I’m reminded why I always shy away from writing about swatching: There are a thousand caveats. I’ve kept this as brief as possible but the fact is it’s an important subject and I want to do it minimum viable justice. It’s a long post. So first let’s talk quickly about whether you need to swatch for a hat, and then I’ll launch into the whole how-to.

The answer to “Do I have to swatch for a hat?” is the same as for anything: Only if you want it to fit. A lot of people don’t swatch for hats, and I’m in this camp. (And it has led to multiple “learning experiences.”) Often you’re trying to squeeze a hat out of single skein and don’t want to give any of it to a swatch. Or you just really want to cast on and knit the hat, and figure you’ll find someone it fits when it’s done. Or you accept ripping as an integral part of knitting, so you let the hat be its own swatch and are prepared to rip and restart if need be.

My friend Rachel was elated to have knitted her first hat last week, and over the weekend I got a series of deflated text messages from her. She had soaked her hat, which was all ribbing, and now that the ribbing had relaxed it was way too big. I told her the hat-that-don’t-fit situation is a rite of passage. She replied plainly, “It’s very discouraging,” and yes, it surely is. But it can be avoided simply by knitting and measuring a gauge swatch first.

WHAT IS A SWATCH?

A swatch is a square of fabric that puts you in control of your outcome. The point of knitting a swatch is to understand how the fabric will behave and to establish your gauge — to see if your stitches are the same size as the pattern drafter’s stitches, and thus whether your finished item will be the same size as the pattern indicates. So the most important thing about knitting a swatch is that it has to be a nearly exact replica of the thing you mean to knit. Therefore:

• Use the exact yarn
No two are the same; a dyed yarn will even behave differently than its undyed version

• Use the exact needles
Your gauge will likely be different if, e.g., you swatch with bamboo and knit with metal

• Use the exact method
Swatch flat if knitting flat; swatch in the round if knitting in the round

HOW TO KNIT A SWATCH

In knitting patterns, gauge is usually stated either in stockinette stitch or in the stitch pattern used for the item in question, and it’s measured over 4 inches but might be stated as the 4-inch measurement or divided by 4 for the 1-inch measurement. I.e., “20 stitches and 28 rows over 4 inches” is the same as “5 stitches and 7 rows per inch.” You measure 4 inches rather than 1 to make sure you’re getting an accurate count of fractional stitches in a given inch, which can add up to quite a lot over the span of a garment.

In order to measure 4 inches, you need at least that many stitches — ideally more. The best swatch is a big swatch (especially if you really want to know if you like the fabric), but at bare minimum, you don’t want to be measuring edge stitches in your 4 inches. Sticking with that 5 sts/inch example above, you know that 20 sts will be about 4 inches (depending on whether you are a looser or tighter knitter), so you want to pad it to give yourself margin for differences and for measuring. I would cast on at least 25 stitches, but again, preferably more. And unless you already know yourself to be a loose or tight knitter, start with whatever needle size the pattern recommends.

Many people like to put a garter-stitch border around their swatches because it looks nice and because stockinette will roll. Others believe (especially if your swatch is small) that the difference in stitch and row gauge between garter stitch and stockinette will affect your measurements. I’m a purist, so I keep my swatches to only the pattern stitch and don’t put a border on them.

Again, if the item you’ll be knitting is knitted in the round, you must swatch in the round, and vice versa. Most people’s gauge varies between their knits and their purls, so if you knit a stockinette swatch flat (knitting on one side and purling on the other), your gauge will be different than when you knit stockinette in the round (knitting every stitch). And vice versa. Garter stitch worked flat is all knits; worked in the round it’s a combination of knits and purls. Here’s a good tutorial on swatching in the round — it’s much easier than it sounds.

HOW TO MEASURE A SWATCH

Here’s the frustrating truth: Some patterns list unblocked gauge and others list blocked gauge. Some don’t specify. (If you don’t know what blocking is, click here.) What you really need to know is: A) is the fabric going to change when you block it? and B) what are the final measurements?

So once you’ve got at least 4 inches of fabric — wide and tall — bind off loosely and measure your swatch. Write it down. Then block it however you will block your garment. If you don’t intend to ever wash your garment, okey doke, you’re done. If you intend to wash your garment by hand, go ahead and soak your swatch. If it’s machine-washable yarn and you intend to use the machine, machine wash your swatch. Then lay it flat to dry.

If you’ve done as I’ve done and knitted a plain stockinette swatch, you’ll have an annoying little rolled up worm of fabric. If you pin the edges while it dries, as pictured, it will flatten out, making it easier to work with and to measure. But you want to know the natural size of the fabric, so don’t stretch it when pinning. Once it’s dry, measure it again.

I’m pretty sure it was in Pam Allen’s “Knitting for Dummies” where I once saw the suggestion that you put a pin in the swatch at the 0- and 4-inch marks before counting. I’d never done that until I did for the sake of these photos, and that’s pretty sharp, so you should do that. First line up your ruler horizontally across the bottom of one row of stitches, with the zero point of the ruler at the outer edge of a stitch, as shown. (A knit stitch, which we’re looking at here, looks like a V. We’re counting Vs.) Put a pin at 0 and at 4. Now count how many stitches are between the pins, and that’s your 4-inch stitch gauge. (Divide by 4 for how many stitches per inch.) Now position the ruler vertically alongside a column of stitches. Again, place two pins and count the stitches between them. You can see here I have 20.5 stitches and 30 rows over 4 inches, or 5.125 stitches and 7.5 rows per inch.

Remember, I’ve knitted a stockinette swatch for this example because that’s the most common case. But your swatch should be in whatever stitch the pattern calls for where it states the gauge.

HOW TO “GET GAUGE”

Assuming you want your garment to match the pattern dimensions, you need to match the pattern gauge. If your stitches are too big (fewer of them per inch), try again on a smaller needle. If your stitches are too small (more of them per inch), try a larger needle. Make whatever adjustments are needed until you’ve matched the pattern’s gauge.

If it’s not important to you to match the pattern dimensions, you can always choose to knit at a different gauge, just make sure you know how different your gauge is and how much of a difference that will make to the finished dimensions. With hats, it’s fairly common to tweak gauge to tweak the size. I have a big head and most hat pattern dimensions are too small for my liking, so I look at every hat pattern and think “can I knit that in the next heavier weight of yarn on a needle one size bigger?” (E.g. if it’s a worsted-weight hat on 7s, I’m inclined to knit it in aran weight on 8s.)

Gauge is a function of yarn weight, needle size and the knitter’s tension, any or all three of which can be varied to get the desired results. It’s delayed gratification — casting on a swatch when you want to be casting on a hat — but you’re that much more likely to be gratified by the hat you wind up with.

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PREVIOUSLY: How to knit a hat, part 1: Anatomy lessons