The underlying difference
A woven fabric does not stretch along the grain. Whatever room the garment needs to be put on, moved in and taken off has to be drafted into it as ease, plus an opening — a zip, buttons, or enough width to pull over the head.
A knit stretches. That means the garment can be smaller than the body and still go on, which is why knit patterns routinely carry negative ease, and why the finished bust on a t-shirt pattern can legitimately be smaller than your bust measurement.
So the two patterns are answering different questions. The woven asks how much room you want. The knit asks how much the fabric will give.
Measuring stretch percentage
Knit patterns print a stretch gauge on the envelope — a small ruler with a mark showing how far a given length must stretch. The measurement behind it is simple:
- Fold a length of your fabric and take a section of the crosswise grain, four inches long.
- Hold one end at the zero mark on a ruler and stretch the other comfortably — the point where it is extended but not straining.
- Read where the four inches now ends. Six inches means the fabric stretched by two inches over four, which is 50 per cent stretch. Five inches is 25 per cent.
Measure across the width of the fabric, since that is the direction most knits stretch most and the direction the pattern is drafted to use. Do it on the actual fabric you intend to sew, not on one you own that looks similar.
Recovery matters as much as stretch
Stretch tells you whether the garment goes on. Recovery tells you whether it still fits at the end of the day. A fabric that stretches to 60 per cent and then stays stretched will produce a garment that grows at the elbows, the seat and the neckline within an hour of wearing.
Test it: stretch a section firmly, hold for a few seconds, release, and see whether it returns to its original length. If it comes back slowly or not at all, treat the fabric as having less usable stretch than it measures, and choose a size with less negative ease.
The same applies to four-way versus two-way stretch. Most jerseys stretch crosswise only. A pattern that expects stretch in both directions — leggings, a swimsuit, a fitted bodysuit — will not behave in a two-way knit, however far it stretches sideways.
How the size decision differs
For a woven, start from the body measurements and the size chart, choosing the upper body size from your high bust. Then check the finished measurements to confirm the ease is what you want, and blend for waist and hip.
For a knit, start from the finished measurements. Find the size whose finished bust sits the intended amount below or above your own, check the pattern's required stretch percentage against what your fabric actually does, and only then look at the body chart as a sanity check.
If your fabric has less stretch than the pattern asks for, size up rather than forcing it. If it has considerably more, the garment will be looser and softer than the sample; sizing down is possible but changes the shoulder and armhole as well, so it is a decision rather than an obvious fix.
A worked example. Your full bust is 38 inches. A knit top pattern gives a finished bust of 36 inches at size M and requires 40 per cent stretch.
Two inches of negative ease is normal for a fitted knit top, so size M is the intended fit. You measure your jersey: four inches stretches to five and a half, which is 37.5 per cent — slightly under. It also recovers well.
Marginally under with good recovery is usually fine. The same shortfall in a fabric that did not spring back would be a reason to take the size up, because the garment would settle at the stretched dimension rather than the sewn one.
The same size number is not the same garment
A company's knit patterns and woven patterns are drafted from the same body block but with different ease built in, so a size M knit tee and a size M woven shirt from one brand are not comparable garments. Comparing across brands compounds it: the body blocks differ as well as the ease, which is the reason the calculator holds each company's published chart separately rather than merging them into one.
Bust adjustments in knits
A full bust adjustment is needed less often in a knit, because the fabric absorbs a certain amount of the difference. Less often is not never. If a knit top rides up at the centre front, pulls horizontally across the bust, or the hem hangs shorter at the front than the back, the fabric is being asked to cover more than it can and the pattern needs the extra length and width that an adjustment provides.
Many knit patterns have no bust dart, which makes a conventional FBA more awkward and pushes the sensible answer toward a small adjustment plus a muslin. Cheap jersey for a test garment is worth the hour.
Two habits worth keeping
Do not size down in a woven to get a fitted look. That removes shoulder width and armhole depth along with circumference. Take the width out at the side seams instead.
Pre-wash and dry before you measure anything. Knits in particular shrink, and cotton jersey can shrink considerably. Measuring stretch on unwashed fabric produces a garment sized for a fabric you no longer own.
General guidance following standard home-sewing practice for knit and woven fabrics, including the conventional four-inch stretch gauge method printed on commercial knit patterns. Individual fabrics vary and a test garment decides. Not professional fitting advice.