Of all the things that have the potential to baffle a person about knitting, yarn weight has to be right up there. So many yarns, so many categories, so little cohesion.

Yes, there’s a generally agreed framework that people use when talking about yarn weight (which, if we’re honest, is really yarn width, since it’s really about how thick a yarn is. Girth matters, people). A 4ply is thinner than a sport is thinner than a DK, and so on.

But the reality is that these boundaries are all a little…well, blurry. Chances are, if you spend long enough around yarn (and you really should spend as much time as humanly possible around yarn) then you’ll come to realise that some 4plys look like some DKs, and some DKs look like some Arans, and some sport weights….well, sport weights are a whole thing of their own.

You see, even though there’re broadly-accepted guidelines based on how many metres per 100g equal which weight, the reality is that not all DKs are created equal. Or even the same width.

Doesn’t this make choosing the right weight yarn even more confusing?

Well, yes. But also – and I’m going to argue quite hard for this one – no. Because in some ways, what weight the yarn claims to be is pretty irrelevant. What matters is what gauge you get when you knit with it.

Hear me out on this one, because it’s going to open up a whole new world of lovely knitting for you. I promise.

Yarn weight is irrelevant? Oh God. Oh, you’re killing me.

I know, I know. Telling you to ignore yarn weight is a bit of a rogue move. And you can’t just chuck it out the window entirely; a laceweight yarn and a super chunky are never really going to be all that interchangeable.

But in and around those yarn-y boundaries you’ve got a lot of leeway, which means you also have a lot of options. And you can make your knitting just how you want it.

Take my Colin, You Flutter Me tee pattern. In it, I say that you need 4ply to DK-weight yarn that meets gauge. And that’s the crucial thing – that it meets gauge.

The pattern – and I – don’t care whether your yarn claims it’s a 4ply, a sport weight, or a DK. All we care about is that you can get 20sts to 10cm with it. And that gives you a lovely bit of scope to muck around with different yarns and get the material of your dreams.

Let’s look at this picking-yarn-by-gauge thing in a bit more detail

I’m going to plonk two photos here to help illustrate my point; on the left we have the Colin, You Flutter Me that I knitted in a yarn that purports to be a 4ply. Next to it, is one knitted in a yarn that claims to be a DK.

Same pattern, knitted in the same size, in two different yarn weights. Both fit in the same way.

What yarn-y magic is this?

Well, it’s no magic at all. Because both of those yarns hit the magic 20st per 10cm gauge that I needed for the pattern, and so both knitted up to the same size.

The difference is in the material itself; 20st per 10cm is a fairly loose gauge for a 4ply yarn, whereas it’s pretty standard for a DK. So the Colin that’s knitted in the 4ply has produced a lighter, airier material than the one that’s in the DK.

Both have the same number of stitches in the same area, but those DK stitches have a little more heft to them than their 4ply counterparts.

So you can get different types of fabric at the same gauge?

Yes. Yes you can. Broadly speaking, the thinner your yarn, the more open your stitches are going to be – and the lighter your fabric. You’ll have less yarn filling up the spaces between – and within – your stitches, meaning you can play around with getting floaty, semi-sheer fabrics.

Using a thicker yarn – or heavier yarn weight if you’re going to go with that lingo – means you’ll have more yarn and less air within those same stitches.

You ready for a shonky illustration? Well, you’re getting one.

See how those there very badly drawn stitches are roughly the same size, but look very, very different? That’s because one of them is drawn to represent a much thinner yarn than the other.

But what does that mean for needle size?

As ever, you’re going for whatever needle size you need to to hit the gauge you’re after. In practice, this means that you’ll often need to go for a larger needle when using a thinner yarn, and a smaller needle when using a thicker one.

Let’s go back to that 20st per 10cm gauge for Colin, You Flutter Me. The needle size my testers used to get that gauge varied wildly based on which yarn they’d picked. One of my testers, who was using a 4ply yarn, went all the way up to a 5.5mm needle. Another, who was using a DK, ended up on a 3.5mm.

And why is that? Simply put (I hope), your stitch size is a combination of two things; the thickness of your yarn, and the thickness of your needle. If you’re using a thin yarn, you’re going to need a thicker needle to boost it up to that stitch size. And if you’re using a thicker yarn, you’ll need a thinner needle.

Think about my terrible drawing again. That bit in the middle of each stitch is the gap the needle will have been in, so of course the thinner yarn needs a bigger needle while the thicker needs a smaller.

So you’re telling me I don’t have to knit with the yarn weight the pattern suggest?

Yes. Yes I am. You just need to get the gauge.

Gamechanger, hey?