How to solve a hard sudoku step by step

Hard sudokus aren't solved by guessing — they're solved with logic and method. If you get stuck every time you move up a level, this guide walks you through the techniques good players use, from the simplest to the most advanced. You can practise each one in our free online sudoku as you read.

Before you start: the golden rule

The goal is to fill the 9×9 grid with the numbers 1 to 9 so that none repeats in the same row, column or 3×3 box. A well-made sudoku always has a single solution, so you never need to guess: if you think you do, it means there's a clue you haven't spotted yet.

Technique 1: scanning (the last remaining cell)

This is the foundation. Pick a number, say 5, and track it across the whole grid:

See which rows, columns and boxes already contain it.
Rule out the cells where it can't go.
If a box has only one possible cell left for the 5, that's where it goes.

Repeat for every number. Scanning solves most easy levels and a good chunk of the hard ones.

Technique 2: hidden singles

Sometimes a cell allows several numbers, but a specific number fits in only that cell within its row, column or box. Even if the cell "could" hold other values, if the 7 can't go anywhere else in that box, it's a 7.

This way of thinking — looking from the number, not from the cell — is what unlocks most hard sudokus.

Technique 3: mark the candidates (pencil marks)

When scanning stops making progress, jot down the possible candidates for each empty cell in small print. Seeing them written reveals patterns you'd miss at a glance. On paper you use a pencil; digitally, many sudokus have a notes mode.

Technique 4: naked pairs and triples

With candidates marked, look for naked pairs: two cells in the same row (or column or box) that have exactly the same two candidates, say {3, 8}. Even if you don't know which goes where, you know that 3 and 8 occupy those two cells, so you can remove 3 and 8 from the rest of that row. That usually frees up new cells.

The same works with triples (three cells sharing the same three candidates).

Technique 5: when you're truly stuck

Switch number or area: if you've been on one corner for a while, jump elsewhere.
Check the boxes with the most clues: they tend to give chains of deductions.
Scan again: every number you place opens new possibilities for the others.
Check for mistakes: a single wrong number blocks the whole puzzle. Our sudoku highlights conflicts in red so you spot them instantly.

Common mistakes that slow you down

Guessing and carrying on: if you're wrong, you drag the error to the end.
Looking only at the cell and not at the number.
Not marking candidates on the harder levels.
Giving up before checking all three constraints (row, column and box) for each cell.

Practise now

The best way to internalise these techniques is to play. Start on medium level to lock them in and move up to hard sudoku once they come naturally. Every game generates a fresh board, so you never repeat the same challenge.

Want a break from numbers? Try minesweeper, which is also pure deductive logic.