In this article you will learn how to hack the Cass T-shirt pattern into a mini-dress with flounce sleeves.

Late in 2021, Michelle Morris — the brains behind That Black Chic and Sewn Magazine — invited me to contribute to the December issue of the mag.

OF COURSE I said yes, because, DUH, how could I pass up the chance to be featured in sewing’s trendiest indie publication?

Sewist contributors were asked to stitch something black and white. I took the invite as a opportunity to hack Sie Macht’s Cass T-shirt PDF sewing pattern.

What you’re getting in this article is the tutorial that appeared in Sewn — how to turn Cass into a flirty party dress.

In case you didn’t know, the Cass T-shirt is a free (for the time being… keep reading!) PDF pattern. It has a relaxed, curvy-skimming silhouette, grown-on sleeves, and unusual shoulder seam placement.

This hack transforms the T into a mini-dress with flounce sleeves!

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The Cass T-shirt, a PDF pattern from Sie Macht that comes in misses and plus size ranges, easily hacks into a fun mini-dress with flounce sleeves.

Flounce Sleeve Dress Hack Supplies and Notes

For this project, you will need the following sewing supplies:

All seam allowances are 1/2 inches (1.3 centimeters). The bottom hem is 1/2 inches (1.3 centimeters).

Cass is drafted for humans with a height of 5 feet 5 inches (165.1 centimeters). The pattern comes in two ranges — misses (B) and plus (D).

For a new-to-you pattern, I recommend sewing a muslin to test fit, techniques, and stitches.

Cass is designed for knit fabrics with fluid drape. Rayon jersey and ITY jersey are good choices. Choose a fabric that flows over your body.


RELATED: How Do I Choose a Knit Fabric?


I sewed this pictured hack in stretch velvet, which I recommend:

Back view of the Cass flounce sleeve dress hack.

Cass Flounce Dress Hack Instructions

0.) Prepare your fashion fabric by washing and pressing it appropriately.

1.) Print and assemble the PDF pattern.

2.) Determine ideal dress length. Measure along your center back from the nape of your neck to your preferred bottom hem location.

3.) Modify the back pattern piece. Slash the back pattern piece above the curved hem and spread it to your ideal dress length. (The pattern’s curved hem includes seam allowance.)

Cass Flounce Dress Hack Slash

4.) Modify the front pattern piece. The front and back pattern pieces are the same length at the sides. Use the side length of the back pattern piece to lengthen the front pattern piece.

Cass Flounce Dress Front Slash

5.) Draw in shoulder seam allowances. At the shoulder of both the back and front pattern pieces, draw a line 1/2 inches from the edge. These lines represent the shoulder seam allowance.

6.) Overlap the seam allowances. Lay the back pattern piece atop the front pattern piece at the shoulder, overlapping the seam allowance lines. Tape the pattern pieces together.

Cass Flounce Dress Shoulder Seam Overlap

7.) Slash the rounded sleeve hem. Draw a line connecting the front and back underarms. Slash along the line, cutting off the rounded sleeve hem and creating a straight sleeve hem.

Cass Flounce Dress Slash Rounded Sleeve

8.) Draw the sleeve seam. The seam is 1/2 inches from the edge of the joined pattern pieces.

Cass Flounce Dress Sleeve Hem Seam

9.) Measure the new sleeve seam. Measure across the sleeve, from the edges of the paper, on the drawn-in sleeve seam line.

10.) Determine ideal sleeve length. Using a flexible measuring tape, measure on your body from the estimated neck opening, over the shoulder, to near the elbow.

11.) Find the center of the sleeve. Fold the edge of the sleeve in half (underarm to underarm) and make a mark. Measure from the pattern’s neck edge to the (marked) center of the sleeve, keeping parallel to the shoulder seam. Subtract 1 inch from this length for seam allowance.

12.) Calculate flounce length. Take your ideal sleeve length (Step 10) and subtract the current sleeve length (Step 11). The difference is the short length of the flounce with no seam allowances.

13.) Do the math to draft the flounce pattern piece. Picture a donut cut into two equal pieces. This is what the fabric flounce pattern piece will look like. First, though, let’s start with a full donut; see the diagram below with Roman numerals.

(i) Circumference of Inner Circle = Length of Sleeve Seam

(ii) Radius of Inner Circle = Circumference of Inner Circle / (2 × 𝜋)

(iii) Radius of Outer Circle = Radius of Inner Circle + Flounce Length

(iv) Circumference of Outer Circle = 2 × 𝜋 × Radius of Outer Circle

Cass Flounce Dress Sleeve Donut

14.) After you’ve made your calculations (i-iv above), draft the flounce pattern piece without seam and hem allowances:

a.) Draw a horizontal line that’s the diameter (2 × radius) of the outer circle.

b.) Mark the center of the horizontal line.

c.) From the center of the line, draw a perpendicular line that’s the radius of the outer circle.

d.) Place the static element of a compass (or other circle-drawing tool) at the center of the horizontal line. Adjust the measure of the compass so it’s the radius of the outer circle.

e.) Using the compass, draw a half circle from one end of the horizontal line to the other end of the horizontal line. The perpendicular line from the center is a landmark to make drawing the half circle easier.

f.) From the center of the horizontal line, draw a perpendicular line that’s the radius of the inner circle.

g.) Follow instructions d and e, substituting the radius of the inner circle.

Cass Flounce Dress Sleeve w/o Allowances

15.) Add seam and hem allowances to the flounce pattern piece. The sleeve seam allowance is 1/2 inches and will be on the inside of the inner circle. The hem allowance is 1/2 inches and will be on the outside of the outer circle.

