Sudoku Techniques Guide

The rules in 30 seconds

Sudoku has exactly one rule: no digit may appear twice in any row, any column, or any box outlined in bold. On a 9×9 board, the digits 1 to 9 each appear exactly once per row, column, and box. The digits filled in from the start (the givens) are fixed.

Smaller boards play by the same rule. A 6×6 fills 1 to 6 into 2×3 boxes, and a 4×4 fills 1 to 4 into 2×2 boxes. With fewer cells to scan, the small boards are great for a child's first sudoku or for practicing techniques.

Starting habits: scanning and notes

Start with the most-filled rows, columns, and boxes. In a row with eight cells filled, the empty cell is simply decided, and a unit with seven filled leaves only two candidates. Every cell you fill narrows the neighboring units in turn, so the crowded spots are the fastest way in.

When you start getting stuck, turn on Notes (the pencil) and jot down which digits could go in each empty cell. Notes in this app aren't filled in automatically — you toggle them yourself, and working out the candidates on your own is exactly what builds skill.

Learn to scan the other way around too. Pick one digit — say, 5 — find every 5 on the board, and check where a 5 can still go in each box. The hidden singles introduced below fall straight out of this scan.

Naked Singles — Starter

Look at an empty cell and cross off every digit already present in its row, column, and box. If exactly one digit survives, that cell is settled. You can judge it by looking at a single cell alone, which makes this the most fundamental technique of all.

Starter puzzles are built to be solvable all the way through with nothing but naked singles. Treat them as practice in sensing which cell to look at next, and build up speed on the small 4×4 and 6×6 boards.

Hidden Singles — Easy

Now look from the digit's point of view instead of the cell's. If a digit has only one possible cell within a box (or a row or column), that cell takes the digit — no matter how many other candidates the cell holds. It hides among the candidates, hence the name hidden single.

Make a habit of asking each box "where can the 3 go here?" and hidden singles start jumping out at you. Easy puzzles are graded to be finishable with just two techniques: naked singles and hidden singles.

Locked Candidates — Medium

If all of a digit's candidate cells within one box fall on the same row (or column), the digit must land in that row of that box — so you can erase it as a candidate from that row's cells outside the box. The reverse direction, narrowing a box from a row or column, holds just the same.

This technique fills no cells directly — it erases notes. The chain where erased notes give rise to fresh naked and hidden singles is the heart of Medium difficulty, and this is where a habit of tidy note-keeping starts to pay off.

Pairs and Triples — Hard

Naked Pair: if two cells in a unit hold exactly the same two candidates, those two digits must be resolved in those two cells — so you can erase both digits as candidates from the unit's other cells.

Hidden Pair: if two digits can only go in the same two cells of a unit, those cells belong to those digits — erase every other candidate written in them. Extend the idea to three cells and three digits and you have a Naked Triple.

Hard puzzles are graded to be solvable by adding pairs and triples on top of singles and locked candidates. These techniques are invisible without notes, so from this level on, notes are no longer optional.

Expert — beyond the ladder

Expert boards can't be finished with the techniques above alone. The single-solution guarantee still holds, though, so you can always reach the answer — with higher-level techniques, or with careful reasoning such as assuming one of the two remaining candidates in a cell and hunting for a contradiction.

When you're stuck, try pressing Hint. Look at the cell it fills and work backwards — "why that cell?" — it's the fastest route to picking up the higher techniques.

Practicing with this app

This app's difficulty levels are a curriculum. During generation, a real grader checks whether the puzzle can be solved completely using only the techniques up to each level — so climbing from Starter upward teaches you the techniques above precisely one at a time.

A hint doesn't copy from the answer sheet — it fills the next cell that is logically forced on the current board. Show conflicts is a pure rule check that only flags duplicates in a row, column, or box, so leaving it on won't spoil the fun of solving. Using either assist is shown alongside your ranking record.

Playing with kids? Learn the rules on the 4×4, which uses only the digits 1 to 4, then move up through 6×6 to the classic 9×9. With A4 printing, the small boards fit several puzzles on one sheet — great as worksheets.