How to Solve Sudoku for Beginners: The 2 Basic Techniques and How to Write Candidate Notes

Sudoku rules and the names for parts of the grid

Sudoku (also called Number Place) is a puzzle in which you fill the 81 cells of a 9×9 grid with the digits 1–9. There is only one rule: every row, every column and every 3×3 box outlined in bold contains each of 1–9 exactly once. No arithmetic is needed. Using the digits already given (the clues), you fill the empty cells by logic.

A properly made puzzle has exactly one solution. That means you never need to guess: every cell can be filled for a reason. If you get stuck, either you have missed something or the position needs a technique you don't know yet.

Technique 1: find the only place a digit can go in a box

The first technique to learn is to focus on one box (or row or column) and look for a digit that has only one cell left where it can go. This is called a Hidden Single, or solving by elimination. Picture drawing lines along the rows and columns that already contain that digit and crossing off the empty cells of the box. Let's try it on this puzzle (. is an empty cell).

1. 1

   Look for a 3 in the top-right box

   The 3 at R2C5 means row 2 of the box can't hold a 3. The 3 at R5C7 rules out column 7 and the 3 at R9C9 rules out column 9. The only empty cell left is R1C8, so R1C8 is 3.

2. 2

   Next, look for a 3 in the top-left box

   The 3 you just placed at R1C8 rules out row 1, the 3 at R2C5 rules out row 2, and the 3 at R6C1 rules out column 1. R3C3 is already 5, so the remaining cell R3C2 is 3.

3. 3

   After placing a digit, look for the same digit in other boxes

   Each digit you place becomes a new "line" that cuts down candidates in other boxes. Once you have placed two 3s, look for the rest of the 3s straight away and they often fall in a chain.

In this puzzle, seven cells can be solved by Hidden Singles right from the start. When we solved Hard and Advanced puzzles made with Sudoku Next-Move Hints all the way to the end, about 90% of the steps were Hidden Singles. Even for hard puzzles, searching thoroughly for these first is the fastest route.

Technique 2: count the digits an empty cell can take

The second technique is to focus on one cell and count the digits already present in its row, column and box. If 8 different digits are there, the cell must be the remaining one. This is called a Naked Single, or a cell with only one candidate.

• They are easiest to spot where a well-filled row, column and box intersect
• Check the three directions (row, column, box) for each digit 1–9 in order, "there" or "not there", so you don't miss any
• Switch to this method when you can no longer find Hidden Singles

An easy way to tell them apart: a Hidden Single goes "from a digit to a place", a Naked Single goes "from a place to a digit". Easy puzzles can be solved with Hidden Singles alone, and normal puzzles can be finished by alternating between the two.

Where to start: the order to look for digits

• Start with digits that appear most often on the grid: if a digit appears 7 times, you only need to find the remaining 2 places
• Look at boxes that are mostly filled: the fewer empty cells, the easier they are to settle
• After placing a digit, immediately re-check that digit, that row and that column: the next placement is usually nearby
• Do a full lap of 1–9 across every box: the last resort to avoid missing anything

Start candidate notes with "two places only"

When the two basic techniques stop working, write candidates small in the corners of cells. But writing every candidate in every cell from the start takes time, and forgetting to erase them leads to mistakes. A step-by-step approach works better.

Intermediate techniques that use notes are explained in how to use Naked Pairs, Naked Triples and Pointing, and advanced techniques used after writing all candidates in how to find an X-Wing. Every time you place a digit, remember to erase it from the notes in the same row, column and box.

Practise with Sudoku Next-Move Hints

1. 1

   Under "New puzzle", choose Easy and press Create puzzle

   You get a puzzle that can be solved with Hidden Singles alone. Once you are comfortable, move up to Normal.

2. 2

   When you get stuck, press Next-move hint

   Only the box, row or column you should look at first lights up. The answer isn't shown, so it trains you to find it yourself.

3. 3

   If you still can't see it, press More detail (see the technique)

   The name of the technique and why the cell must be that digit are shown. Press Place this move on the board to apply it.

4. 4

   Finally, press Check answers

   Cells that differ from the solution turn red. If you find a mistake, see also what to do when you get stuck.

FAQ

Are Sudoku and Number Place the same thing?

Yes, they are the same puzzle. It first appeared in an American magazine as "Number Place", and in Japan the puzzle publisher Nikoli named it "Sudoku" and made it popular. Because "Sudoku" is a Nikoli trademark in Japan, other Japanese publishers often call it "Nanpure" (short for Number Place).

Where should I start solving a sudoku?

Pick a digit that appears many times on the grid, then look in the boxes that don't have it yet for one where it can go in only one place. Starting from mostly filled boxes makes these easier to find.

Should I write candidates in every cell?

You don't need to write them all from the start. Once the basic techniques stop working, write only digits that can go in two places or cells with 2–3 candidates; this saves time and reduces forgotten erasures. Writing every candidate is only needed for advanced puzzles.

Is it OK to guess in sudoku?

A properly made puzzle has one solution, so in principle you can fill every cell by reasoning without guessing. If you guess, you may not notice the mistake until many moves later, and undoing it is a lot of work. If you truly can't progress, the article on what to do when you get stuck explains how to make a trial placement.