And I do mean pretty darn exact. Check out this photo of my finished hoops with my draft laid over the top for comparison!
You can use the following method to create any type of round hoop, whether cone shaped, bell shaped or a crazy undulating shape. The key is that only round style hoops can be done like this; bustle/hoop combos, elliptical hoops and panniers won't work with this method. But if the silhouette of your intended hoop design is identical from the front, sides and back then this will work for you.
To start, you're going to need a largish sheet of paper, a ruler or an architect's scale, a pencil and the following measurements in inches:
your waist to floor measurement
your corseted waist measurement
desired circumference of hoop (optional; you could just draw what you like and go with the resulting circumference)
desired distance between floor and bottom edge of hoop
We're going to make our drawing to scale; that is, it will be drawn to the exact measurements we need, only smaller. For instance, you could draw it so that 1" = 1/4". If I am 65" tall and want to draw my figure in 1/4" scale I would draw it 16 1/4" tall. Choose the largest scale that will allow you to fit a figure your height on your paper. You'll be taking measurements later, and it's harder to take accurate measurements off a tiny drawing. Once you've picked what scale you'll be drawing at, stick with it all the way through. I'll be using my real life measurements as examples, but assume that from here on I'm converting them to scaled measurements when I actually draw them (so when I say I drew a line 65" up from the floor line, I'm actually drawing it to my scaled height of 16 1/4").
I've already got my figure drawn to scale, but here's how I got there-
To begin with, draw a line at the bottom of your page to indicate the floor. You'll also want to draw a vertical line down the center of your page so you can center your figure and hoops exactly. The top of your figure's head will be [your height in inches] above the floor. In my case, I'm 65" tall, so I drew a mark 65" up from the floor to indicate the top of my figure's head.
Make another mark where your waist will be using your waist to floor measurement. My waist to floor measurement is 41", so I measured 41" up from the floor and made a mark.
You can guestimate the diameter of your waist by dividing your corseted waist measurement by π (3.14). Usually your waist is more of an oval than a circle, but a corset tends to compress your waist on the sides more than front and back, making your waist very close to a circle. My corseted waist measurement is 25". Divided by π, I get a diameter of 8", so I drew a 8" wide line centered at waist level (green line in the above picture).
At this point I sketched in my figure from the waist up to that mark I made for the top of the head. I got a bit fancy and drew undergarments and whatnot, but don't worry too much about details. It's also no big deal if you can't draw people very well; the point is to just get a very rough idea of the shape of the body above the hoop.
I know I want my hoop skirt to have a total circumference of 95". I can find the diameter of my bottom hoop by dividing 95" by π, which gives me 30.25". I also want my hoop skirt to be 6" off the floor, so I drew a line 30.25" wide 6" up from the floor (make sure it's centered!). I added in some feet to complete my figure.
Now you can draw the shape of your hoop skirt! So long as it starts at each end of the line you drew for your waist, finishes at each end of the line you drew for the base and is symmetrical you're golden. I made sure my shape was exactly the same on both sides by drawing it on one side, then copying it to the other side by folding the paper in half on the centerline and tracing it. Whatever shape you draw is exactly what you'll get, so take some time to get a shape that pleases you. I wanted a very bell type shape reminiscent of the early 1850's just before hoops were invented, when layering multiple petticoats gave women's skirts a shape just like an upside-down U.
Once you've got your overall shape down, you'll need to draw in where your hoops go. Keep in mind that you can space hoops farther apart in areas where there's little change in shape, but you'll need to space them closer together in areas where the shape changes drastically. In the drawing above, the most drastic shape change takes place in the top half of the hoop skirt, so those hoops need to be closer together to maintain that shape. Also consider the overall size of your hoop skirt. Larger hoop skirts will require more support than smaller hoop skirts. I once made a giant hoop with a nearly 180" circumference that failed because it only had 6 hoops holding it up. It's better to err on the side of caution and add too many hoops than to have to few and watch your hoops collapse under the weight of your skirts!
Now that you've got your hoop skirt drawn as you like it, you'll start taking measurements. You are going to measure each space between hoops at the side edge of your hoopskirt (see photo above for example). Don't forget to measure the distance between the top hoop and your waist too! Measure straight from the edge of one hoop to the next, disregarding the curves. Remember, you don't want to measure in the middle of your drawing or you'll end up with too little space between your hoops. In the photo above, look at the area between the top hoop and the waist. If measured on the edge, the result is 5". If measured in the middle, it's 2". That's a big difference! You can write these measurements directly on your drawing or put them on another sheet of paper, but make sure you have them for later when you make your actual pattern.
Next you'll measure the length of each of the lines you drew for your hoops. This will be the diameter of each hoop. You'll only need these numbers long enough to find the circumference of each hoop.
Now that you have the diameter of each hoop, you can easily find the circumference by multiplying the diameter x 3.14. The result is the length each of your hoop bones needs to be (well almost; we're going to add a bit extra for overlap when we actually make the hoop skirt, but ignore that for now), so write each one down next to its corresponding hoop for easy reference.
Now you've got all the measurements you need to make your pattern! You can draw your pattern out on paper full size, or you can save time and draw it directly on your fabric.
First, you'll need to make a rectangle as long as the circumference of your bottom hoop (so 95" in my case) and as high as all of the measurements in between each hoop combined (in my case, 6+6+6+6.25+3.25+3.5+2.25+5= 38.25).
You can also draw in lines for each of your hoops, using the measurements you took in between each hoop to space them properly (see picture).
Add however much you want on each side for your seam allowance (yellow area in picture).
Add a bit on the bottom to fold up to make a neat hem (green area in picture). If you add enough on the bottom, you can also use it to encase your bottom hoop.
Add enough on the top to fold down to make a casing for a drawstring (pink area in picture). Ta-Da! You're ready to start sewing up your hoop skirt.
I made this pattern at TechShop. I've just posted the instructions on how to use this pattern and sew up your hoopskirt here.