Knitting calculations: yarn requirement for your tension

I seem to have a knack for a mismatch between the tension in the pattern and that of my preferred yarn. While I can of course change the needle size to match the suggested stitch gauge, I seldom get both stitch and row gauge as in the pattern, and sometimes I get neither, even if I am using the exact same yarn as the sample for the pattern.

How to turn this mismatching two into the perfect couple for a project? Bearing in mind looks, namely the fact that a pattern might use stitches that would look very different depending on the yarn weight, I find that the “suggestion” I’ve read at times to simply check the yardage used and translate it into a different yarn is not very reliable.

So my rule of thumb (and of course, everything is approximate here) is to look at the square footage of a pattern, and translate that.

So for instance, suppose I want to knit this jumper and the tension in my yarn that gives me my preferred consistency for the fabric is 22 sts x 36 rows for 10cm square in stocking stitch.

Then I need:

1. tension of yarn used in the pattern: in this case, 20 stitches and 24 rows using 4.5mm needles = 10 cm in stockinette stitch
2. pattern schematics, to work out the total “surface” covered by the sweater
3. two swatches, blocked: 
   1. a  big one to figure out your real tension: you need to knit a large surface to make sure you measure the 10cm square at the centre, to avoid your swatch lying to you. In my case it was 22sts x 36 rows
   2. a smaller one, with the exact number of Sts and rows for your 10cmx10cm tension square, that is I cast on 22 Sts and work for 36 rows. Just to be sure, make a provisional cast on and do not bind off, and remove the provisional cast on after swatching. No need to cut the yarn here, so that this can 

To work out the total surface covered, I need the schematics from the pattern for your project, so this can’t be done before procuring the pattern. For this simple pattern looking e.g. at the smallest size, the body is 65cm wide x 64cm high each, while each sleeve is 50cm long and measures roughly 25cm  at the cuff and 50cm cm at the top (101 stitches which divided by 20 stitches give us 5 times the size of the swatch, that is 50 cm). Modifications to the pattern will obviously change the size.

In this example the total “surface” covered by the sweater is 65×64=4,160 square cm for each of front and back, and ((25+50)x50)/2=1,875 square cm for each sleeve. Adding up it yields: 4,160+4,160+1,875+1,875=12,070 square cm. Each smaller swatch covers 10×10=100 square cm, so if you divide the total coverage of the jumper by the size of the swatch, it means that you have to knit the equivalent of 120.7 swatches. This excludes the yarn needed for seams, cast on and general finishing, which I’ll have to eye-ball separately.

For a quick  measurement, I just weight the small swatch, and then multiply by 121 to figure out the total weight of the yarn I want to use. For a more accurate measurement, undo the small swatch, measure the length of the yarn used, and multiply by 121.

Yarn length is more accurate, as especially with natural fibres the weight will be affected by humidity, and in manufacturing yarn, producers base their balls/skeins/cones on yarn length.

Again, this excludes any additional yarn that might be needed for finishing the garment. Also, for garments with multiple textures, one has to swatch for all of these, and repeat the “knit/block” and then weigh or unravel-and-measure process for each separate swatch – something I’d do anyway.