Common Sudoku Mistakes and How to Get Unstuck

Mistake 1: checking two directions instead of three

Every square belongs to a row, a column and a 3×3 box. The most common slip, especially early on, is checking the row and column and forgetting the box, or the other way round. The result is a digit that seems to fit but repeats somewhere you didn't look.

Fix: make it a fixed order. Row, column, box, every time, before you write anything. It feels slow for a few puzzles, then it becomes automatic.

Mistake 2: guessing without noticing

Hardly anyone guesses on purpose. It happens when "this is probably a 4" turns into writing a 4. On a well-made puzzle you never need to guess. Every puzzle on this site has exactly one solution, reachable by logic (our guide on how puzzles are made explains how we check that). So a digit you aren't sure of is a sign you've missed something, not that the puzzle wants a guess.

Fix: before writing a digit, say the reason in one sentence: "it's the only 4 left in this box", "this square sees every digit but 7". If you can't say it, don't write it. Pencil it in as a note instead.

Mistake 3: letting one wrong digit spread

A wrong digit rarely shows itself straight away. It sits there quietly while you build on it, and ten moves later two squares in a row both need to be 6. By then it's hard to tell which entry started the trouble.

Fix: when the grid stops making sense, suspect your recent entries first, not the puzzle. In our game:

• Undo steps back one move at a time, up to 200 moves (Ctrl+Z on a keyboard).
• Show mistakes (on by default) marks any digit that doesn't match the solution in red and counts mistakes in the status bar. With it off, the game still marks a digit red when it repeats in its row, column or box, but it won't tell you about a digit that is wrong without a direct clash.
• Hint checks for wrong digits first. If there is one, the hint points to it by row and column instead of filling a square, because every later step depends on it.

Mistake 4: notes that go stale

Pencil marks are only useful if they're current. When you place a digit, it has to come out of the notes in its row, column and box. Miss one, and a square looks like it has two options when it really has one.

Fix: our game removes the placed digit from the notes in the same row, column and box automatically, which takes care of most of this. If you play on paper, clear notes as you go, not in batches. Our pencil marks guide covers note-taking systems that stay manageable.

Mistake 5: only asking "what goes here?"

Looking at one square and asking which digit fits finds naked singles. It misses hidden singles, where a square could hold several digits but one digit has nowhere else to go in its row, column or box. Medium puzzles on this site need at least one hidden single, so players who only look from the square tend to hit a wall there.

Fix: alternate the question. After a pass over the squares, take each digit from 1 to 9 and ask, box by box, "where can this digit go?"

Mistake 6: working in only one area

It's natural to keep pushing at the part of the grid you've been working on. But when an area stalls, its next digit usually depends on something elsewhere.

Fix: after two minutes without progress in one area, move to the most crowded row, column or box elsewhere and start fresh. New digits there often unlock the area you left.

Mistake 7: notes too early, or too late

Filling every square with candidates at the start of an easy puzzle wastes time, because most squares solve without notes. Refusing notes on a hard puzzle makes it nearly impossible, because techniques like pairs and locked candidates only show up in the notes.

Fix: on easy and medium, solve without notes until you stall, then note only the squares with two or three candidates. On hard and expert, fill in notes once the obvious singles are gone.

A routine for when you're stuck

1. Check your work. Turn on Show mistakes for a moment, or scan for any digit that repeats. A wrong entry is the most common reason for a stall.
2. Sweep by square. Go along each row and check each empty square for a naked single.
3. Sweep by digit. For each digit 1 to 9, look at every box that's missing it and see whether only one square is possible.
4. Update notes. Make sure every empty square's candidates are complete and current.
5. Look for the next technique. With notes in place, look for two squares in a unit sharing the same two candidates (a naked pair), or a digit confined to one row inside a box (locked candidates). The techniques page lists them in order.
6. Take a hint. On easy and medium puzzles, the hint names the single it found and links to how it works. On harder puzzles, when no single is left, it fills one square and tells you the next step needs pairs or locked candidates. Either way, ask yourself why that digit had to go there before you carry on.

When to move down a level

Being stuck is part of learning, but being stuck on every puzzle isn't. If you need several hints on most puzzles at one level, spend a week on the level below. Solve it without hints, then try again. The step from medium to hard in particular is a big one, because hard is where notes stop being optional.

And if you're new, our step-by-step beginner walkthrough shows a whole puzzle solved with good habits from the first move.