How to Play Sudoku: A Beginner's Guide
Sudoku is a 9×9 grid split into nine 3×3 boxes. Fill the empty squares so that every row, every column and every 3×3 box contains the numbers 1 to 9 exactly once. There's no maths involved and no guessing needed: each number can be worked out from the ones already there, starting with the rows, columns and boxes that are most nearly full.
The rules of Sudoku
That one rule is all there is. A few words you'll see in every guide:
| Word | Meaning |
|---|---|
| Cell | One square of the grid. There are 81. |
| Row / column | A horizontal line / vertical line of 9 cells. |
| Box | One of the nine 3×3 blocks, marked with thicker lines. |
| Givens | The numbers printed at the start. You can't change them. |
| Candidates (notes) | Small pencil marks for the numbers a cell could still be. |
A proper Sudoku has exactly one solution, which is why you never need to guess. The numbers are just symbols: you could play with nine letters or colours and nothing would change.
Technique 1: scan a box for one number
This is the technique that solves most easy puzzles, often called scanning or cross-hatching. Pick a number and a box that doesn't have it yet. Then look along the rows and columns that pass through the box: wherever that number already appears, it rules out the whole line. If only one empty cell in the box is left, that's where the number goes.
Take the top-middle box and the number 7. There's a 7 in row 2 (on the far left), so the 7 can't go in that row of the box. There's a 7 in the middle column further down, and another at the bottom of the box's left column. That rules out every empty cell except one.
Work through the numbers in order, 1 to 9, scanning each box. Then do it again: every number you place can unlock another one.
Technique 2: the last number left in a cell
Sometimes it's easier to start from a cell instead of a number. Look at everything in its row, its column and its box. If eight different numbers are already there, the ninth is the only one that fits. This is called a naked single.
The centre cell of the puzzle is a good example. Its row has 1, 2, 3, 4, 8 and 9. Its column adds 6 and 7. Only 5 is missing, so the centre is 5.
Technique 3: what a row is missing
When a row, column or box has only a few gaps, list what's missing and check each gap. In the top row of the example, the missing numbers are 1, 7, 8 and 9. Pencil in what each empty cell could be and you'll find that 1 fits in only one of them, so it goes there even though that cell has other options too. This is a hidden single.
A step-by-step method for beginners
- Start where it's busiest. Find the row, column or box with the most givens. Fewer gaps means fewer options.
- Scan numbers 1 to 9 across all nine boxes (technique 1). Place every number you're sure of.
- Check nearly full lines. For any row, column or box with two or three gaps, list what's missing (technique 3).
- Check single cells in crowded areas (technique 2).
- Repeat. Each number you place gives new information, so go round again.
- When nothing is left, pencil in candidates and look for pairs. That's where what to do when you're stuck picks up.
How to get better at Sudoku
- Never guess. If you can't prove a number, leave the cell empty. A guess that turns out wrong can ruin the rest of the grid without you noticing.
- Use notes on harder puzzles, but only once the easy numbers are in. Pencilling every cell too early is slow and hides the obvious moves.
- Keep notes tidy. Each time you place a number, remove it from the notes in its row, column and box.
- Play a little every day. A daily puzzle is a good habit: GameLake's Daily Sudoku is the same for everyone, with a streak that builds each day you solve it.
- Move up a level when Easy feels quick. On GameLake, Easy can be solved with singles, Medium needs pairs, and Hard needs box-line techniques, so each level teaches the next idea.
- Use hints to learn, not to finish. GameLake's hints explain the next logical step instead of just filling in a number.
Frequently asked questions
What are the basic rules of Sudoku?
Fill the 9×9 grid so every row, every column and every 3×3 box contains the numbers 1 to 9 exactly once.
Do you need to be good at maths for Sudoku?
No. Sudoku uses numbers only as symbols. There is no adding up; it's pure logic.
Where should a beginner start in Sudoku?
In the most crowded row, column or box. Scan each number 1 to 9 across the boxes and place any that can only go in one cell.
How many numbers do you need for a Sudoku to have one solution?
At least 17 givens. A computer search completed in 2012 proved that no standard Sudoku with 16 or fewer givens has a unique solution. Most puzzles you'll meet have quite a few more.
Is it OK to guess in Sudoku?
Try not to. A well-made Sudoku can always be solved by logic, and a wrong guess can spoil the grid. On GameLake a wrong number also costs one of your three lives.