Understanding Negative Ease and Fabric Stretch Percentage

In this post, I explain what ease means, why it's important for our clothing and how to measure Fabric Stretch Percentage.

Hello my mathy sewing friends! This Thursday is Thanksgiving in the US and it always feels like even though it’s only a one day holiday, nothing actually gets done this week. Everyone is kind of checked out and ready to coast on to the New Year and I totally identify. I feel like all of my attention needs to be on crafting and cooking for the next month, preferably while watching a Christmas movie. Fortunately, my work is slower this week so I can get caught up on all of my sewing projects.

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Today’s post is a pre-amble to the drafting of our Fitted Knit Skirt (link below). These concepts might be old hat but they are really important to understand for sewing with knits, even if you’re not drafting patterns.

Introducing Project 005—Fitted Knit Skirt

What is Ease?

At its most basic, ease is the difference between the body measurement and the garment measurement. When we wear fabrics that don't stretch, we need some extra room between the body and the garment so that we can move and breath without tearing the fabric. Ease can also be used for style and design purposes. For example, an oversized top could have 10" of ease or more. Some of that ease is for moving but most of it is for style.

In math speak, GARMENT - BODY = EASE. The ease can be positive, zero or negative.

When the garment is bigger than the body, there is POSITIVE EASE. Most woven garments will have some positive ease.

When the garment exactly matches the body measurement, then there is ZERO EASE. You’ll find Zero Ease in garments that use a stable knit (i.e. a little stretchy but not super stretchy). In fact, our Fitted Knit Skirt project can be successfully drafted with zero ease.

When the garment is smaller than the body, then there is NEGATIVE EASE. Negative ease is used in garments like swimsuits, leotards, leggings or even sports bras. They are usually smaller than your body but have a lot of stretch so that they tightly hug the skin. A so-called "bodycon" dress is the same idea—super tight and super stretchy.

Why does the fabric need to stretch?

There are two factors here. First, the fabric will need to stretch enough to fit each area of the body when it is worn. For example, the bust area of a dress or top needs to stretch enough to fit the bust measurement.

Second, all parts of the garment, in particular the openings, need to stretch enough to put on and take off the garment with ease. For example, if you're making a top that goes over the head, the neckline needs to stretch as big as your head. And the openings at the wrist need to stretch enough for your hand to easily pass through. Likewise, the waist of a skirt needs to stretch as big as your hips.

If you're making a one-piece swimsuit, the neckline opening will need to stretch enough to go over the thighs, hips and bust. The smaller the opening and the less stretchy the fabric is, the harder it will be to put on your swimsuit. And when it's hard to put something on (or take it off) the stitches can break and the fabric can tear.

For this second "getting dressed action" reason, fabric stretch is important to think about even if there is positive ease in the body of the garment. For example, with a crewneck t-shirt the neckline is relatively small while there is some positive ease in the body. This style of t-shirt has positive ease everywhere WHILE you are wearing it. But when that neckline goes over the head it will need to stretch.

I have made things multiple times where I was not careful about how much the fabric stretched and it is a huge bummer. It is so disheartening to make something beautiful and find that it's too tight to go over your head. So, please double and triple check your fabric stretch!

Which leads us to the next section.

Fabric Stretch Percentage

If you've ever sewn a garment that uses knit fabric or stretch woven fabric, then you've probably heard the term "Fabric Stretch Percentage". In short, fabric stretch percentage is the amount (or percent) that your fabric is able to stretch beyond its original size. Let’s call it FSP for short.

I always felt like the terminology around FSP was a little confusing. Like, what does it actually mean when they say a fabric has 25% stretch? Taking a percentage makes something smaller, not bigger. Well, in sewing, it means that your fabric has the ability to stretch 25% bigger than its original size. Mathematically, the stretched fabric would be 125% or 1.25 times the original size. 

