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How to solve

Solving Sudoku, from first move to last

Every Sudokufy puzzle is solvable by reasoning alone — there is always a next logical step, and never a need to guess. This guide walks from the rules to the 21 techniques the app uses to grade puzzles and shape its hints.

The rules, and where to start

Fill the 9×9 grid so that every row, every column, and every 3×3 box contains the digits 1 through 9 exactly once. That single rule — no repeats in any row, column, or box — is the whole game.

Start where the grid is most constrained. Scan rows, columns, and boxes that are already nearly full: often a cell has only one digit it can legally take. Place those, and each placement narrows the cells around it. When the obvious moves run out, switch to pencil marks (see the Notes button below) to track the remaining candidates in each cell, then look for the patterns described next.

The 21 techniques

Sudokufy grades every puzzle with the same logical techniques a person would use, applied hardest-first, so a puzzle's rating reflects the toughest step it actually requires. Here they are, grouped from simplest to most advanced.

Singles — the foundation

Naked Single

A cell has just one candidate left. It must be that digit.

Hidden Single

A digit can go in only one cell within a row, column, or box — even if that cell has other candidates.

Intersections

Pointing (Pairs / Triples)

When a digit in a box is confined to one line, it can be removed from the rest of that line.

Box–Line Reduction

When a digit in a line is confined to one box, it can be removed from the rest of that box.

Subsets

Naked Pair

Two cells in a unit share the same two candidates; those digits leave every other cell in the unit.

Naked Triple

Three cells collectively hold the same three candidates, clearing them elsewhere in the unit.

Naked Quad

The same idea with four cells and four candidates.

Hidden Pair

Two digits appear in only the same two cells of a unit; every other candidate in those cells can go.

Hidden Triple

Three digits restricted to the same three cells, letting you clear their other candidates.

Hidden Quad

Four digits restricted to the same four cells.

Fish

X-Wing

A digit forms a rectangle across two rows and two columns, removing it from those columns (or rows).

Swordfish

The X-Wing pattern extended to three rows and three columns.

Jellyfish

The same fish pattern across four rows and four columns.

Single-digit patterns

Skyscraper

Two strong links on one digit share a line, eliminating that digit where the far ends see each other.

2-String Kite

A row link and a column link on the same digit meet in a box, clearing the cell their ends both see.

Empty Rectangle

A digit's candidates in a box form an L, combining with a line link to make an elimination.

Wings

XY-Wing

Three linked bi-value cells force a digit out of any cell the two "wings" both see.

XYZ-Wing

An XY-Wing where the pivot shares the eliminated digit too, tightening the target.

W-Wing

Two matching bi-value cells joined by a strong link remove their shared digit from cells both see.

Colouring & uniqueness

Simple Colouring

Chain a digit's strong links in two colours; contradictions in a colour clear those candidates.

Unique Rectangle

Avoids a pattern that would give two solutions — proof a candidate must be removed, since the puzzle has exactly one answer.

The difficulty ladder

Six tiers, each calibrated to the techniques it genuinely demands — a Grandmaster puzzle isn't just an Expert with more blanks, it actually requires the harder patterns above.

The buttons

Options

Open Options from the app menu to tune the game to you.