How to solve a sudoku step by step (beginner's guide)

By Miguel Arechaga

If you've ever started a sudoku full of enthusiasm and abandoned it halfway through with the board covered in crossed-out numbers, this article is for you. Solving a sudoku has nothing to do with luck or being "good at math": it's pure method. Here, step by step, is the system experienced players use to complete any easy or medium sudoku without guessing a single cell.

Before you start: remember the three rules

A sudoku is a 9x9 grid divided into nine 3x3 blocks. The only rule is that every row, every column and every 3x3 block must contain the numbers 1 through 9 without repeating any of them. There's no adding up or calculating involved: just pure logic.

Step 1: find the most repeated number

When you open the board, don't start with the first empty cell you see. Instead, quickly count which number appears most often among the starting clues. If the 5 is already placed in six of the nine blocks, only three fives are left to place, and the options will be very limited. Starting with the numbers that are almost complete will get you your first solved cells within seconds.

Step 2: scan by rows and columns (the "laser" technique)

This is the most important sudoku technique, and the one that solves most cells on an easy puzzle. Here's how it works: pick a number, say 7, and look at a 3x3 block where it isn't placed yet. Now imagine "laser beams" shooting out from every 7 already on the board, along its rows and columns. Those lines forbid the 7 in the cells they cross. If the block you're looking at has only one free cell left that isn't crossed, that's where the 7 goes. Guaranteed.

Step 3: look for cells with a single candidate

When scanning stops producing results, switch approach: instead of following a number, follow a cell. Pick an empty cell that has many already-completed neighbors and ask yourself: which numbers could it hold? Check its row, column and block, and rule out anything already present. If only one number is left after ruling those out, that cell is solved. This is called the "single candidate" technique, and combined with the scanning from step 2, it's enough to complete any easy sudoku and most medium ones.

Step 4: check rows and columns that are nearly full

Whenever a row, column or block has seven or eight numbers placed, stop and look at it. With eight numbers, the missing cell solves itself (it's the only number not present). With seven, the two empty cells can only hold two values, and almost always one of them will be forbidden by that cell's column or block. This check is extremely quick and unlocks entire areas of the board.

Step 5: start the cycle again

Here's the secret beginners don't know: every number you place creates new clues. That 7 you just wrote now shoots its own "laser beams" and can solve cells that were impossible before. That's why the method is a cycle: scan numbers → look for single candidates → check nearly-full lines → and start again. If you get stuck, don't worry — repeat the cycle from step 1, because you've almost certainly missed something.

The golden rule: never guess

If you're torn between two numbers for a cell, don't fill in either. A guessed number that turns out wrong poisons everything you place afterward, and you'll end up with an impossible board with no way to tell where you went wrong. Well-built sudokus (like every one on MundoSudoku, which are guaranteed to have a unique solution) always have at least one cell solvable through pure logic. Your job is to find it, not to guess it.

What about when the difficulty goes up?

These techniques will get you through easy and medium levels. For hard and expert sudokus you'll need more powerful tools, like naked pairs or the X-Wing, which we cover in our advanced sudoku techniques article. But there's no rush: master the basic cycle first, because it's the foundation for everything else.