Sudoku HowTo

Sudoku Rules

A standard Sudoku Puzzle is a 9 by 9 grid of cells or squares. Each cell will eventually contain a single value. I have used the term "cell" here to refer to the container for each value. A Sudoku grid of cells is further divided into units or regions. I have used the term "unit" on this site to refer to these groups of cells.
Units are rows, columns or boxes. A "row" is a horizontal line of 9 cells. A "column" is a vertical line of 9 cells. A "box" which usually has darker lines around it in published puzzles, is a 3 x 3 area of cells. Other names have been used for these units in the Sudoku world. I have used these names because they appear to be the most consistent.

A puzzle starts out with some values already filled in. These are referred to as the "given values" or "givens". Using these values and the basic Sudoku Rules it is possible to determine where to place all the other values.

The Basic Rule of Sudoku - Each unit (row, column or box) has to contain all the values, 1 to 9.

This also means that each unit will only ever contain one instance of any value. When you have 9 cells and 9 values have to be used, you cannot use a value twice.

Solving a Sudoku puzzle is the process of finding a cell that can contain only one value or finding a value that can only go into one cell of a unit.

Most puzzles published in Sudoku books or newspapers can be solved using the 3 or 4 simpler solving techniques described on this site. The simpler techniques are at the top of the strategy list. It is only the extreme puzzles that need the more complicated techniques.

If you are new to Sudoku start out with some simple or moderate puzzles and practice the simpler techniques. Get familiar with cross hatching and counting. These techniques in their basic form use the values already placed in a puzzle and do not require candidates for any cell be tracked.

Once you have mastered these, move onto the more difficult puzzles to practice box claims and row column claims. You can start tracking candidates and looking for naked pairs or hidden pairs. These techniques will solve a large portion of the Sudoku puzzles printed in puzzle books.

Only extreme puzzles will require the more complicated solving techniques. All of these techniques require the tracking of candidates for each cell. Some of the more complicated techniques get very difficult to identify when using pencil and paper.

If you have been working on Sudoku for a while and are here to learn more solving techniques try some of the sample puzzles or test yourself on the more difficult puzzles. Enable just some of the solving techniques so you can see how they work.

When you use this program to create puzzles it will create a puzzle of random difficulty. If you are trying to learn a particular technique or want easier puzzles disable some of the solving strategies. If you create a puzzle on this site while some solving strategies are disabled then the created puzzle will be able to be completed using only those strategies that are enabled along with Cross Hatching. The possible strategies in the right hand column are ordered by my perception of the difficulty to use the strategy.

Not every technique will work in every puzzle. You will need to experiment a bit to identify the signs that a particular solving technique might produce results in any given puzzle.

Once you have mastered the standard 9 x 9 Sudoku puzzles see if you can head around the same process using a 16 x 16 grid. These puzzles use the same logic but have 16 possible tokens for each cell and 16 of each type of unit.

Have Fun
30 July 2015
by Tony Hughes
RE: Comments and feedback for basicRules

What is the answer to the Qs on 9/7/14 ?
20 September 2014
by Arlette
RE: Comments and feedback for basicRules

Thank you so much for this site. It is a lot easier to understand than some of the other sites when it comes to more complicated techniques. I still have a long way to go but you are certainly helping. Keep up the good work.
9 July 2014
by John
RE: Comments and feedback for basicRules

It looks like I have a logic flaw here. If a number is placed in a cell while entering a puzzle, it can be changed to a different number by clicking on that cell again, but at this time it can not be removed altogether.
9 July 2014
by David Steward
RE: Comments and feedback for basicRules

Thanks for the program.

How do I remove an error when entering the initial numbers for a puzzle from a magazine?
13 January 2013
by Helen Eichman
RE: Comments and feedback for basicRules

I love this site! It really helps to know the next step in solving a puzzle even though some techniques elude me. Hopefully I can catch on using your information. Thanks!

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This page last updated on March 24, 2015