This is the first practical step for all of the catsuit patterns. In the diagram, you can see a human figure with circumferences marked around its limbs at the joints and along the body. The idea is to take sufficient measurements to be able to transfer the shape/area of the skin to the cloth.
Draw a table of circumferences around your arms, legs, body, and neck; and the distances of those circumferences from the first circumference. Like this for an arm:
|Circumference||Distance from wrist|
|mid upper arm||279mm||368mm|
Much as I like inches, I would encourage you to measure in mm because it makes it much easier when you scale the measurements down for a skin-tight fit. If you prefer measuring in inches, multiply by 25.4 to convert to mm.
If you have a bust, or some exceptionally bulgy musculature, you should measure more circumferences in that area to capture the shape of your body. If the fabric's stretch isn't enough to compensate, you will need to add darts or gussets.
It would be best only to wear underwear (or something very thin and close-fitting) while measuring.
The pink lines running along the arms from the neck in the diagram demonstrate measurement of the raised and dropped arm length. The difference between these two lines is the length a sleeve's shoulder will need to be modified - the dart for a raglan sleeve, or extra sleeve cap (ease) for an in-set sleeve.
Don't make the topmost arm circumference too snug, or it will tend to cut into your underarm and acquire other common underarm characteristics. An extra inch will help.
For my geometric-style construction of raglan sleeves (and optionally for the surplice neckline), you will need to take two unconventional measurements of your chest at armpit level. Notice the oval with three lines above it. This oval is supposed to represent a cross-section of a chest at armpit level. The topmost line is half of the chest's circumference. The middle line is the width of the chest. The bottom line is the width of the chest that is within an inch of a ruler held against its front [which I now realise amounts to the distance between your nipples, if you're a guy]. (Of course, these measurements should be scaled like circumferences too.)
While bodies are mostly symmetric between left and right, they're decidedly not symmetric between front and back - particularly the neck/shoulders, chest, and bum regions.
Since spandex is stretchy, you can choose to ignore a few inches' difference between the front and back in the chest and bum regions, but you must take account of the neck/shoulders or your collar will be too low at the back or uncomfortably high at the front. Either take front and back measurements while wearing a comfortable T-shirt, from the T-shirt's relatively snug collar to your armpit line. (You can just measure the T-shirt itself if this is too hard.) Or you can just assume that a third of your neck circumference is at the back, and two thirds at the front.
If you want a particularly nice fit, you should take the chest and bum differences into account by measuring half-circumferences from one side of your body to the other and using those when drafting patterns: if plotting from a line running along the side of the body, use the appropriate back or front measurement. If you have a T-shirt with side seams, those seams can help with half-circumference measurement.
Bodysuits are supposed to be form-fitting. With spandex, this means that you make them smaller than full-sized in order that the body stretches the spandex to a nice wrinkle-free fit, but the garment must also allow a full freedom of movement, so there's a balance to reach.
It's best to measure the body circumferences and distances while standing straight (or at least while the bit of the body being measured is not flexed), because that's a pose that's easy to make consistent for more accurate measurements, but those measurements will need to be scaled (multiplied), to accommodate the body's flexing (as when the back is bent forwards, it grows longer) while not exceeding the fabric's ability to stretch.
The first step in scaling is to measure the fabric's stretch. This is determined by folding the cloth across the direction you want to gauge, and holding 10 inches (or centimetres) of taut but unstretched fabric between finger and thumb. You then pull one end out only so far that it still relaxes to 10 inches (centimetres). If you can stretch it by 2 inches (centimetres), it has 20% stretch.
Fabrics that stretch can often be overstretched: that is, you can pull them too far so they don't return to their original size. This results in baggy catsuits, which are unsightly. Generally, if you stop when you feel the fabric's resistance to stretching increasing, that will be fine. To avoid overstretching while being worn, it's probably best to pretend that your fabric stretches 1/10th less than it really does: that is, treat a 70% stretch as 70-7=63%.
