If you’re solving Sudoku on paper, consider making notes with a pencil so you can easily erase them later. Alternatively, you could use a pen with very light pressure. Another option is to jot down small numbers in the corners of the cells to keep them unnoticeable for future reference.

When you are solving Sudoku on our website, you can utilize a special note feature by activating the ‘Pencil’ mode. It is necessary to accurately fill out candidate numbers because any mistakes can increase puzzle difficulty and consume more time. By writing down possible values in the empty cells using figures that are not there yet, you get a better view of the solution.

Image 1 is a Sudoku puzzle you are going to solve. Now focus on the 3×3 box highlighted with the purple frame as shown in image 2. Turn the ‘Pencil’ mode ON. Now write candidate or possible numbers in all the empty cells.

Once finished you can see that the numbers 5 and 7 are the possible numbers for the red frame. Since 7 appears once in the entire 3×3 box, it is the clear winner for the red frame. Also, numbers 5 and 6 are the possible numbers for the green frame and 6 is the clear winner since it appears once in the 3×3 box.

Candidate numbers help you identify where each digit should go besides allowing you to apply advanced Sudoku solving methods. These techniques largely depend on information that comes from candidate numbers thus simplifying one’s way through the puzzle.

For more strategies and lessons about solving Sudoku see the Tips section on our website. Also, download our free printable Sudoku puzzle books.

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In example 1 below there is a Sudoku puzzle you are going to solve. Now as shown in image 2 identify a 3×3 square where only one number is missing. As we know every 3×3 square must contain numbers 1 through 9 only once. Hence the missing number is ‘2’.

Now as shown in image 3 identify a row where only one number is missing. As we know every row must contain numbers 1 through 9 only once. Hence the missing number is ‘2’. As shown in image 4 identify a column where only one number is missing. As we know every column must contain numbers 1 through 9 only once. Hence the missing number is ‘2’.

Thus, if there is only one blank cell within any of these nine squares or columns or rows, it’s easy to determine which digit (from 1 through 9) is missing and place it into this gap. Look at the ‘Last free cell’ technique as your initiation into the world of Sudoku. This basic strategy once mastered opens doors to advanced techniques that will make you solve even the most difficult puzzles. It’s like learning the alphabet before writing essays; on achieving this phase one becomes more skillful in dealing with Sudokus.

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Let’s consider an example. Image 1 is the puzzle you are going to solve.

As shown in image 2 focus on the area with white background. Apply the three Sudoku rules here. The white area highlighted with purple frame cannot contain number 1, since number 1 appears only once in the horizontal row and the vertical column. Hence, the only possibility for the question marked cell is 1.

Number 1 is the final answer as shown in image 3.

This way we can ignore cells where number 1 cannot go until we find ourselves with only one choice for placing this digit which is done by systematic exclusion. In other words, you solve the block by identifying ‘Last Remaining Cell’ containing digit 1 among others; this strategy may also be adopted across all blocks and digits existing within the framework.

All but one possibility is eliminated when using this approach to identify any digit’s location in each cell. Most aspiring Sudoku solvers see this particular technique as part of their essential toolbox due to its simplicity and effectiveness.

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For example, as shown in image 1, we need to find the missing number in the question-marked cell. Focus on the area with a white background and check out all the numbers presented in the 3×3 block as well as the shared row and column. Follow the below steps:

1. Write numbers 1, 2, 3, 4, 5, 6, 7, 8, 9

2. Now cross all the numbers already present in the 3×3 box: 1, 2, 3, 4, 5, 6, 7, 8, 9

3. Again cross all the numbers already present in the row: 1, 2, 3, 4, 5, 6, 7, 8, 9

4. Again cross all the numbers already present in the column: 1, 2, 3, 4, 5, 6, 7, 8, 9

What you are likely to discover is that only one number remains which is the number 5. Since each number can only appear once per block, row, and column, then it means that 5 can be the only possible figure for this cell. By using this method of elimination we can confidently put 5 into our question-marked cell as shown in image 2.

The ‘Last Possible Number’ technique makes Sudoku easier by eliminating numbers that have already been written using this method. Thus, it can hold on to the rule that no number should appear twice within any 3×3 block, column and row.

The ‘Last Possible Number’ together with the ‘Last Remaining Cell’ and ‘Last Free Cell’ are core strategies for solving Sudoku. In other words, getting good in the above three techniques will provide a good foundation for you to be ready for more complicated puzzles.

Practice more with our free printable Sudoku puzzle books. Download now!

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First, fill in the Sudoku grid with what can be written in every cell, some might have more than one candidate. The aim is to locate any cell with only one number that can fit therein. As shown below, if you come across a cell in the puzzle that has only 5 as its potential value, then don’t hesitate to drop it there because this will enable you to go on and solve other parts of the puzzle.

‘Obvious Singles’ or ‘Naked Singles’ are common names for this method. It is an easy technique that makes solving quite easy by identifying cells where only one number fits.

Using this strategy of ‘Obvious Singles’ will mean that you won’t despair when tackling Sudoku puzzles since they will now seem easier and faster to complete than before. Why not try? Download our free printable Sudoku puzzle books and see how much better you become!

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For example, in image 1 below, you have now filled all the cells with candidate numbers. Now consider a 3×3 block highlighted with the purple frame. Now you see that the numbers 5 and 8 are recurring in two cells highlighted with the green frame. Since these two cells can only be filled by either of them (5 or 8), this is our ‘Obvious Pairs.’

The next step is to figure out which other cells hold the number 5, and which one contains the number 8 as shown in image 2. As such, you can remove both 5 and 8, thus leaving you with one unique digit in each cell. This process identifies ‘The Obvious Single’ for you to fill in as shown in image 3. Now write down the winner numbers 6, 7, and 9 into their respective cells as shown in image 4, after that you can proceed to solve this puzzle using other techniques.

By mastering the ‘Obvious Pairs’ technique, you can solve more complex puzzles with confidence. To know more about this technique, refer an article from University of Cambridge. Once you have a good grasp of this technique, download our free printable Sudoku puzzle books!

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