Can Every Sudoku Be Solved Without Guessing?

We put 10,000 random Sudoku through our app's hint engine: most need no guessing at all, about one in five needs a careful what-if chain, and a few go further.

Updated 2026-11-05

What counts as guessing

Guessing means writing in a digit you cannot justify and carrying on, hoping it was right. If it was wrong, you find out many moves later and have to undo everything since. A forcing chain is different: you suppose a digit, follow what must follow, and if that leads to a square with no possible digit, you have proved the supposition wrong. Nothing unproven stays in the grid. Some solvers count that as logic and others as guessing in disguise, so this page keeps the two apart.

What we measured

We made 10,000 random puzzles the way a simple program does: fill a grid at random, then remove clues one by one, in random order, for as long as the puzzle keeps exactly one answer. They start with 22 to 27 clues. Our app's hint engine then solved each one, always taking the easiest step it could find among its techniques. It is the same engine that gives hints in the app and in our solver.

The result

79.7% of the puzzles were solved with human techniques alone, from singles to almost locked sets; for 32.6% of all puzzles the hardest step was only a hidden single. 19.3% needed at least one forcing chain. The remaining 1.0% (102 puzzles) reached a point where none of the engine's techniques applied: solving them takes a technique beyond the ones it knows, or a longer chain of what ifs.

Why the puzzles you play are different

Puzzles in newspapers, books and apps are not random: they are chosen and graded. Every one of the 60,000 puzzles in our app's bank is solved without a forcing chain. The famous hardest puzzles are the opposite: they were built to need chains, and our page on them shows where our engine has to use one.

The world's hardest Sudoku →

If you are stuck anyway

When you have tried every technique you know, a careful what if is a fair next step. Pick a square with only two possible digits, write one of them in lightly, and follow what it forces. If that leads to a contradiction, the other digit is right. If it does not, you have proved nothing, so rub it out and try the other digit or another square.

Stuck on a Hard or Expert Sudoku? The Technique You Need Next →Sudoku solver →

Practice

Every Find the Mistake puzzle on this site has exactly one wrong digit, and you can play each board right on its page.

Play the puzzles →