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Diagnostic guide

Sudoku tips for when you're stuck

Being stuck almost always means one specific technique is missing — not that you've hit a wall. Find your difficulty below.

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"Stuck" almost never means the puzzle is unfair or that you've reached your personal ceiling. In the vast majority of cases it means one of three things: a specific technique you haven't learned yet is required, a small bookkeeping mistake happened a few moves back, or the candidate grid simply isn't complete enough to see the next move. This page walks through all three, starting with the fastest diagnosis: which difficulty are you stuck on?

What's missing, by difficulty

Easy — You're missing a Hidden Single, not a Naked Single.

Easy puzzles are built to fall to Naked Singles (a cell with one candidate left) and Hidden Singles (a digit with only one legal home in a unit). If a cell has stalled, you've probably only been scanning for Naked Singles. Pick one digit and scan every row, column and box for it specifically — Hidden Singles hide in plain sight because the cell itself still looks 'busy'. Read the Hidden Single guide →

Medium — You need pencil-marks, and probably a Naked Pair or Hidden Pair.

This is the most common stalling point on the entire site. Mental scanning stops being enough the moment a puzzle needs a Naked Pair (two cells sharing exactly the same two candidates) or its mirror, the Hidden Pair (two digits confined to the same two cells). Without a full candidate grid, both are essentially invisible. Pencil-mark every empty cell before doing anything else. Read the Naked Pair guide →

Hard — You're missing Pointing Pair or Box/Line Reduction.

Hard puzzles lean on the interaction between a 3×3 box and the row or column running through it. If a digit's only candidates in a box all sit on one line, it's a Pointing Pair; if a line restricts a digit to one box, it's a Box/Line Reduction. Both are easy to miss if you only ever scan rows and columns for pairs and never think box-first. Read the full Hard Sudoku tutorial →

Expert — You need a whole-grid technique: X-Wing, Swordfish or XY-Wing.

Once every simpler technique has been exhausted and candidates still won't budge, Expert puzzles usually need a pattern that spans the entire grid rather than one unit. X-Wing looks for a digit locked into two rows and two columns; Swordfish is the same idea with three; XY-Wing uses a three-cell chain instead of counting. Check X-Wing first — it's by far the most common of the three. Read the X-Wing guide →

Five mistakes that stall a solve

Before assuming you need a new technique, rule these out — they cause more "impossible" positions than any missing technique does.

1. A digit was placed wrong several moves ago

The single most common cause of a Sudoku that 'has no solution' from here. If every remaining deduction leads to a contradiction (two of the same digit forced into one unit), the puzzle isn't broken — a digit went in wrong earlier. Restart from your last fully-confident position rather than pushing forward.

2. Candidates weren't updated after the last placement

Every time a digit goes into a cell, it should immediately be erased as a candidate from every cell in its row, column and box. Skipping this step is the single biggest source of 'phantom' Naked Pairs and Hidden Singles that don't actually exist yet.

3. Only rows and columns were scanned — boxes were skipped

Beginners often scan rows and columns out of habit and forget that the 3×3 box is an equally valid unit for every technique. Locked-candidate techniques (Pointing Pair, Box/Line Reduction) specifically live at the row/column-and-box intersection, so skipping box scans hides them entirely.

4. The same technique was tried for ten minutes straight

If a technique isn't turning anything up after a couple of minutes, the puzzle probably needs a different one, not more patience with the same one. Move up the ladder: Singles → Pairs → Locked Candidates → Fish → Chains. Don't get stuck re-scanning the same pattern.

5. No break was taken

Pattern recognition — which is what Sudoku actually tests — degrades with fatigue faster than almost any other cognitive task. Ten minutes away from the board resets your eyes more reliably than any technique. Many 'impossible' cells resolve instantly on a second look.

The technique ladder

If pencil-marks are correct and no mistake is hiding upstream, work through techniques in this order — re-checking Singles after every single placement — rather than jumping straight to the most advanced pattern you know:

  1. Hidden Single (and plain Naked Single)
  2. Naked Pair → Hidden Pair
  3. Pointing Pair → Box/Line Reduction
  4. X-Wing → Swordfish
  5. XY-Wing

FAQ

Why can't I solve hard Sudoku even though I finish Easy and Medium fine?

Hard puzzles require two techniques that Easy and Medium never demand: Pointing Pair and Box/Line Reduction. Both need a full pencil-marked candidate grid to spot. If you've been solving Easy and Medium mostly by eye, Hard is the first puzzle level where that habit stops working — not a sign you've hit a ceiling.

Is it ever OK to guess in Sudoku?

A properly constructed puzzle never requires guessing — every step should be provable by logic alone. If you feel forced to guess, it usually means a technique higher up the ladder (Pointing Pair, X-Wing, XY-Wing) is available but hasn't been found yet, or an earlier digit was placed incorrectly. Guessing also risks an unrecoverable contradiction ten moves later.

How do I know which Sudoku technique to try next?

Work up a fixed ladder in order and don't skip steps: Naked Single → Hidden Single → Naked Pair → Hidden Pair → Pointing Pair → Box/Line Reduction → X-Wing → Swordfish → XY-Wing. Re-run the Singles pass after every placement before trying anything higher on the ladder — most 'stuck' moments are a missed Single, not a missing advanced technique.

Why does my Sudoku suddenly have no valid digit for a cell?

This means a digit was placed incorrectly earlier — a valid published puzzle always has exactly one solution, so a genuine dead end can only come from a mistake already on the board. Trace back to your last confident position (ideally where you last fully re-checked every unit) and re-solve from there instead of guessing forward.

I've pencil-marked everything and I'm still stuck — what now?

Double-check the candidate grid itself first: a single missed elimination early on cascades into dozens of wrong candidates. If the marks are correct, work through the technique ladder above in order rather than jumping straight to advanced patterns — most stalls are a missed Pair or Locked Candidate, not a missing Fish or Chain technique.

Still stuck?