It is always difficult to estimate how much yarn you will need for a project because you cannot know for certain until you have finished knitting it, and by then you will already have made your decisions. Of course, a mixed yarn project is easier because you do not have to worry about matching dye lots and even if you cannot obtain more of a yarn you can simple add in something else which blends in colour, texture and yarn weight. However, if you are using only one yarn, there are various ways of estimating approximate yarn quantities at least.
You can weigh a similar garment in a similar yarn. If you then add an amount of the yarn ‘wastage’ during knitting, this will tell you how much yarn to buy. You could also look at a pattern resembling your design in shape and yarn type. The colour is also important. Differing dye intensities make even different shades of the same yarn vary in weight. If you are using a pattern, compare the length measurement per ball if stated on the ball band or cone band. This gives a more accurate guide than weight alone.
Knit up a ball
Knit up a ball of yarn or a certain quantity – say 50 g. Multiply the height of the knitted piece by the width to get the area of the piece of knitting produced from this quantity of yarn. Now, divide the area of knitting produced into the approximate area of the entire garment as calculated from your measurement plan, roughly converting each pattern piece into rectangular shapes for easy calculation. This will give you the approximate number of balls (or 50 g) needed to complete your garment.
Number of stitches
Calculate the number of stitches knitted from one ball by multiplying the number of stitches per row by the number of rows knitted. If you are knitting from cones or yarn then wind off 50 g. Calculate the number of stitches in the back and one sleeve of the sweater in the same way, by using your measurement diagram, and double the result. Divide one into the other as above.
Keep a check on the number of balls used (or the weight used) to knit the back and one sleeve of a sweater. Approximately the same amount will be needed to complete the rest of the garment, not including edgings or additional features such as pockets and collars.
Keep a record of your tension details and how much yarn is used for each project for future reference. This makes future estimates easier.
When you knit a garment record all the details of the garment size, yarn specifications and quantity used. One day you may see something similar and need to decide whether there is sufficient yarn for your purpose.
Machine knitters use industrial yarns and these provide the knitter with special problems when estimating the amount of yarn required for a garment. Industrial yarns usually come in cones and are often treated with oil to help the yarn knit on a machine. After the garment is knitted it will need to be washed to remove the oil. This means that weighing the garment after it has been washed will perhaps be different to the weight before washing. These oiled yarns usually shrink in the first wash. So, if you are using only one yarn throughout the knitting you need to weigh the garment, before it is washed if the yarn is oiled, and record the pattern size and quantity used. If you are using more than one yarn, then weigh each cone before you start and again when you have finished knitting, and record the results in your notebook.
Use the tension square
Buy a small quantity of the yarn and knit a tension square. Weigh it on a postal or kitchen scale. Record the weight. Assuming it is 14.5 g. Also, record the area. If your tension square is 10 cm square, it’s area is 100 square centimetres (10 x 10 = 100). Next, calculate the approximate area of each garment section by multiplying widest dimension times overall length. Add these figures together to get the approximate area for the garment. Then, divide the result by the sample area.
Here is an example of such a calculation:
across the chest 45 cm
length 50 cm
total area for back 2,250 cm
total area for front 2,250 cm
width at upper arm 30 cm
length 58 cm
total area for sleeve 1,740 cm
total area for both sleeves 3,480 cm
Total area for sweater:
2250 + 2250 + 1740+ 1740 = 7,980 cm
Divide by sample area (100)
7980 ÷ 100 = 80 (approx.)
The figure 80 represents the number of your samples needed to knit the sweater. Sample weight was 14.5 g. So, 14.5 x 80 = 1,160 g. From this you could deduct 10% to allow for shaping decreases. The resulting 1,044 g is the estimate of yarn needed.
Ask for assistance
If in doubt, consult an assistant in a local yarn/wool shop. A good assistant (if you can find one) will have the experience and access to pattern books from which to make an estimate of the amount of yarn your project will need.