Play hard Sudoku online
Hard Sudoku rewards method over speed. With only about 28 givens, plain counting will not get you far, here clean candidate lists and techniques like naked pairs or locked candidates are what crack it. At Sudoku Sagax you play them ad-free, and mastering them lifts you noticeably through the fox ranks.
What makes a Sudoku hard
A hard puzzle starts with around 28 givens. Naked and hidden singles only carry you part of the way, after that you are stuck unless you track candidates. The key is being systematic: consistently enter the possible digits for every open cell, then work with patterns in those candidates. Even so there is never any guessing, every move can be justified.
The technique: pairs, triples, locked candidates
- Keep candidates clean. For hard Sudoku you note the possible digits for every free cell. This list is the basis for every further technique, keep it up to date after each move.
- Naked pair / triple. If two cells of a unit hold exactly the same two candidates, those digits belong in exactly those cells, and can be removed from the rest of the unit. Likewise with three cells and three digits.
- Hidden pair. If two digits appear in a unit only in the same two cells, you can remove all other candidates from those cells.
- Locked candidates. If a candidate lies in a block within a single row or column only (or vice versa), remove it from the overlapping part of the other unit.
These four tools solve practically every hard Sudoku. You will find a detailed guide on our techniques page.
Playing Hard at Sudoku Sagax
The button above starts a random Hard Sudoku right away, so you land in the game instead of a level grid. Every solved puzzle earns points for your fox rank and the leaderboard, and there is a shared daily challenge every day. Everything runs in the browser, ad-free, without trackers and without forced sign-up. A good Hard puzzle takes around 20 minutes.
Other difficulty levels
Frequently asked questions
Why are hard Sudoku barely solvable without notes?
Because only about 28 cells are given. Naked and hidden singles are not enough throughout; you need candidate lists to spot patterns like naked pairs or locked candidates.
Do you have to guess in hard Sudoku?
No. A properly generated hard Sudoku has exactly one solution and is solvable by pure logic. At Sudoku Sagax every solution is verified on the server.