Be aware that a fabric stretch of 70% can in principle sustain a scaling of body measurements down to 100%/170%=59%, but in practice you will use something looser.
Fabrics often stretch rather differently along their selvage (machined) edge as perpendicular to it. If your fabric is like this, then conventionally whichever is stretchier should go around the body.
Different parts of the body stretch differently: Touching your toes causes your back to become several inches longer (and shrinks your front by a similar amount), while bending your knee only lengthens its front by an inch (and shortens its back by an inch or so), and of course you can't bend your shin or thigh.
If you want a particularly snug fit all over, it may be worth changing the scaling factors for different parts of the body - most likely, the neckline may be looser than the rest of the body - but be careful to make any transitions smooth (to avoid baggy bits).
Bear in mind that if you make the longitudinal body scaling looser than the leg scaling, the crotch will drop, and if you make the longitudinal body scaling tighter than the leg scaling, the crotch will be tighter. Similarly, making the legs too tight will tend to pull the heel up the leg and scrunch your toes. Less obviously, if you make the knees/thighs tight, that will tend to anchor the leg cloth and pull the crotch down.
The circumferential scaling factor can be the same for most of the body. I tend to use a third of the fabric's stretch, but so long as you make the narrow bits stretchy enough to allow any wider bits to go through (eg the heel at the ankle, the calf at the knee), you should be fine.
If you're working for the tightest fit possible, you will want to take account of the length across the back of your shoulders changing when you bring your elbows together: If your fabric has a 70% stretch, and your shoulder-back changes from 18" to 26", the amount of fully stretched fabric measuring 26", when released, is 26"/170%=15". The circumferential scaling taking 18" of body measurement to 15" on the pattern is 15"/18"=79%. (But you can choose a looser scaling - nearer to 100% - if you prefer.)
In fact, you can scale a little more tightly than this (ie, pretend your shoulders' curved length is an inch or two shorter) because the tops of the shoulders will slide, relieving some of the stretch.
Most important: remember that you still need to breathe! It's unlikely that a chest expanding from 38" to 40" is going to strain the fabric, but a suit that tries to throttle you is not a good suit. I would advise scaling the neckline only half as much as its surrounding area - (79%+100%)/2 = 90%
To determine the lengthways scaling factors to use, you must work out how much your body stretches when you flex it, how tight you want the suit to feel, and any other factors such as appearance and practicalities (like getting into it). Once you've worked those out, you simply scale your measurements as appropriate before you start plotting.
The main flexion you should factor into your suit is your back length while touching your toes: the other movements - knees, ankles - are relatively trivial in comparison.
Measure the length of your back both when you stand straight, and when you bend over: With a straight back, hold one end of a tape measure just below your buttocks, and bring the rest over your shoulder behind your back, parallel to your spine, holding it level with the back of your neck. Keeping the end near your rear steady, read the length shown at your shoulder, then bend far as you're likely to while wearing the suit, letting the tape slide between your fingers at your shoulder, then read it again. (If you can't turn your head enough to read at the back of your neck, read a little further around the front of your shoulder, and subtract the extra from both readings)
So my back measures 31" straight, 36" bent. If my fabric has a lengthways stretch of 70%, then I need to know how much fully stretched fabric is 36" long (that's 36"/170%=21") in order to check that bending my back won't overstretch the suit's back (21" is smaller than 31", so it's OK). In fact, 36"/31" says that the fabric must have at least 16% stretch, which will work if I use a pattern scaling of 100%.
The tightest scaling I can use depends on the fabric. If 21" of fully stretched 70% stretch fabric is long enough (for my 36" bent back), I will have to scale my straight back measurements to 21". Since my back measures 31", scaling it by 21"/31"=68% will achieve this. In fact, scaling a little shorter than this will work, because when you bend, your front measurement will shrink, and the garment will slide somewhat at your shoulder and crotch.
