How to solve sudoku
The techniques behind Sudoku Sunrise’s five daily levels, simplest first, each with a real example.
Before you start
Fill each row, column and 3×3 box with the numbers 1 to 9, each exactly once. A good puzzle has one solution and never needs a guess: every step follows from what’s already on the board. The solver that grades our puzzles uses only the techniques on this page, always trying the simplest first, and a puzzle’s level is the hardest one it needs.
Most of the later techniques work with notes: the small numbers you pencil into a square for what it could still be. In the examples, notes are the small numbers; a ring marks the squares the step is about, a tint marks the row, column or box it works in, and a crossed-out note is one the step rules out. The words under each board are the same ones a hint gives when you play today’s puzzles. How the puzzles are made and graded is in how sudoku puzzles are made.
Gentle
A relaxed start, about five minutes. Every next number can be found by looking at one square at a time.
Last number left
Look at one empty square and the numbers already in its row, column and box. If eight different numbers are there, the ninth is the only one left for it.
Easy
About ten minutes. Now you look from the other side too: where can this number still go?
Only spot left
Pick a number and a row, column or box that still needs it. If every other empty square there is blocked by that number in a crossing row, column or box, the one spot left must hold it.
Medium
A steady think, about fifteen minutes. Notes start to matter, because these steps rule numbers out rather than placing them.
Pointing
When a number’s possible squares inside a box all lie in one row or column, the box will put that number on that line. So it can’t go anywhere else on that line, outside the box.
Claiming
The other way round: when a number’s possible squares in a row or column all lie inside one box, that line will put it in that box, so it can come out of the box’s other squares.
Pair
Two squares in the same row, column or box that can each only be the same two numbers will share those two between them. No other square there can hold either one.
Hidden pair
When two numbers can only go in the same two squares of a row, column or box, those squares must hold them, so any other notes in those two squares can go.
Hard
A real challenge, about twenty-five minutes. Patterns stretch across several squares, rows and columns.
Triple
Three squares in one row, column or box whose notes all come from the same three numbers (each may have two or three of them) will use up those three, so they come out of the other squares there.
Hidden triple
When three numbers can only go in the same three squares of a row, column or box, those squares hold them, and their other notes can go.
X-wing
Take one number. If, in two rows, it can only go in the same two columns, it sits at two opposite corners of that rectangle, and either way it fills both columns. So it can’t go anywhere else in those columns. It works with rows and columns swapped, too.
XY-wing
Three squares with two notes each: a pivot with X and Y, and two wings it can see, one with X and Z, the other with Y and Z. Whatever the pivot turns out to be, one wing ends up Z, so a square that sees both wings can’t be Z.
Expert
For seasoned solvers. Bigger patterns, and following one number through the whole grid.
Swordfish
An X-wing across three rows and three columns: if a number fits only in the same three columns in three rows, those rows fill all three columns with it, so it comes out of those columns everywhere else.
XYZ-wing
Like an XY-wing, but the pivot has all three numbers, X, Y and Z. One of the three squares must be Z, so a square that sees all three of them can’t be.
Coloring
Follow one number through squares where it has exactly two places in a row, column or box: if one isn’t the number, the other is. Color the links in two alternating colors. One color is right and the other wrong, so a square that sees both colors can’t hold the number, and a color that turns up twice in one row, column or box is the wrong one.
Master
The toughest we make: chains of squares, and trying a square both ways. Master puzzles are in the Sudoku Sunrise app.
XY-chain
A chain of squares with two notes each, every square sharing one number with the next. If both ends of the chain could be Z, then whichever way the chain falls, one end is Z, so a square that sees both ends can’t be Z.
Forcing
In a square with just two notes, try one of them and fill in the squares that then have only one choice. If that runs into a clash, such as the same number twice in a row, the other note must be the right one.
Practice
Five new puzzles arrive every day, Gentle to Expert, the same for everyone: play today’s puzzles, free in your browser. When you’re stuck, Hint shows the next of these steps on your own board. Sudoku Sunrise for iPhone and iPad adds hands-on lessons for each technique and Master puzzles.