How to solve a hard sudoku

Almost everyone who gets stuck on a hard sudoku is stuck at the same place: the point where scanning for obvious digits stops producing anything. That is not the puzzle being unfair. Hard grids are defined by that wall, and getting past it is a different skill from filling in the easy part faster.

First, accept that scanning is finished

The habit that solves easy grids — looking for a cell where only one digit fits — is exhausted long before a hard puzzle is done. Continuing to sweep the grid a fifth time feels productive but is not. The moment two full passes yield nothing, the method has to change rather than the effort.

Fill in the candidates, properly

Every technique beyond scanning operates on candidates, so a grid without pencil marks hides all of them. Write the possible digits into every empty cell, or turn on notes if you are solving on a screen. This step feels tedious and is the single largest difference between people who finish hard puzzles and people who abandon them.

Look for pairs before anything clever

With candidates visible, scan for two cells in the same row, column or box holding exactly the same two candidates. Those two digits belong to those two cells in some order, so they can be struck from every other cell in that unit. This one pattern unlocks more hard grids than every advanced technique combined, and it is the cheapest thing to check.

Then work upwards, cheapest first

If pairs yield nothing, go to triples, then to the patterns where a digit is confined to one row inside a box, and only then to the rectangle-shaped patterns. Always spend the cheapest tool that can work: a hard puzzle rarely needs an exotic technique, and reaching for one first usually means missing something simpler.

When you truly cannot see it

Comparing your grid against a full explanation teaches more than a revealed digit, because it names what you missed. Enter the position into the solver and it walks the whole path, saying which technique decided each step and linking its lesson — so the next grid of that shape is one you solve yourself.

Put it into practice

Hard grids are where these steps pay off. Start one, or read the pattern library if you want the named techniques in order.

Play Hard Techniques Solver

Frequently asked questions

Why does a hard sudoku have fewer starting digits?

Fewer givens leave more candidates alive, so the easy deductions run out sooner. Difficulty comes from which techniques are needed, though, not from the count alone: a puzzle with twenty-four givens can be harder than one with twenty-two.

Do I have to write candidates in every cell?

Not always, but on hard grids the exceptions are rare. Partial notes hide exactly the pairs and triples you are looking for, which makes the search unreliable rather than faster.

Is it ever right to guess and backtrack?

It will reach an answer, and it teaches nothing. Every correct puzzle is fully deducible, so a guess is a way of skipping the pattern you would otherwise learn to recognise.

How long should a hard puzzle take?

Between ten and thirty minutes is normal once candidate notes are a habit. Before that it can take considerably longer, and that is a comment on practice rather than ability.