How many clues does a hard sudoku have?

Twenty-seven to twenty-nine here, placed in rotationally symmetric pairs — the dig aims at twenty-seven and stops when no further pair can come out. 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](/sudoku/expert), which digs as low as 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 to twenty-nine clues with exactly one solution. On about a third of these grids singles get you a few cells and then stop, and what breaks the position 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. The other two thirds do come apart to patient scanning.

Time0:00Clues27Empty54Puzzle#20260726

New puzzle generates a fresh grid at the level you last chose, Hint fills one square and names the single it used — a naked single if a cell has one candidate left, a hidden single if a digit has one home left in a unit, and otherwise it simply reveals the answer, which happens on about a fifth of hard grids and a third of expert ones. The three-dot menu at the end of the bar holds pencil marks, fill-in-all-marks, check-as-you-go, today’s shared puzzle and the difficulty level.

Your statistics · hard

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

This level

27–29 clues. About a third of grids need naked or hidden pairs or triples before they open up.

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: 27–29
Techniques: Singles on 2 grids in 3, else pairs and 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 whenever a naked or hidden single is available, which is the difference between being told and being taught; on about a fifth of hard grids none is, and it says plainly that it is reading the answer off the solution instead.

Is 27 clues always hard?

No, and it is worth being straight about that — twice over. The generator digs towards twenty-seven and stops wherever a symmetric dig runs out, which is twenty-eight on most grids and twenty-nine on a few, so the count itself is a range. And it proves uniqueness after every removal without ever classifying the finished puzzle by technique: across a 200-seed sample, naked and hidden singles alone finished 136 of 200 hard grids. 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 to twenty-nine here, placed in rotationally symmetric pairs — the dig aims at twenty-seven and stops when no further pair can come out. 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, which digs as low as 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.