How to play Sudoku

Sudoku is a number puzzle you solve with logic, not maths. This guide covers the three rules, the two moves that crack most easy grids, pencil marks, and your first real technique. Every example board is a real move from a puzzle you can play here.

🔢 Play Easy Sudoku Today’s Sudoku

The rules of Sudoku

A Sudoku is a 9 by 9 grid, split into nine rows, nine columns and nine 3 by 3 boxes. Some squares start with a digit in them. These are the clues, and they never change. Your job is to fill every empty square so that:

every row contains the digits 1 to 9 exactly once,
every column contains the digits 1 to 9 exactly once, and
every 3 by 3 box contains the digits 1 to 9 exactly once.

That is the whole game. There is no arithmetic: the digits are just nine different symbols. A well-made puzzle has exactly one solution, and you can reach it by reasoning alone.

A finished grid. The shaded row, column and box each hold 1 to 9 exactly once, and so does every other row, column and box. Clues are in dark ink; the digits the solver filled in are blue.

Reading the grid

Rows are numbered 1 to 9 from the top, columns 1 to 9 from the left, and the boxes are named by position: top-left, top-centre, top-right, and so on down to bottom-right. So “row 2, column 2” is the square second from the top and second from the left. This guide and the in-game hints both use these names.

Move 1: the only square left

Pick a digit and a box. Every copy of that digit elsewhere on the board rules out its whole row and its whole column, including the squares where they cross the box. If only one empty square in the box survives, the digit has to go there. Solvers call this a hidden single, or cross-hatching. Our hints call it only square left.

Here is one from a real puzzle. Look for the 2 in the top-left box. The box has 4 empty squares:

row 1, column 2 is out: row 1 already has a 2 (in column 9).
row 2, column 1 is out: column 1 already has a 2 (in row 9).
row 3, column 2 is out: row 3 already has a 2 (in column 6).
That leaves row 2, column 2 as the only place for the 2.

The ringed 2s each rule out a square of the shaded box (crossed). Only row 2, column 2 is left, so the 2 goes there.

Do this for every digit and every box and you will place a surprising number of squares. On easy grids this move alone often gets you halfway.

Move 2: the last digit standing

Move 1 starts from a digit. This one starts from a square. List the digits already in its row, its column and its box. If eight of the nine are there, the missing one is the answer. Solvers call this a naked single; our hints call it last digit standing.

Take row 2, column 2 in this puzzle:

its row already has 1, 3, 4, 5 and 9,
its column has 1, 5, 7, 8 and 9, and
its box has 1, 2, 3, 4, 5 and 7.

Between them that is 1, 2, 3, 4, 5, 7, 8 and 9. The only digit missing is 6, so row 2, column 2 must be a 6.

The shaded row, column and box of row 2, column 2 hold every digit but 6.

This move is quickest on rows, columns or boxes that are nearly full. A row with only one or two gaps is always worth a look.

Pencil marks

Sooner or later neither move gives you anything. That is the moment for pencil marks, also called notes or candidates: small digits in the corner of a square listing every digit that could still go there. In our Sudoku, tap Notes or hold a number to pencil it in.

Two habits make pencil marks work. Fill them in for a whole box or row at a time, not one square here and there. And whenever you place a digit, clear it from the notes of every square in the same row, column and box. Stale notes are the most common reason a grid stops making sense.

Your first technique: locked candidates

With pencil marks in, a new pattern appears. If every square in a box that could hold a digit sits in one row or column, then that digit must be in that row or column inside the box. So no other square in that row or column, outside the box, can hold it.

In this medium puzzle, every square in the centre box that can still take an 8 sits in column 5 (rows 4 and 6). So the 8 for that box is somewhere in column 5, and the rest of column 5 can’t have one: cross the 8 off rows 1, 2 and 3 of column 5.

Blue: where the 8 can still go in the shaded box. Red: the 8s this rules out further along column 5.

Locked candidates never place a digit on their own, but the squares they clear often turn into a Move 1 or Move 2 straight away. It is the first thing Medium Sudoku asks of you.

A routine for any grid

Scan each digit from 1 to 9 through each box (Move 1). Start with the digit that appears most.
Check the fullest rows, columns and boxes for a square with one digit missing (Move 2).
Repeat both until they dry up. Every digit you place can unlock another.
Pencil in the candidates for a box, row or column.
Look for locked candidates, then go back to step 1.
Still stuck? In our Sudoku, Hint names the rule that proves the next move, then shows the square, so you learn the pattern rather than just the answer.

Beginner mistakes

Guessing.A wrong guess can sit unnoticed for twenty moves. If you are about to guess, there is a move you haven’t seen yet.
Forgetting the box. A digit can be legal for its row and column and still clash with its box. Check all three every time.
Writing in a digit you only think is right. Place a digit when you can say why no other digit fits, not when it looks likely.
Stale pencil marks. Clear a placed digit from its row, column and box notes straight away.

Where to go next

The fastest way to get better is to play a grid just slightly harder than the last one you finished. Our levels climb one technique at a time, and Sudoku strategies explains each technique the harder levels need.

Sudoku questions

Do you need to be good at maths to play Sudoku?

No. Sudoku uses the digits 1 to 9 only as symbols. You never add or multiply anything, and the puzzle would work the same with nine letters or nine colours. It is a logic puzzle, not a maths one.

Can a Sudoku have more than one solution?

A proper Sudoku has exactly one. Every puzzle on free2mobi is checked to have a single solution before it ships, which is what lets you prove each square instead of guessing.

Do you ever have to guess in Sudoku?

Not in a well-made puzzle. Every free2mobi grid is solved by a logical solver, with no guessing, before it is published. If you feel you have to guess, there is a move you haven't spotted yet: pencil marks usually reveal it.

What is the best way to start a Sudoku?

Pick the digit that already appears most often and scan it through each box. Every copy on the board rules out a row and a column, so the most common digit is the one most likely to leave a box with only one square free.

🔢 Try it on an easy grid