Sudoku basics

The rules of sudoku in plain English, how the grid is laid out, and a simple way to make your first moves on any puzzle.

What it is

Sudoku is a number puzzle played on a square grid of 81 cells. The grid is split into 9 rows, 9 columns and 9 boxes. Each box is a small square of 3 by 3 cells, marked off by thicker lines.

Some cells already have a digit in them when you start. These are called the givens, and you cannot change them. Your job is to fill every empty cell so that:

• each row contains the digits 1 to 9 exactly once,
• each column contains the digits 1 to 9 exactly once,
• each box contains the digits 1 to 9 exactly once.

That is the whole rulebook. There is no maths involved. The digits could just as easily be letters or colours. All you are doing is making sure nothing repeats.

Every puzzle on sudoku.ie has exactly one correct answer, and you can always reach it by reasoning. You never need to guess.

How we describe the grid

To keep our guides clear, we number everything from 1 to 9:

• Rows are numbered from top to bottom, so row 1 is the top row.
• Columns are numbered from left to right, so column 1 is on the far left.
• Boxes are numbered left to right, then top to bottom, like reading a page. Box 1 is top left, box 3 is top right, box 5 is the centre and box 9 is bottom right.

When we say “row 4, column 7”, we mean the single cell where row 4 and column 7 cross. That cell sits in box 6.

How to spot your first move

A good way to start any puzzle is to pick the digit that appears most often among the givens. If there are already six 8s on the grid, there are only three 8s left to find, and the existing ones give you lots of information.

Take that digit and visit each box that does not have it yet. For each one, ask: which cells in this box could the digit go in? Rule out any cell whose row or column already has that digit, and rule out any cell that is already filled. If only one cell is left, the digit must go there.

A worked example

Say you are looking at box 1 (rows 1 to 3, columns 1 to 3) and you want to place a 9.

• There is a 9 in row 1, out in column 5. So no other cell in row 1 can be a 9.
• There is a 9 in row 3, out in column 8. So no other cell in row 3 can be a 9.

That leaves row 2 as the only row in box 1 where the 9 can go. Now look at the three cells of row 2 inside box 1. Row 2, column 1 already holds a 6. Column 2 has a 9 lower down, in row 7. So the 9 cannot go in column 2 either.

Only row 2, column 3 is left. That is where the 9 goes.

You have just used the most important skill in sudoku. Every harder technique builds on this same idea: rule things out until only one choice remains.

A common mistake

The most common slip for new players is forgetting about the box. It is easy to check a row and a column and feel sure a digit fits, only to find the same digit already sitting in the box. Before you write anything in, check all three: the row, the column and the box.

The second common mistake is guessing. A guess that happens to be right teaches you nothing, and a wrong one can spoil the whole grid without you noticing until much later. If you are unsure, leave the cell empty and look elsewhere.

When you will need it

Always. Every puzzle on sudoku.ie, from Easy on Monday to Expert on Saturday, uses these rules. Once you are comfortable, read our guides to scanning and pencil marks, which turn the ideas above into a steady method.