Calculating gauge from a swatch

I'm teaching a few machine knitting beginners at the moment, and I think the maths can be a bit daunting, so I figured it was high time I did a post about how to calculate gauge from a swatch. This also works for handknitting or crochet!

First you should make some swatches, using your desired yarn, stitch pattern and tension. For example, if I was knitting with 4ply acrylic on a standard 4.5mm knitting machine, I'd knit a tension swatch of 40 sts wide by 60 rows in tensions 5, 6 and 7, seperating each one with 6 rows of contrast yarn. I would actually cast on 50 stitches in waste yarn, and then transfer needle 21 on both sides and leave that needle out of work, to give an easily visible "gutter" to measure from. I mark what tension I've used by making 6 holes above the T6 section in the waste yarn - if you knitted T6 and one dot, mark this with six holes on the right and 1 hole on the left. The swatch should be cast off and washed/treated in the way you would treat the finished garment. Marking the swatch with holes is useful - you think you'll remember what tension you used, but believe me you'll probably forget!

Top tip: If you are knitting a tuck pattern, the bigger the swatch the better - for the Passap I knitted 100 sts by 100 rows. Note what you did for future reference, I keep a notebook for this!

The next step is to measure your swatches, top to bottom and left to right. I'll use the Passap sample mentioned above, the maths works the same no matter how you worked your swatch.

100 stitches = 32.5 cm wide

We want the amount of stitches per 1 cm, and to get that we divide the width by itself on both sides of the equation:

100/32.5 = 1 cm wide

3.076 sts = 1 cm wide

Knitting patterns tend to give the gauge per 10cm, so then you divide both sides by 10:

30.76 sts = 10 cm

You do exactly the same for the rows:

100 R = 15 cm

100/15 R = 1 cm

6.666R = 1 cm

66.6R = 10 cm

So 31 sts x 67 rows equals 10cm by 10cm (I round down if it's .4 or less, up if it's .5 or more). As this is a tuck fabric, it's a lot of rows to 10 cm. You can now check this against your pattern and see if you are close to the desired gauge. Personally I'd try and match stitch gauge over row gauge - it's much easier to add or lose a row every 10 rows, for example, than to try and recalculate stitch counts, especially around necks and armscyes.

You can also work this out another way - you can calculate the width and height of one stitch. This is useful as a double check. In this case, you start as before, but rearrange for stitches/rows not cm. I think this was known as "literal equations" in GCSE maths when I learned it, but I could be wrong! The pattern I was following called for 30 s x 67 r = 10cm square

100 stitches = 32.5 cm wide

1 s = 32.5/100

30 s = 30 x (32.5 / 100)

30 s = 9.75cm

100 R = 15 cm

1 R = 15/100

67 R = 67 x (15/100)

67 R = 10.05cm

I decided 9.75 cm x 10.05 cm was close enough to 10 cm square to go ahead with the pattern as written.

I hope this post helps with your calculations!