This page lists some basic sudoku solving tips and strategies that you might find usefull before you play our free sudoku puzzles. Other pages that you might find useful include a page covering sudoku history and a page with the basic sudoku rules.
Broad Focus  Rows and Columns
One basic
strategy for any Sudoku game is to look at the rows and columns of the
9x9 Sudoku grid and to identify squares in which an integer must be
placed. To do this, choose any number and look across the
columns to see which columns already contain the particular
numeral. For instance, you may choose to start with 1, and
look from left to right for any columns with a 1 already in
it. If you find any of the three sets of three columns in
which two columns already possess a 1, you may be able to logically
assume where to place the 1 in the third column.
To do this, find which 3x3 subsection in the set of three columns does
not contain a 1. Since we have established that two columns
already have the number, and the numbers cannot be in the same 3x3
square, only one should remain. In this 3x3 square, two
columns have been already eliminated, leaving a maximum of three
individual boxes to choose from, or, if luck is on your side, one empty
box and two occupied boxes. Of course, if you find only one
empty box, you will place the 1 in the empty box with certainty.
If there are two or three empty boxes in the column within the 3x3
square without the 1, you will need to look at the rows for clues to
guide you to the correct box. Of your three rows in this set,
look to see if any 1’s exist in the other sets.
Hopefully, you will find one or two, which will narrow your
options. If a 1 already exists on the same row as one of your
empty boxes, that box will not receive another 1, since only one of
each integer can be on the same row.
Narrow Focus 
Individual Boxes
While the method
above looks first at the whole grid, then narrows the focus to just one
square, this strategy chooses one square and then broadens the scope to
include the rest of the grid.
On your puzzle, choose an empty square with a moderate amount of given
numbers around it. First, look at the 3x3 subsection in which
your empty square resides. You can eliminate any givens
within that subsection as contenders for this particular square, since
each subsection cannot have more than one number of the same
value. For example, let us assume that the subsection of a
particular empty square already contains a 2, 5, and 6. We
can definitely rule these integers out of our consideration set for our
chosen empty square.
Now, look at the row on which your empty square is found. You
can safely rule out any numbers on that row. For example, our
chosen square (which we already know cannot contain a 2, 5, or 6) has
an 3, 7, and 8 on its row. Therefore, just from looking at
the row and the subsection surrounding our chosen square, we have
eliminated six of the nine possible numbers for our square.
Finally, look at the column above and below the empty square.
Whatever givens exist on that column cannot exist in this particular
square. For instance, if the same square we have mentioned
has a 4 and a 9 in the same column, we can rule out those two
possibilities. So, for our chosen square, 2, 5, and 6 were
eliminated by looking at the 3x3 subsection, 3, 7, and 8 were
eliminated by looking at the row, and 4 and 9 were eliminated by
looking at the column. By the process of elimination, then,
only one possibility remains for our square – 1.
