Expert sudoku

21 clues — what actually changes

Expert is the hardest level on the site: 21 clues, roughly a quarter of the grid. It is not that more techniques are involved than on hard — it is that fewer of them are available at each step. With 25 clues there is usually more than one way forward. With 21 there is often exactly one, and you have to find it.

The second effect is purely mechanical: with a grid this empty, every cell starts with many candidates, so the first few minutes are not solving at all. They are building a picture of the board.

What solving an expert board looks like

1. Write full pencil marks before you enter anything. On this level that is not overkill, it is the entry fee. Without them you will revisit the same cells ten times.
2. Find every pair first. Cells with two candidates are the skeleton of the board — even when nothing can be written, each pair eliminates candidates from its neighbours.
3. Work by units, not by squares. Take one row or box and squeeze everything out of it instead of hopping around the grid.
4. Record progress, not guesses. The board has exactly one solution and is reachable by logic — if guessing feels tempting, there is an error or a missed pair behind you.

A realistic time for an experienced solver is 8–20 minutes. Under five minutes belongs to people who do this daily and play from the keyboard.

Expert, extreme, evil — the same thing or not?

Names for the hardest levels are completely inconsistent across the web: one site calls it expert, another extreme, a third evil or diabolical, and a fourth adds master and nightmare on top. There is no shared standard behind any of it — identical boards can carry three different labels.

Here, expert means one specific thing: 21 clues and a board solvable by logic, without guessing. If you came looking for extreme sudoku, evil sudoku or simply the hardest puzzle that can still be solved honestly, this is the level. One thing you will not find: boards with multiple solutions where you have to gamble. The generator rejects those by design.

Time, keyboard and three mistakes

• Play from the keyboard. Arrows or WASD, digits 1–9, Del, pencil on space, undo on Z. On expert, the gap between mouse and keyboard is minutes per board.
• Conflict highlighting is off by default, because at this level it does a meaningful share of the work for you. H turns it on if you want it.
• Three mistakes end the board. With 21 clues, one digit written on instinct can cost ten minutes of work, so genuinely put doubts in pencil.
• Pause hides the grid — a break is not free analysis time.

If 21 clues are too much for today, hard sudoku gives you 25 and the same toolbox in a gentler dose.