For consistent Medium solvers who avoid guessing.
Hard Sudoku online
Slow down. Read the whole grid.
Hard Sudoku offers fewer obvious placements and asks you to work with relationships between candidates. It rewards tidy notes, patient rescanning and precise eliminations rather than risky guesses.
Advanced practicePairs and locked candidatesFree, no sign-up
Candidate-driven solving29 clues
Use patterns to remove candidates before placing values.
Maintain an accurate candidate map in difficult regions.
Find one justified step, then rebuild the picture.
Why Hard Sudoku feels different
A Hard grid may begin with a few direct singles, but the straightforward path soon disappears. Progress often comes from proving that a candidate cannot occupy certain cells. That elimination creates a new single or reveals another pattern several units away.
The word “Hard” describes the solving path, not just the number of givens. A sparse puzzle can be surprisingly simple, while a fuller grid may hide a difficult sequence. SudokuGG’s Hard setting reduces immediate clues and expects comfortable use of candidates, pairs and interactions between boxes and lines.
A disciplined Hard Sudoku process
- Complete a clean opening scan. Take every naked or hidden single before moving to advanced eliminations.
- Build accurate candidates. Record legal values and update all three connected units after every placement.
- Inspect bivalue cells. Cells with two candidates are easier to compare and often participate in pairs or chains.
- Check box-line interactions. Look for a candidate confined to one line inside a box, or to one box inside a line.
- Search for subsets. Pairs and triples can reserve values and remove them from the rest of a row, column or box.
- Return to singles. Every successful elimination should trigger another basic scan before you seek a harder pattern.
Core Hard-level techniques
Locked candidates
Use the overlap between a box and a row or column to eliminate a digit from one side of that relationship.
Naked pairs and triples
A small set of cells reserves the same number of candidates, removing those values from other cells in the unit.
Hidden pairs
Two digits appear only in the same two cells of a unit, allowing all other notes to be removed from those cells.
X-Wing preparation
Track one candidate by row and column; repeated two-position patterns may create a rectangle elimination.
What to do when no pattern is visible
First verify the candidate map. One forgotten elimination can hide a single; one incorrect note can create a pattern that is not real. Check givens, user entries and candidates unit by unit. Then choose a single digit and mark where it appears in every box rather than staring at individual cells.
If that does not help, inspect rows and columns containing several bivalue cells. Compare candidate pairs and look at box boundaries. Change the order in which you read the grid, but do not introduce a guess. A legitimate Hard puzzle has a logical continuation even when it is not immediately comfortable to see.
Taking a short break is also useful. Returning with a fresh scan often exposes a constraint you repeatedly skipped.
Accuracy before speed
Hard Sudoku punishes rushed candidate work. A fast opening means little if one wrong note forces a restart later. Time yourself only after you can solve reliably. When reviewing a slow solve, identify the first pattern you missed rather than judging the total duration.
Expert is appropriate when locked candidates and subsets feel routine and you want longer chains or patterns such as X-Wing. If Hard currently requires guesses, strengthen pairs and box-line interactions first.
Hard Sudoku questions
Does Hard Sudoku require guessing?
A well-formed standard puzzle with a logical solution path should not require a guess. If you feel forced to choose between candidates, audit the grid and search for a missed elimination.
Should I use automatic candidate notes?
Automatic candidates can save input time, but manual notes develop awareness. Whichever method you use, understand why each candidate remains legal and why an elimination is valid.
Why did a pair not produce a placement?
Many techniques only remove candidates. The value of that elimination may appear in a different unit, so rescan the whole grid after applying it.
Written and reviewed by the SudokuGG editorial team against the current Hard game. Last reviewed July 21, 2026. See our editorial policy.