The Tin Foil Crotch Curve Method
You may have heard of the tin foil crotch curve method. There are some of us whose crotch curves are very different than standard drafting. If you have attempted smaller adjustments to your muslins and you feel that although the fit is ‘better’ it’s not where you want to be, this method can be very illuminating. However, you will need to have a basic understanding of how your body fits into the pants fitting adjustments. You can find a handy guide in identifying your individual needs HERE. It is a more advanced fitting skill, but I believe anybody can at least gain some insight into their body curves by giving it a try. Honestly! It’s just paper and fabric!
To make the curve, take a generous piece of tin foil (24” or so) and roll it into a long rope (a flex ruler works very well too). I put a clip on the tin foil and a rubber band on the flex ruler. We’ll use that in the next step.
Wearing a pair of leggings, thread the foil through your legs and mold it to your curves, front and back. Mark the foil with that clip (or rubber band if using a flex ruler) at the point where your inseams come together.
Carefully remove the foil or ruler and lay it on a piece of paper. Trace your curve on to the paper. Now repeat the entire process two more times. Draw each line on top of each other. You can see my 3 attempts here in the three different colors. I am going to cut the ‘average’ of the three. The thing to notice here is that although it varies quite a bit up at the top (because foil moves when you take it off the body), the lower front and back curves are all more or less the same. That is what I will be focusing on.
Now we’re going to get our pattern pieces ready. I’ve traced mine on tissue so that you can see better how I am going to do this.
You’ll need to line up your crotch points and overlap them by the seam allowance (in this case 3/8”). Do not worry about lining up your legs. It won’t happen and it will skew your curve. The overlap seam allowance needs to be flat. Otherwise you will end up with weird points or dips when you put your legs together. Place a little piece of tape there to hold it while we work.
Just for fun, place your tin foil or flex ruler over your pattern pieces for your first look at what kind of a crotch curve you have. This is mine. HAHA. Wow. That doesn’t look like it belongs there at all!
Before we start making pattern alterations, this is where we get a sense of what we’re going to be doing based on your first muslin. I have compared my drag lines to my Fitting Guide and have some sort of idea what I’m looking for. If you have not assessed your muslin to the Fitting Guide, go do that now. Go on. I’ll wait…
…Ok. Here’s what I know from my muslin:
1) I have extra fabric there in the front because my pubic bones are set further back than ‘average’ (flat pubis). Making that whisker look even worse is the fact that I have a low butt and full outer thighs (saddlebags – that’s a worse term than pubis, if you ask me). Bringing me to point two.
2) Looking to the back, you can clearly see that even though I graded out one size in my back pattern to accommodate my booty, it’s just not enough. The fabric is pulling and clinging in unflattering ways. I also have a sway back that makes things look weirder up at the top, but that’s independent of this particular tutorial.
Let’s see how the visual cues of my muslin compare to the foil crotch curve in front of me. I’m going to use my cut-out now so we can start to make changes.
Your crotch curve is NOT an exact science. You do NOT want to trace it exactly into permanence for ever and ever. It’s a guideline to illuminate your path to better fitting pants.
First, I’m going to draw a horizontal line at the crotch line across my entire pattern (the bright pink highlighter). Then a second line, this one vertical, from the top of the back rise, perpendicular to my first line. You can do the same to your front pattern if you are going to be need more extensive alterations up there. These are strictly guidelines so I can keep my curve on the flat and from getting too out of control.
Next, position your crotch curve so the inseams line up and rotate it a bit so that the bottom curve is flat-ish down there at the bottom. Trace it out. Mine is in RED marker.
I’m going to ignore what’s going on up on the rises. I already know I will have to address my back rise independently (the sway back). And my front fits great up top with the built in negative ease so I will leave that part alone. My focus is the bottom curves. Looking at the front there, I can see where my flat pubis is. I clearly need to flatten the front curve a tad. That tiny bit will make all the difference for me.
Aaaaand let’s look at that back. Yikes. No WONDER my pants look like that. That’s about 1 1/4” scoop beyond ‘average’. My booty is anything but average!
Now that I have a good idea of what changes I need to make, I’m going to begin smoothing and sketching. I’ll be doing this with GREEN. I am going to start with the front. I need to make that front curve approximately 1/4” shallower (translates to less fabric on my body). I’m going to smooth that curve up into the front rise.
That back… yowza. I’m going to add 1” into the scoop instead of the full 1 1/4”. I can always take more out later if I want to and my gut tells me I don’t need quite that much. Again, I’m not exact. Just using a bit of drafting intuition and smoothing the curve from the crotch point up to the rise. Ish.
Lastly, I need to add back the fabric I scooped out of my back crotch curve to the side seam. If you have a flat bum and are scooping out your curve, you may not need to add it back. Or not add as much. Like I said, trial and error. For me, I KNOW my ‘saddlebags’ (yuck) need the room, so I am going to curve that outseam back out by 1” to retain the width of the fabric.
*NOTE* Anytime you are extending or reducing outseams, inseams or crotch points, your leg seams will no longer match up. You will have to do some evening out before finishing your leg. It may not be noticable if your changes are small, but changes like this will absolutely throw it off.
That’s it. Make your next muslin and see what effect your changes made. You may want to make more. I went on to make my sway back adjustments, as well needing to slim my outer thigh back DOWN a lot more after my fullest point. But just LOOK how much better my second muslin is!
If you want to see me explain the process in action, check out this VIDEO.
Disclaimer – I am not comfortable on camera so I apologize in advance for any awkwardness and bad jokes.