Medium Sudoku

What makes a puzzle medium

Every puzzle on this page has been solved by our grader before you see it, using human techniques and always trying the simplest first. A puzzle counts as medium when the grader finishes it with singles alone, but needs at least one hidden single along the way. In other words, there will be at least one moment where no square has only one possible digit, and the way forward is to look from the digit instead.

Medium puzzles start with at least 30 digits, compared with at least 38 on easy. Each has exactly one solution, so you never need to guess.

The technique that unlocks medium

A hidden single turns the question around. Instead of asking "what can go in this square?", ask "where can this digit go in this row, column or box?"

An example. A box is missing a 4. One of the box's rows already has a 4 further along, so no square in that row of the box can be 4. A column through the box also has a 4 elsewhere, which blocks another square. If only one square in the box is left unblocked, that square must be the 4, even if 2, 6 or 9 could also fit there on their own.

The same works along a row or a column: if a row is missing a 7 and every empty square but one can see a 7 in its column or box, the last one is the 7.

Getting faster

• Sweep digit by digit. Take 1, then 2, and so on to 9. For each, look at every box that doesn't have it yet and see whether only one square is possible.
• Start with the most common digits. A digit that already appears six or seven times has the fewest places left, so its hidden singles are easiest to see.
• Follow your last move. After each placement, look at the row, column and box you just changed. New singles often appear right there.
• Keep notes light. Filling every candidate is slow at this level. Note only the squares with two or three options. Our pencil marks guide has a system for this.

When you get stuck

If nothing seems to fit, the most likely cause is a wrong digit placed a few moves ago. Show mistakes (on by default) marks it in red, and Undo takes you back as far as you need, up to 200 moves. A hint checks for wrong digits first; if there are none, it fills one square and tells you whether it was a naked or a hidden single, with a link to how that works. Our guide to common mistakes and getting unstuck has a full routine.

Ready for more?

When you can finish medium puzzles without hints, try hard sudoku. Hard puzzles need locked candidates or naked pairs, techniques that work on your notes rather than placing digits directly. There's also a daily medium puzzle, the same for everyone on the same date.