More wool is required for each colour when stitching a pattern than a block of solid colour.  This is because there is more finishing and the thread is carried on the back of the canvas between areas of the same colour.  The table shows two figures for each combination of canvas and stitch type.  The high figure represents the number of square inches you can stitch in a solid block with a skein of wool.  The lower figure is a guide to the likely number of square inches you could cover with a skein if you are stitching a pattern.  This is purely indicative as it will be very dependent on the pattern.

10hpi 12hpi 14hpi
Half cross stitch 10.5-13 sq inches 9-12 sq inches 7-9.25 sq inches
Continental tent stitch 7.25-9.5 sq inches 5.75-7.5 sq inches 4.5-6.25 sq inches
Basketweave 6.75-9 sq inches 5-6.5 sq inches 4-5 sq inches

To work out the amount of wool you will need:

1.            Work out the area of the finished piece of work;
2.            Guess what proportion is each colour;
3.            Use your guess to calculate the number of square inches you need for each colour;
4.            Use the table above to work out how many skeins you will need for each colour;
5.            If you are planning to use Appleton’s wool, choose either hanks or skeins.
With Appleton’s there are 6 skeins in a hank, but a 4 skeins cost about the same as a hank.