Wednesday, January 18, 2012

Drafting your own half circle skirt pattern. (Math is your friend)

My Greetings!  No new sewing for a bit, but I've been planning things out for when I'm back to normal.   A few days ago I came upon Vivat-Veritas' half circle skirt pattern (from the Grosgrain free pattern month).   It was lovely, very simple, and solved a problem I've been having about a lovely but tricky print.

I didn't want to spoil it with shaping seams, nor do I find gathered skirts to be a flattering look for my body, so its been sitting in my stash for well over a year, just hanging out and hoping for inspiration.

Well! a half circle would mean only one seam, at the center back - and everything would be pre-alligned by the fold, so, perfect!   Now, I know I linked to a downloadable pattern at the top, but if you are like me and prefer geometry to endless taping, here's the math I worked out for mine.  (I promise you, this is easy.  It looks wordy, but that's only because I am defining all my terms.  The short version is: R1 = (AL*4)/2pi, where AL is half your waist measurement plus a seam allowance.  SL is the desired skirt length, also plus a seam allowance, and R2 = R1 +SL)

1) Measure your waist where you want the skirt to sit (natural or otherwise) and add one inch for seam allowances.  We will call this value WL (waist length) in my case this value is 31 inches.

2) Divide WL by 2, for the quarter arc length in the image above.   (Why divide by two for a quarter arc?   We are making only a half circle skirt, so that one quarter of the full circle will go around half your waist.   Should you choose to expand on the pattern and make a full circle skirt, then you would divide by 4.)  For me, this is 15.5" and we will call it AL (arc length).

3)  We now need to calculate the radius of this arc from the center of rotation (i.e. the fold at the top of the fabric)  which means its time for comparing the circumference of a circle to its radius.   The radius is C/(2*pi) where the circumference is four times the arc length we're using (remember, half circle skirt vs full circle math)  so, to define our terms both algebraically and with my measurements, C = 4*AL = 62 inches.  2*pi = 6.28, which means the radius R1 = 62/6.28, or 9.8 inches.   The fabric inside this circle is excess.

4) Now decide how long you want your skirt to be - this is the length from wherever you picked as your waist point to where you want it to hit your leg.  This will depend partially on personal preference and partially on the width of your fabric - your skirt can be no longer than half the full width, due to the placement of the fold.   Now, personally, I like things a bit below my knee, and  from my chosen waist point, that is 30 inches, which is defined as SL (skirt length) in the picture.  (Don't forget, SL must include your seam allowence

5) Add R1 and SL and you've got your total length from the center of rotation (i.e. top of fold) now known as R2. For me, R2 =  9.8 + 30, or 39.5 inches.   (For narrower fabric, try folding it the opposite of the way it was on the bolt, so you get the full woven width in one direction, and  the length you bought in the other, which will allow a nice long skirt.)

6)  Pin a measuring tape in place at the top of the fold and mark every inch or so at lengths R1 and R2 as you rotate it from one side of the fabric to the other.   (Take a look at the red arcs in the picture - see how the smaller one lines up with R1 and the larger one lines up with R2?   You can also use bits of string that are the right length, tie chalk on the end, pull it tight and draw.)

7)  THAT IS IT for the math.  Cut it out, pop in the zipper of your choice, hem it, bind the waist-band, and you are done!

EXTRA)  Adding a more standard waistband can be achieved either with a short rectangle the length of your waist (very narrow, not much more than a binding, and best done on the bias)  or for something wider, measure the height you want it to go up your torso, the change in width between that point and your waist point and draft the band based on the arc of the original skirt.  You will not end up with full half-circle pieces, more like rounded trapezoids, but you get the general idea.   (If not, feel free to bug me and I will draw more pictures.  But I am sleepy and don't really feel like it just now.)

Instructions on how much fabric you will need here.


  1. This is so good Allison! You have explained it so neatly. I think even I can understand it - & it brings back all those happy days in the classroom learning geometry. Thank you & hope you are feeling OK :-)
    What would be even more interesting, & just kind of top it off - what kind of yardages do you end up needing for a half circle skirt? I know it will be different for everyone. I never know when I look at stash fabric whether there's a circle or half circle skirt in there.

  2. Thank you - exactly want I needed for my replica attempt of an American Apparel skirt.

  3. Very clear! Thank you so much!!
    Miriam :)

  4. The instructions are very clear and exactly what I needed!
    Thank you so much!! :)


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