Hidden Single sudoku technique

A hidden single turns the naked single around. Instead of asking "what fits in this cell?", you ask "where can this digit go in this row, column or box?". When the answer is a single cell, the digit belongs there, no matter how many other candidates that cell seems to have. The single is "hidden" because the cell itself still looks undecided until you check the unit.

How to find a hidden single

1. Choose a unit — one row, one column, or one box — and a digit missing from it.
2. Mark every cell in that unit where the digit could still legally go.
3. One possible cell means you have found it: place the digit there.
4. Digits that already appear six or seven times in the grid are the best hunting ground, since they have few homes left.

The pattern on the grid

Worked example

Ask where digit 3 can go in box b2. The 3s already placed elsewhere strike out every cell of the box except r2c4 — even though r2c4 itself still shows 34689 as pencil marks. The unit leaves the digit a single home, so r2c4=3.

When it fails

The classic slip is checking two of the three unit types and stopping. A digit that looks placeable in two cells of a box often loses one of them to a column you have not scanned yet.

Puzzles that need it

These grids come from rated games on this site — our solver confirmed that hidden single was part of the shortest solving path for each one.

Every string is the standard 81-character format (0 = empty) — paste it into any solver to verify the rating and the solving path yourself. Want to try one under the clock? Play a free puzzle — no account needed — or create an account to train specific difficulty tiers.

Related techniques

• Naked Single
• Hidden Pair
• Claiming

All solving techniques