Determining fabric stretch is pretty easy, you only need a ruler. To measure the FSP, fold your fabric along the crossgrain and using your fingers pinch 4 inches or 10 cm of fabric. Pull the fabric and measure how far the fabric stretches. Then divide the stretched length by the original length.

FOR EXAMPLE:

A = Original Length = 4"

B = Stretched Length = 6"

B/A = 6/4 = 1.5 = 150%

In this the example, the fabric stretch percentage is 50%, meaning the fabric can stretch 50% larger than its original size.

I have a pretty thorough blog post and video explaining fabric stretch percentage on my other blog. Click here to check it out.

What do 2-way stretch and 4-way stretch mean?

Two-way stretch means that the fabric stretches on the crossgrain, perpendicular to the selvage. Four-way stretch means that the fabric stretches along the crossgrain and along the grainline (parallel to the selvage). I think this terminology is a little confusing but you can think of it as "two-way stretch is left to right" and "four-way stretch is left to right AND up and down".

It's important to note that with a 4-way stretch fabric, the stretch percentage can be different on the crossgrain versus the grainline. For example, a fabric may have 60% stretch on the crossgrain and 25% on the grainline.

You will want to use 4-way stretch on garments that have a lot of negative ease and/or need to stretch up and down. For example, with a swimsuit or leotard, the garment is wrapping up and down around the torso, so you will be thankful for 4-way stretch. Similarly, leggings are very form fitting and more comfortable to move in with 4-way stretch.

For our fitted skirt project, a two way stretch fabric should be fine as the body will not be putting as much strain on the up/down axis of the garment. 

Considering Fabric Stretch when Pattern Drafting

After all that talk about Fabric Stretch Percentage, you might assume that pattern drafting uses formulas to determine negative ease, based on the FSP. Sadly, most pattern drafting books do NOT give us magical mathematical formulas.

The books that I use for pattern drafting by and large, use the same negative ease for all sizes, rather than a percentage. One of my favorite pattern drafting books is Metric Pattern Cutting for Women’s Wear by Winifred Aldrich. When drafting a close fitting body block (page 200), it instructs to use a total of 12cm (4.75”) negative ease at the bust for very stretchy fabric and 4cm (1.5”) negative ease for medium stretchy fabric. For a knit skirt, they recommend 4cm (1.5”) negative ease.

With this approach to drafting, a 1” negative ease on a size 2 is going to stretch the fabric a lot more than a 1” negative ease on a size 32. I think this approach of equal negative ease across sizes generally works. It’s certainly easier to communicate on a sewing pattern envelope. But, it should also be noted that most pattern drafting books don’t take larger sizes into consideration. (This is really an unfortunate industry oversight.)

That said, I found one resource that does use a percentage to determine negative ease when drafting—the book Designing and Patternmaking for Stretch Fabrics by Keith Richardson. I don’t own this book (it’s priced like a textbook and is around $90 used online) but someone has uploaded photos of most of the book to Issuu. (I don’t want to link to it because I think there are some copyright issues with it being posted. But, it’s easy to find with a quick google search of the title and author.)

This book has TONS of charts for the percentage of negative ease based on the FSP, body part and size range (i.e. Misses vs. Junior vs. Tall Misses). However, I’ve read a few reviews that say the book and the charts in particular have a lot of errors. I have found multiple instances that can only be explained as errors. The errors are big reason why I haven’t bought the book. So, it’s potentially a good resource but not a reliable one, unfortunately.

That said, I found a couple of items that will be really helpful for our purposes. In the chart above, you can see different categories of fabric in column one with the FSP in columns 2 and 3. Then in columns 4 and 5, is the information on how to calculate the finished garment measurement (or how much negative ease to use).

The diagram above shows example skirt slopers for different types of knit fabric. Basically, the more stretch the fabric has, the more negative ease you can use.

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Thank you so much for following along! I hope that this post was helpful. As always, let me know in the comments if you have any questions. And if you enjoyed this post, you can tap the "heart" at the top or bottom to help other people find it.

Happy mathing!

Beth

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