A Friendly Guide to Tiered Skirt Maths

When I read sewing blogs I’m interested in learning new sewing techniques, how to make French seams or sew knit fabric or trace pattern pieces, that kind of thing. I don’t notice lovely explanations of how to calculate things so much, because, err, numbers are my friends (after all I have spent a ridiculous 8 years at Uni playing around with them, so we got to know each other a little).

So when I wrote about making a tiered skirt (here), I concentrated on the (rather rough and ready) sewing and forgot about the calculations, until I read Beth’s comment. So Beth, this tutorial is for you.

So, say you want to make a skirt …

Geeky maths drawing of a skirt

Geeky maths drawing of a skirt

Ok, so maybe you don’t want to make that skirt, but if you use your imagination the red line is the waistband, the yellow is the hem (the skirt is spread out flat on the floor), and the blue line is the length. Lets work out some numbers.

First choose your length (blue line), get your tape measure out and measure your favourite skirt or yourself/chosen victim. So far so easy.

Next, work out how big the waistband needs to be (the red line). This is actually the size of the waistband casing. To enable the skirt to get pulled on and off properly this needs to be the hip measurement plus 1 inch (2.5 cm) ease. If it were any smaller we’d need to start faffing about with zips or something, rather than nice easy elastic.

Now onto the hem. Here there is some choice, depending on how full you want your skirt to be. The absolute smallest it could be would be the same hip measurement plus 1 inch ease you’re making the waistband, which I guess would make some kind of pencil skirt with an elasticated waist.

The fullest kind of skirt you’re likely to want is a full circle (my daughters ideal), so lets work out the measurements of the fullest that you’d want it to be. Ready for some maths? I’ll hold your hand. We need to start by working out the distance from the waistband to the middle of your body, but don’t worry, I wont be talking about different types of fat therein and trying to sell you amazing new diet foods. Just find the calculator app on your smart phone and divide your waistband measurement (that’s the hip measurement plus 1 inch, remember) by 6.3 (if you’re interested what we’re doing here is finding the radius of the waistband circle by dividing the circumfrence by twice pi). Now add this answer to your chosen length (to find the radius of a full circle hemline), then multiply the answer by 6.3 (approximately 2 pi again) to give you the hem circumfrence of a full circular skirt of your chosen length.

If you want a half circular skirt, your hemline will be half this length, for a quarter circle skirt, a quarter of this.

Quick recap time, with an example for a child about 5:

  • I choose my length of 12 inches
  • I measure hips at 20″, so the waistband casing needs to be 21″
  • Then I divide 21 by 6.3 to get 3.3 (rounding it to one place after the point)
  • Adding 3.3 to my 12 inch lengthgives me 15.3
  • 15.3 times 6.3 is 96.4
  • So for a full circle my hemline is 96.4, for a half circle it’s 48.2 and for a quarter circle it’s 24.1, although I would round these to 96 and a half inches, 48 inches and 24 inches respextively for my sanity (plus my cutting is not that accurate).

Next, lets work out the panel sizes for a skirt like this. I’ll assume you have 3 panels, but it’d be easy to adapt for 2 or 4. Choose how deep you want each panel, for my 12″ example I’m going to have each one an equal 3″ deep, but you don’t need them all the same depth. My panels are going to look something like this (imagine they’re laid next to each other before gathering and seaming).

No, not building blocks, panel pieces.

No, not building blocks, panel pieces.

Start with the bottom panel. The width of this panel needs to be your hem circumfrence (24″/48″/96.5″ in my example above depending on how twirly you feel) plus your preferred seam allowance each side. The height needs to be your panel depth (mine is 3″) plus seam allowance at the top and hem allowance at the bottom. So with a half inch seam and hem allowances I would cut a piece 4″ by 97.5″ (for a full skirt on my example). You could also cut 2 panels at half the width and have 2 side seams, if so add seam allowance at each side to both of them. So I’d cut 2 pieces both 4″ by 49 and a quarter inches.

Top panel next. This will be gathered in at the waistband, so if there’s a difference between this and your actual waist the easy option is to stick to the hip plus ease measurement plus seam allowances for the width. 22 inches in my example. Or if you want to stick to a full circle circle then you can use the waistband radius that you found (my 3.3″ above), add it to your top panel depth (3″) and multiply by 6.3 (i.e. ( 3.3 + 3 ) × 6.3 = 40″ ). Plus seam allowances of course. If you’re not adding a seperate waistband casing but folding over the top of your panel piece to make the waistband then add this to the depth. So mine would be 3″ (depth of panel) plus 2″ (twice my waistband depth) plus 1″ (two lots of my half inch seam allowance), which makes 6″.

Finally the middle panel. The height is just the panel height plus twice your seam allowance. Easy option, add the width of the bottom and top panels together and divide by two to get a width half way in between. Or for the full circle method, waist radius plus depth of top two panels multiplied by 6.3 (e.g. ( 3.3 + 6 ) × 6.3 = 58.5″ ).

Still with me? Now you can make your own tiered skirt. If you have a question please put it in the comments box below.

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