Sudoku difficulty is not about how many digits you start with. It's about which techniques a puzzle forces you to use — which is why a grid with 30 givens can be harder than one with 24.
The intuitive assumption is that fewer givens means a harder puzzle. It's roughly true at the extremes and unreliable everywhere in between. What actually determines difficulty is the hardest single deduction the puzzle requires. A grid can hand you 32 digits and still contain one step that needs a locked-candidate elimination — and that one step will stop most players for longer than a sparse grid that yields to scanning the whole way down.
Placement matters more than count. Clues clustered into a few boxes leave large regions under-constrained; clues spread evenly give you footholds everywhere. Two puzzles with identical clue counts can sit two difficulty bands apart.
The minimum number of givens for a Sudoku with a unique solution is 17. That was proved by exhaustive computer search in 2012 — no 16-clue puzzle with a single solution exists. Most published puzzles carry between 22 and 30 clues, well above the floor.
Every puzzle on this site is generated, then solved by a program that only knows human techniques, applied in order of difficulty. The rating is whatever the hardest technique it needed turns out to be. Puzzles are also checked for a unique solution before being served, so guessing is never required at any level.
Solvable end to end with crosshatching and naked singles. You should never need to write a pencil mark. Expect a steady rhythm where each placement immediately suggests the next, and a finish in five to ten minutes once you're comfortable. This is the level to use when you want the pleasant part of Sudoku without the thinking-hard part.
Introduces hidden singles as a requirement. At least once, you'll hit a point where no cell has an obvious answer and you have to ask where a specific digit can go within a row, column or box. Pencil marks become useful, though a confident player can often hold it in their head. Ten to twenty minutes is normal.
Requires candidate-based elimination: naked and hidden pairs, naked triples, and usually at least one pointing pair or box/line reduction. Pencil marks are no longer optional — the deductions are invisible without them. Expect to stall genuinely, several times, and to find the way through by systematic search rather than by noticing. Twenty to forty-five minutes.
Everything above, plus at least one step where the simpler methods are all exhausted and you need something structural — often an X-Wing, sometimes a Swordfish or a chain. Expert grids reward careful bookkeeping over speed; a single stale pencil mark will send you down a false path that survives twenty moves before collapsing. Some take over an hour, and abandoning one halfway is a normal outcome.
The Daily Challenge sits at a fixed moderate difficulty — roughly Medium, occasionally tipping into easy Hard. It's the same board for every visitor and resets at midnight. Because everyone is solving the same grid, it's the fairest way to compare a time with someone else, and the streak counter tracks consecutive days completed.
Practice mode is unlimited at any of the four levels, with no account required.