Every puzzle on Sudoku Challenge is solved by our grader step by step, the way a careful person would: at each step it uses the simplest technique that works. A puzzle's level is the hardest group of techniques it cannot be finished without.
Easynaked singles only: every step is a cell with one possible digit.
Mediumadds hidden singles (a digit with one possible cell in a row, column or box) and locked candidates.
Hardadds pairs, triples and quads, naked and hidden, the unique rectangle and BUG+1.
Extremeneeds at least one advanced pattern, such as the X-Wing, XY-Wing, Skyscraper or Swordfish.
Diabolicalneeds the longest chains and largest patterns our solver knows, such as the XY-Chain, Jellyfish or ALS-XZ.
So a Hard puzzle is one that singles and locked candidates cannot finish on their own, yet never needs an Extreme pattern or a guess. Its score sits between 2,000 and 4,999 points, shown before you start.
The number of clues says little: our Hard puzzles have 20 to 32 clues, 25 on average, about as many as our Extreme and Diabolical ones. What counts is the techniques.
The techniques you need
First the ones every level uses: Naked Single, Hidden Single and Locked Candidates. Then the Hard techniques, with how often our grader used each one across our 70,000+ Hard puzzles:
Fill in every candidate first: pairs and triples only show when each cell lists what it can still hold.
Clear the singles and locked candidates as far as they go. Most of a Hard puzzle still falls to them.
When nothing is left, look for two cells in a row, column or box that hold the same two candidates: a naked pair. Those two digits can be eliminated from the rest of that row, column or box.
Still stuck? Look for a digit that fits in only two cells of a house (a hidden pair), then for three cells sharing three candidates (a triple).
Check every elimination before you write a digit. One wrong candidate early on is what turns a Hard puzzle into a guess.
Not the number of clues but the techniques it needs. A Hard puzzle on this site cannot be finished with singles and locked candidates alone: somewhere it needs a pair, a triple or one of the other Hard techniques. Our grader checks every puzzle step by step to place it.
Do hard Sudoku puzzles need guessing?
No. Every Hard puzzle here can be solved by logic alone with the techniques listed on this page, and every puzzle has exactly one solution.
How many clues does a hard Sudoku have?
Our Hard puzzles have between 20 and 32 clues, 25 on average, about the same as our Extreme and Diabolical ones. That is why clue count tells you little about difficulty.
Is the Classic Daily Challenge a hard Sudoku?
Yes. The Classic Daily Challenge is a Hard puzzle every day, the same for everyone, with one attempt and a leaderboard. The daily practice also has a Hard puzzle you can replay with every tool.