Beginner’s Guide to Underwear Sewing // Part One: Underwear Block & Calculating Negative Ease

Hello and welcome to the first installment of our new “Beginner’s Guide to Underwear Sewing” series, a fun collaboration with my friend Joy (@byjoymao on Instagram). In this two-part blog series, she documents and shares her experience as a beginner underwear sewer.

It’s pretty easy to get started drafting and sewing your own underwear. All you need is some basic pattern-making tools, a little fabric and trim, and a home sewing machine that can perform straight and zig zag stitches. 

What we’ll need:

  • Measuring tape
  • Pattern paper (regular paper will do, ideally at least 18” x 24”)
  • Clear ruler
  • French curve
  • Pencil and eraser
  • Invisible tape
  • Scissors for cutting paper
  • Scissors or rotary cutter for cutting fabric
  • Pins and optional pattern weights (Joy uses tuna cans as pattern weights)
  • 1/2-1 yard fabric of your choice + organic cotton jersey for crotch lining (Joy uses 2 colors of organic cotton jersey)
  • 2-3 yards elastic of your choice (Joy uses ¾” fold-over elastic)
  • Thread (Joy uses Gutermann mara 120 polyester thread)
  • Sewing machine with straight stitch and zig zag capabilities, or a serger if you have it (Joy uses Bernina 1008 with straight stitch and zig zag)

Underwear Block Draft

Joy: To get started, I watched Beverly Johnson’s “Sewing Panties: Construction & Fit” class on Craftsy to learn the basics of drafting and sewing underwear, and found it super approachable and informative. I won’t dive too deep into the actual pattern-drafting in this post, but you can check out Beverly’s class, among other great resources, to learn how to draft an underwear block.

We’ve also linked a few other Underwear Block or Underwear Pattern Drafting resources here to get you started and these are all free:

I started by gathering my pattern-making supplies: paper, measuring tape, pencil, french curve, and clear ruler. With my measuring tape, I took four measurements:

  • Waist
  • Hip
  • Waist to hip
  • Crotch depth (while sitting on a flat surface, the distance from your waist to the surface)

Some snaps from my pattern-drafting process:

Pictured here is my finished underwear block pattern drafted based on my own measurements, with the materials I chose for my first sew (“Bright Petrol” Organic Cotton Jersey Spandex and “Blossom” Fold Over Elastic).

Important note: I chose not to include any negative ease in my block pattern, because I wanted to create a master pattern which could be adapted for different fabrics that have varying levels of stretch. Once you have a master block pattern that fits well, you can adapt it for any fabrics, trims, and styles!

Calculating Negative Ease

Joy: For those new to sewing with knits (like me!), “negative ease” refers to the portion of a pattern or garment removed to account for the stretch of a knit fabric. Garments with negative ease measure smaller than the body’s measurements, but stretch to fit appropriately.

In this step of my underwear-making journey, I’m measuring the stretch percentage of this fun “Bright Petrol” Organic Cotton Jersey, and then calculating the negative ease to be taken out of the master pattern I created above. This results in a finished pattern that’s tailored for a specific fabric and fit.

To measure negative ease (in either vertical or horizontal directions), you first identify a stable area towards the center of your fabric (the outer edges can yield a skewed measurement) and place two pins 10 inches apart.

Then, holding one pin at 0, you pull the fabric along a ruler and stop when you meet resistance. Take note of the measurement of the second pin.

This jersey fabric has stretch in both horizontal and vertical directions, so I repeated the process for the vertical stretch.

Calculating Stretch Percentage:

In the case of this jersey fabric, I started feeling resistance around 11 ½” (11.5″) in the horizontal direction, and 11 ¼” (11.25″) in the vertical direction. We’ll use these numbers to calculate the stretch percentages of our fabric, which we will then use in our negative ease calculations.

Percentage negative ease = (measurement when stretched – 10”) / 10” x 100

Horizontal: ((11.5-10)/10 ) x 100 = 15% negative ease

Vertical: ((11.25-10)/10) x 100 = 12.5% negative ease

(Pro-tip: a quicker way to make this calculation once you understand the theory behind it is to simply remove the first “1” from the measurement you took and shift the decimal point one figure to the right to get your stretch percentage. For example, if your measurement was 11.5, you can remove the first “1” to get 1.5, then shift the decimal over to get 15%)

Calculating Negative Ease:

The next step is to define where you’ll be taking out the negative ease, and calculate how much you’ll be removing using the measurements of your pattern. Because my fabric had both horizontal and vertical negative ease, I made two calculations to determine how much negative ease to remove from my pattern:

Horizontal: 15% x 6.75″ (waist measurement) = ~1”

Vertical: 12.5% x 12.625″ (center back measurement) = ~1.5”

Based on my calculations, I need to remove about 1” in the horizontal direction, and 1.5” in the vertical direction.

Because most of the stretch will happen around the waist and hips, I drew a horizontal line about halfway down between waist and hip to represent where I’ll remove vertical ease (confusing, I know), and a vertical line about halfway along the waistline for removing horizontal ease. I chose not to adjust the ease in the crotch piece, because that piece is relatively small, and because there is relatively little stretch happening there.

I then measured across 1” from my vertical line to take the horizontal ease, and measured down 1.5” from my horizontal line to take out the vertical ease.

The next part is super fun:

Cut along one of your vertical lines (it doesn’t really matter which one), and shift it over the meet your second vertical line. Make sure that the horizontal lines match up, and then tape the cut edge down. Do the same for your horizontal lines, and voila! You’ve taken out your negative ease!

Now it’s time to reshape your lines. I used my clear ruler to redraw my side seam and leg openings from point to point, and my french curve to reshape my waist seam.

After you repeat this whole process for the front pattern, you can true your patterns (make sure the side seams still measure the same where they need to be joined) and add your seam allowances. 

I added ¼” seam allowance to the side seams and crotch seams, but removed elastic allowance from the waistband and leg openings to account for using Fold Over elastic, which doesn’t require any elastic allowance.

In the next part, I sew up this pattern in the Bright Petrol Organic Cotton Jersey and share some basic style changes I made to the basic style underwear pattern once I nailed the fit. Stay tuned for construction details on how I sewed my underwear.

Much love & happy sewing!

Joy Mao
@byjoymao

Joy Mao is a Chinese-American designer uncovering wonder in the simple and strange. Moved by fashion’s ability to make us sensitive to our environments, emotions, and dreams, she creates clothes that support and inspire life.

Joy holds a BA in Fundamentals from the University of Chicago and an AAS in Fashion Design from the Parsons School of Design. Originally from Shanghai, China, she now lives in New York with her husband and her Silver Reed SK 280 knitting machine.

You may also like