Cass Flounce Dress Hack Sleeve w/ Allowances

16.) Cut out the flounce pattern piece. Label the pattern piece, making note to cut two pattern pieces on the fold. Flounces, because they’re circles, are cut on the bias, grain, and cross grain. In other words, don’t worry about flounce pattern piece placement in relation to the grain.

17.) Cut apart the taped front and back pattern pieces at the shoulder seam. The shoulder will not have a seam allowance. Make a note on the front and back pattern pieces to add a 1/2 inch seam allowance at the shoulder.

18.) In your fashion fabric, cut out:

  • 1 dress front
  • 1 dress back

19.) Cut out the knit neckband. You may refer to the original Cass T-shirt instructions for pattern piece dimensions. Or, you can cut a rectangular piece of fabric, with the long side on the cross grain, that’s 2 inches (5.08 centimeters) by 85 percent of the length of the neck seam. (If your fabric is especially stretchy, you might want to reduce the percentage.)


RELATED: How to Sew a Knit Neckband


20.) Sew the short ends of the neckband, right sides together.

21.) Fold the long sides of the neckband, wrong sides together. Gently press to set the fold.

22.) Baste the long sides of the neckband. Use a long and narrow zig-zag stitch 1/8-1/4 inches (0.3-0.6 centimeters) from the edge. This will make sewing the neckband to the neck opening easier. Instead of controlling three layers of fabric, you’re only controlling two.

23.) Sew the shoulders of the front and back, right sides together. (Remember to add seam allowances!) Press the seam allowances toward the front (down toward the hem).

24.) Divide the neckband loop into quarters.

25.) Divide the neck opening into quarters.

26.) Align the quarter markings of the neckband and neck opening, right sides together.

27.) Pin the neckband to the neck opening at the quarter markings. The neckband fold should point toward the dress hem on the right side of the dress.

28.) Sew the neckband to the neck opening. Stretch the neckband between the pins to make it the same length at the neck opening. Do not stretch the neck opening.

29.) Press the neck seam allowance toward the body of the dress from the wrong side.

With the seam allowance, you can:

  • Topstitch it down from the right side.
  • Trim it away.
  • Finish its raw edge with a zig-zag or serger stitch.

Note: If you please, to help the seam allowance lay flatter against your body, clip the allowance at 2-3-inch intervals (to the seam stitching but not through it). Clip the allowance before you topstitch or serge it.

Flounce sleeves add drama to a dress that might otherwise be for casual wear only.

30.) Cut the two flounces on the fold. You have two donuts. Yum, donuts.

31.) Cut into the donuts without cutting them in half. Each flounce will have two short ends that will be sewn with the side seams. Cut from the edge of the outer circle to the edge of the inner circle (negative space). The cut must be perpendicular to the inner circle.

32.) Finish the raw edge of the flounces. Each flounce has a 1/2-inch hem allowance. Hemming a circle, especially a circle cut from a drapey fabric, is difficult. I suggest:

  • Leaving the edges raw. Knit fabric will not fray. (You’ll want to trim away the hem allowance.)
  • Finishing the edges with a serger or zig-zag stitch. (Trim away the hem allowance.) That’s what was done with the pictured Cass dress.
  • Serging a rolled hem.
  • Sewing a facing. If you don’t like the wrong side of your fabric, this could be a good option. Choose a facing fabric; I recommend facing with your main fabric. Cut a facing piece for each flounce using the flounce pattern piece. Sew the facing to each flounce with a 1/2 inch (1.3 centimeter) hem allowance, right sides together. Turn the flounce unit right sides out and gently press the hem seam. Baste together the raw edges of the flounce and flounce facing. Note: Before sewing a facing to a flounce, test how the two layers of fabric will work together. The faced flounce could get heavy.

33.) Sew flounces to the sleeves, right sides together. For best results, divide flounces and sleeves into quarters, match the quarters, and pin in place. You shouldn’t have to stretch any fabric layers, because the sleeve seam allowance and flounce seam allowance are the same length… if you’re a careful fabric cutter and pattern measure-er.

34.) Sew the side seams between the sleeve hem and dress hem. Press seam allowance to the back.

35.) Hem the dress. The dress has a 1/2-inch/1.3-centimeter hem allowance. Because the hem is gently curved (and not a circle), you can turn the raw edge to the inside and stitch without trouble. Or, you can match the bottom hem finish to sleeve hem finish.

I sewed the Cass flounce sleeve dress hack in stretch velvet from the brand Telio. I bought the fabric on Amazon.

Final Thoughts on the Flounce Dress Hack

Congrats! You hacked Cass! Please post your finished Cass dress to Instagram with #smcass. Tag @sie.macht.sewing.

If you liked this hack, stay tuned this spring for an e-book of five more (!) hacks for Cass. Yup, I’ve cooked up a book of hacks for both B and D ranges, so all the beautiful bodies will have an opportunity to wear fashionably hacked Casses.

And remember how I mentioned that the pattern was free… for now? When the Cass Book of Hacks goes on sale, the pattern will be included with it. That means it won’t be a freebie anymore.

P.S. I plan to share a coupon code via email for the e-book to sewists who got Cass for free. So (sew!), if you like to save cash money, get your complimentary Cass while the getting’s good.

P.P.S. Check out these articles to help you have the most fun sewing your Cass:

Thanks for reading, hacking, and sharing your makes! I’m so excited to see what you come up with.

Please leave your best color, fabric, design, etc. ideas for a flounce sleeve Cass dress in the comments! Let’s stoke the fire of this sewing inspiration train, choo choo!