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Circle skirt maths – explained!

∏ = 3.141592653589793238462…

How many times in Maths class did/do you ever wonder when in the hell you’d actually need to know equations like this in real life? Well, to my dismay, it turns out that when you like to make stuff, Maths is kind of fundamental. Especially when it comes to making circle skirts. One of the easiest and quickest garments you can whip up on your machine, provided you can first figure out the radius of your waistline circumference… Ugh. Fear not, my mathematically challenged friends – we have figured it out for you. And made some pretty skirts along the way!

Ready class? Then let’s begin.

We’re going back to GCSE Maths here guys. Remember pi?? That crazy 3 with endless decimal places is what we need to figure out the radius of our waistline circumferences. And why exactly do we need to know the radius? We need the radius in order to measure, mark out and cut the perfect little circle -that will be our waistline- onto the fabric. Without the radius, the only way to mark out the curve would be by shaping your measuring tape into a quarter circle on your folded piece of fabric (this ad hoc method I wouldn’t even suggest to lazy stitchers as the results would inevitably be inaccurate, leading to a whole lot of time consuming fixing and tweaking and hair pulling and fabric wasted…).

For each type of circle skirt, be it full circle, half circle or quarter circle, the mathematical equation we need in order to find out the radius will be slightly different. Before we look at each one individually, we first need to know the foundation equation: Circumference (c) ÷ ∏ (3.14) = diameter (radius × 2). We’d also like to point out now that we will be using metric measurements ie. centimetres, not inches. Nothing against Imperial, only that our brains simply can’t cope with Imperial decimals, what with them being in eighths as opposed to tens…!

A note about seam allowance: seam allowance needs to be added on to both your waistline seam and side seams. For the waistline, simply subtract 1.5cm from your final radius measurement. For the side seams, be sure to add 1.5cm for each raw edge to be seamed to your initial waistline circumference measurement: half and quarter circle skirts will have only one back seam so add 3cm to your waistline; a full circle skirt cut from 2 pieces will have 2 side seams so add 6cm altogether).

Full circle skirt

Let us begin by pointing out that it is unlikely you will be able to cut a whole circle skirt from a standard piece of 45″ width fabric (unless making a miniskirt). The following diagram assumes that you will be cutting 2 semi-circles and joining them at the side seams, with your zipper inserted into one of those side seams – please see our invisible zipper tutorial when you get to that part!

Example: Your waistline measures 66cm (equivalent of a 26″ waist). You are making a full circle skirt from 2 semi circles so you need to factor in the 4 raw edges that will be your 2 side seams. 66 + 1.5 + 1.5 + 1.5 + 1.5 = 72cm – this is your Circumference. 72 ÷ 3.14 =  30cm (diameter). Your radius is 13.5cm (half of your diameter minus 1.5cm seam allowance).

Tip: cut a piece of string the length of your final radius measurement and holding one end at the corner, use it to accurately mark out your curve.

Half circle skirt

Our waistline measurement now becomes a semi-circle, so in order to find the radius with our little equation we need to double the waist measurement.

Example: (66cm x 2) = 132cm (2C) ÷ 3.14 = 42cm (diameter). Your radius is 19.5cm.

Tip: before hemming your circle skirt it’s a great idea to put it on a mannequin and leave it to “drop” overnight. Seeing as parts of a circle skirt hang on the bias, they’ll need some time to stretch out naturally so you can then sew an even hem. If you don’t give the fibres time to drop, you’ll end up with a wavy, uneven hem line.

Quarter circle skirt

Just as we doubled the waistline as we halved the circle skirt, so we need to quadruple the waistline for a quarter circle skirt. The following diagram shows the quarter circle piece being cut from a single piece of fabric, no fold.

Example: (66cm x 4) = 264cm (4C) ÷ 3.14 = 84cm (diameter). Your radius is 40.5cm.

Phew… Broken out in a bit of a mental sweat there guys, and words like radius and circle have lost all meaning. We hope this has fully explained the maths behind constructing circle skirts and their variations – if not, let us know and we’ll do our best to clarify anything further. We leave you with the pretty fruits of our experiments…and one final tip: hemming a curve can be tricky; try our rolled hem tutorial for a quick, slick finish!

Over and out x

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18 comments on “Circle skirt maths – explained!

  1. Heather Lou
    September 6, 2012

    I’ve always wanted an explanation for the difference between a full/half/quarter skirt so THANK YOU! Also Elisa, loving your pink tips and those GIVE THEM TO ME purple bow pumps. SO into ladylike heels again – clunky platformy things are dead to me at the moment.

    • byhandlondon
      September 7, 2012

      Yay for pink hair! Going violet next… Nice and Autumnal. Only fun thing do do really, with long hair is to colour it I reckon 😉

  2. ooobop!
    September 6, 2012

    Brilliant explanation, thank you. I have made a full circle skirt before but never a half or quarter so this will come in really handy… when I eventually get round to it!

    • byhandlondon
      September 7, 2012

      So happy to be of help! And you know, I almost prefer half and quarters – quarter circle can be so slinky and elegant and half just a little less likely to blow up in the wind!

  3. Msanhle
    September 8, 2012

    Oh thank you so much for this explanation! Awesome 🙂

    • byhandlondon
      September 8, 2012

      Glad to be of service!!

      • Msanhle
        September 8, 2012

        Just pinned 🙂

        • byhandlondon
          September 8, 2012

          Yay spread the word! Thanks 🙂

  4. yesmissy
    September 8, 2012

    Hey Hun! I find your blog so inspiring I decided I nominated you for the 7 Things About Me Blog Award.
    http://yesmissy.com/2012/09/08/7-things-about-me/

  5. Marie
    September 9, 2012

    Aaah, excellent! I’m dying to try out a quarter circle skirt. Thanks for all the tips and for making thinks simple!

  6. LM
    September 16, 2012

    Hi Thankyou so much! Can you explain the easiest way to add the waist band?

    • byhandlondon
      September 17, 2012

      You’re most welcome! We’ve got a “perfect waistband” tutorial coming up real soon, so stay tuned for all you need to know…

  7. Chelsea
    September 25, 2012

    Great tutorial, thanks!

  8. sharon
    September 27, 2012

    for a full circle skirt the diameter assumed was 30cm so radius is 15 = 15-1.5= 13.5cms how did you get 14.5cm
    for a half circle skirt the diameter assumed was 42cms so radius is 21=21-1.5=19.5cms how did you get 22.5 cms
    for a quater circle the diameter assumed was 84 cms so radius is 42= 42-1.5= 40.5
    how did you get 4.5 cms
    also In all case you said to simply decrease by 1.5 cms whats the reason

    Please explain the above

    • byhandlondon
      September 27, 2012

      Thank you for bringing this to our attention!! You’re absolutely right – just goes to show my lack of mathematical talent! I will amend this immediately.

  9. craftylittlebugger
    September 28, 2012

    I am in love with your quarter circle skirt, and plan to make my own. I don’t have enough width in my fabric to cut it the length I want. Is it possible to cut it on the fold? I’m pretty sure I need to do some recalculations, I just don’t know what! Can you help?

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This entry was posted on September 6, 2012 by in DIY, Sewing tutorial and tagged , , , , , .

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