How many clues does a hard sudoku have?

Twenty-seven here, placed in rotationally symmetric pairs. Every removal is verified to leave exactly one solution.

What is a naked pair in sudoku?

Two cells in the same row, column or box that both have exactly the same two candidates. Between them they use up both digits, so those digits can be eliminated from every other cell in that unit.

Do you ever have to guess on a hard sudoku?

No. Every puzzle here has a unique solution reachable by logic. Guessing is what people do instead of finding the elimination, and it usually costs more time than the elimination would have.

What is harder than hard sudoku?

Expert, at twenty-four clues — approaching the 17-clue proven minimum, where every additional removal has to be checked individually for uniqueness.

Hard Sudoku

Hard sudoku here is twenty-seven clues with exactly one solution. Singles will get you a few cells and then stop, and elimination by pointing pairs alone usually will not finish it either. What breaks a hard grid open is spotting sets — two cells in a unit that between them can only hold two digits, or three that can only hold three — and using them to clear candidates elsewhere.

Time

0:00

Clues

Empty

Puzzle

#20260726

Your statistics · hard

Played: 0
Won: 0
Win rate: 0%
Streak: 0
Best streak: 0
Best time: —

This level

27 clues. Naked and hidden pairs or triples are usually what break it open.

Keyboard

Arrows: Move the selected cell
1 – 9: Enter a digit
Backspace: Clear the cell
P: Pencil marks on or off
F: Full screen

Clues: 27
Techniques: Usually naked and hidden pairs or triples
Solutions: Exactly one, verified
Guessing: Not required

What makes a hard sudoku hard?

Not that there is less information — the puzzle still has exactly one solution and a logical path to it. What is harder is that the next certain move is rarely about a single cell. You have to reason about groups:

A naked pair: two cells in the same row, column or box whose candidates are the same two digits. Those two digits belong to those two cells, so they can be struck out of every other cell in that unit — even though you cannot yet say which is which.
A hidden pair: two digits in a unit that can only appear in the same two cells. Those cells therefore hold exactly those two digits, so every other candidate in them can go.
Triples, the same argument with three cells and three digits. Harder to see and far more productive when you find one.

How do you solve a hard sudoku without guessing?

Fill in the pencil marks first. At this level you cannot hold the candidate sets in your head, and every technique above is stated in terms of candidates.
Work the units with the fewest empty cells. A row with three blanks is where a naked pair is easiest to see.
When you find an elimination, follow it immediately — one removal frequently creates a single two units away, and that single creates another.
Use the hint if you stall. It names the deduction rather than just filling in the answer, which is the difference between being told and being taught.

Is 27 clues always hard?

No, and it is worth being straight about that. The generator targets the clue count and proves uniqueness after every removal; it does not classify the finished puzzle by technique. Twenty-seven clues usually lands in the territory described above, and occasionally produces one that comes apart much faster. Clue count is a good proxy for difficulty and a bad measurement of it.

Hard Sudoku questions people ask

How many clues does a hard sudoku have?: Twenty-seven here, placed in rotationally symmetric pairs. Every removal is verified to leave exactly one solution.
What is a naked pair in sudoku?: Two cells in the same row, column or box that both have exactly the same two candidates. Between them they use up both digits, so those digits can be eliminated from every other cell in that unit.
Do you ever have to guess on a hard sudoku?: No. Every puzzle here has a unique solution reachable by logic. Guessing is what people do instead of finding the elimination, and it usually costs more time than the elimination would have.
What is harder than hard sudoku?: Expert, at twenty-four clues — approaching the 17-clue proven minimum, where every additional removal has to be checked individually for uniqueness.

The full rules of sudoku

This page is one rule set. The complete rules, with worked examples and the questions beginners actually ask, are on how to play sudoku, and every rule set is switchable on the main sudoku page.