Sudoku Difficulty Levels Explained

What actually changes between Easy, Medium, Hard and Expert? We generated 1,600 puzzles with the same engine that builds every board on this site, graded each one by the solving techniques it needs, and compared the results with 1,790 real solves by our players.

By Sachin Jagtap, developer of Sudoku Leader · Published October 22, 2026

Every Sudoku site has a difficulty menu, and almost none of them tell you what the labels mean. A “Hard” puzzle on one site can be easier than a “Medium” on another. So instead of describing our levels in general terms, we measured them. The short version: on a 9×9 board, Easy, Medium and Hard mostly differ in how much work there is, while Expert is the first level where you regularly need more than simple logic.

How a level is set

A Sudoku starts life as a completely filled grid. The generator then removes numbers one at a time, and after every removal a solver checks that the puzzle still has exactly one answer. If a removal would allow a second answer, that number goes back. The level decides how many numbers the generator tries to remove: 40% of the 81 cells for Easy, 50% for Medium, 60% for Hard and 70% for Expert.

That leaves a fixed number of starting numbers (also called clues or givens) on the lower levels. Expert is the exception. Its target is 56 empty cells, and on some boards the generator reaches a point where any further removal would create a second solution, so it stops early.

400 freshly generated 9×9 boards per level, October 2026. “Singles” means naked and hidden singles, explained below.

The techniques behind the numbers

To grade a puzzle, our engine solves it the way a person would. It starts with the simplest technique and only moves to a harder one when the easier ones stop producing new numbers. The grade is the hardest technique it needed at any point.

• Naked single: a cell where only one digit is still possible, because every other digit already appears in its row, column or box.
• Hidden single: a digit that can only go in one cell of a row, column or box, even though that cell has other candidates too.
• Locked candidates: inside a box, a digit is limited to one row or column, so it can be ruled out of the rest of that row or column.
• Naked pairs and triples: two cells in a unit that can only hold the same two digits (or three cells sharing three), which removes those digits from the other cells in the unit.
• X-Wing: a digit confined to the same two columns in two different rows, which rules it out of those columns everywhere else.

Some boards need something beyond all of these. Our grader puts them in one bucket labelled “beyond X-Wing”. That bucket can mean a Swordfish, a chain or a forcing argument. The grader doesn’t try to tell them apart.

Easy and Medium: the same logic, different length

Every one of the 400 Easy boards could be finished with naked singles alone. Medium was nearly identical: 394 boards needed only naked singles and the other 6 needed a hidden single somewhere. Neither level ever required anything harder.

What changes is the amount of searching. Medium starts with 8 fewer numbers, so there are 8 more cells to fill and fewer obvious placements at each step. Our players’ times show it clearly. The median 9×9 Easy solve takes 2 minutes 30 seconds, and the median Medium takes 3 minutes 31 seconds.

Hard: where hidden singles take over

Hard boards start with 33 numbers. About 61% of them can still be done with naked singles alone, but 36% need at least one hidden single, and that is the step that slows people down: you have to look at a whole row, column or box and ask where a digit can go, instead of looking at one cell. Only 14 of the 400 Hard boards needed anything more advanced. The median time for our players on 9×9 Hard is 5 minutes 40 seconds.

Expert: the first real jump

Expert boards keep 25 to 29 numbers. Half of them can still be solved with singles, but the other half cannot. 37 of the 400 boards needed locked candidates, 27 needed naked pairs or triples, one needed an X-Wing, and 136 (34%) went beyond everything our grader knows. This is the level where pencil marks stop being optional. The median Expert solve on the site is 8 minutes 54 seconds, with a wide spread: the middle half of solves fall between 3:33 and 12:49.

Pro, Evil and Evil Pro: graded by technique, not by clue count

Because clue count alone guarantees so little, our three hardest tiers are chosen differently. The generator builds a board, grades it, and throws it away unless it is hard enough.

• Pro (9×9 on the home page) uses Expert’s clue count but keeps a board only if it needs naked pairs or triples or something harder. 164 of 400 Expert-depth boards (41%) pass that test.
• Evil removes more numbers than Expert. Across 400 Evil boards the starting count ranged from 21 to 28, with a median of 24.
• Evil Pro keeps only the Evil boards that our grader cannot finish at all with the techniques listed above: 36% of them.

Other grid sizes

Small grids behave differently. A 4×4 board cannot be made hard: when we carved 120 of them all the way down to 4 starting numbers, every single one could still be solved with naked singles. With only four digits per row, the constraints run out almost at once. On 6×6, even carved down to as few as 9 starting numbers, only 4 of 120 boards needed anything beyond basic logic. That is why our 4×4 and 6×6 levels are about pace and accuracy rather than technique. They’re a good place to learn the rules.

Larger grids are hard mostly because of their size. A 12×12 has 144 cells and a 25×25 has 625, so even simple logic takes a long time to apply across the whole board.

How long our players take

The times below come from every solve saved on Sudoku Leader between March and October 2026. They are raw solving times, before any hint or mistake penalties. Some levels are played by only a few regulars, so we show how many solves and how many different players each figure is based on. Treat the smaller samples as a rough guide.

Medians from saved solves on sudokuleader.com, March 22 to October 22, 2026. Only puzzles with at least 13 saved solves are listed.

Two more things stood out in the data. 97% of saved solves used no hints at all, and 59% were finished without a single mistake. The Daily Challenge, which starts with 37 numbers, lands between Medium and Hard in both clue count and solving time.

Which level should you pick?

• New to Sudoku: start on 4×4 or 6×6 to learn the rules, then 9×9 Easy. Everything there can be solved by finding cells with only one possible digit.
• Comfortable with the basics: Medium and Hard teach you to scan whole rows and boxes for hidden singles. If Medium feels slow rather than difficult, move up.
• Ready for pencil marks: Expert, then Pro on the home page. Expect about half of Expert boards to need a real technique.
• Looking for the hardest boards here: Evil Pro, or a long session with 16×16 or Samurai Sudoku.

If you want to know how the puzzles themselves are built and checked, we wrote that up too: how Sudoku Leader makes its puzzles.