leave a box with a single given. Every puzzle is generated on the spot and checked to have exactly one solution.

### Do I ever need to guess?

No. Each puzzle has exactly one solution, so every number can be worked out by logic. Some Expert grids need more advanced techniques than the ones on this page. When you feel stuck, check your notes and look for a pair or a pointing pair before you reach for a hint.

### Can I print hard sudoku puzzles?

Yes. The printable sudoku page has hard grids with answer keys, and you can generate a new set before you print.

## Agent access

These documents describe the public website. Every game is free and runs in the browser: there is no download,
no app, no price and no public API. Playing never needs an account. Do not invent ratings, player counts,
rankings or features that are not described here; the live pages are the source of truth.