My belly is 20" top to bottom when my back is straight and 16" (as best I can measure it) when my back is bent. If I want the suit's front to be taut when I'm sitting, say, I can't make it any longer than 16" (setting a loosest scaling 16"/20"=80%). The tightest scaling would be the theoretical tightest.
The tightest practical scaling for the back is 68% (the loosest is 100%), the loosest scaling for an unwrinkled belly (with a bent back) is 80% (the theoretical tightest is 59%). So I have to choose 68%-80%. (If I don't care about the belly being taut while bent, it's 68%-100%.)
If you're going for a tight suit, scale the legs longer than the body, because a snug crotch works best with taut fabric.
If you plan to use a surplice neckline, the fabric must have sufficient stretch available to allow you to draw the neckline over your rump, your chest (with your elbow while you're trying to slide your arm into the sleeve) and over your shoulder while one side of the neckline is at your underarm. In particular, for a single-surplice, the neckline and the length across the back of the garment will effectively form a doubled loop which must stretch once around the chest and once over the shoulder from the armpit. That will inform your circumferencial scaling: What length do you need to get in, how little cloth will stretch that far, and if your fabric is stretchy enough, what pattern scaling do you need to use? Remember: if it's hard to get into, it will be similarly hard to get out of, which could be embarrassing if needs are pressing.
Getting into a tight garment generally involves some wriggling around, so you would be best advised, for your first attempt at least, not to try for the very tightest fit lest it prove unwearable.
Your stitching - zig-zag, overlock, or something else - will have a natural stretch. For zig-zag, sew once to hold the seam for the next stitching, stretch the seams to break the thread, then sew again while stretching the fabric. For overlock, make sure the tension on the needle-thread(s) is correct (less for more stretchy fabrics). You will need to experiment with this.
Finally, if your fabric has a particular effect that is controlled by how far it's stretched, you will want to incorporate that factor into your calculations: some silver velour I bought has a gold backing which shows through when stretched beyond 15%; and some blue spandex I have is particularly shiny when stretched almost fully. Cosmetic factors shouldn't be allowed to interfere with making sure that you don't overstretch the fabric - indeed, since fabric with special effects is likely to be expensive, it's probably best to tend towards the loosest fit that will satisfy your requirements (if you can afford to make a potentially unwearable suit, then feel free to go for the tightest and please send feedback to let me know what you did and how it went! :)
I have little experience, hence little advice, regarding fabric that only stretches in one direction (so-called two-way stretch). Since I can readily buy full-stretch (four-way) fabric, I have little incentive to experiment with half-stretch; but the conventional wisdom, that the degree of greatest stretch goes around the body, seems counterintuitive to me for half-stretch. I'd be happy to write a page based on feedback from readers with experience of it, but I it seems to me that a half-stretch bodysuit would either be wrinkly or limit body motion.
To draft a full catsuit pattern, I normally use four A3 sheets of tissue paper (they're 10p each at my local art shop): two for the body, one for the neckline, and one for the sleeve. I usually have to cut spare bits off to extend the rectangles here and there.
I use a mechanical pencil, a ruler, and a trisquare to mark, a pair of compasses for drawing the circle for the neck, and a soft artists' rubber for mistakes (I generally find that the rubber that manufacturers put the other end of the mechanical pencil rips the tissue).
My cutting mat is squared - inches on one side, centimetres on the other - which is very useful.
I also have a french curve ('designer's curve') which I'm experimenting with.
It is important to clip small (3mm) notches perpendicularly into cut fabric edges to help make sure that edges that are sewn together are aligned. It's particularly important when the edges are only the same length, not the same shape, such as an in-set arm seam.
It's common practice to clip double notches on the back sleeve/body seam edge, to help distinguish it from the front sleeve/body edge.
I will try to mention where these notches should be made, but in general you will want one or two on any long curved edge where the ends of the edges are more than 18" away, or more frequently on short very curved edges, and also more frequently if two curves that are supposed to be sewn together don't have the same shape (such as the in-set sleeve).