Down Pants/Jacket Help

It will be more than you would think. Let's assume you want pants that have 1" of loft, and you want them to have some room for ease of movement. Simplifying the shape of your legs to circular cross section, allowing for 1" of loft and 1/2" all around the leg for ease of movement means allowing for 1.5x2x3.14= 9.42" more circumference than the leg itself. Sounds like a lot but that's what it takes. If you are shooting for less loft you need less additional circumference. It might be worth it to make a pair of test pants out of cheap fabric the size that you think will work, and then try them on over some down pants in a store if you can. If they are tight over the down pants then your pattern is too small; if they are loose then it is too big. Might help. Patterns for insulated clothing are tricky – I made some polarguard insulated pants and pullover, and despite having other insulated garments to measure for the basis of my patterns, I still had to make adjustments as I went.

You're basically making two pairs of trousers. There's the inner pair that should fit you as normal, though if you're planning them as over pants, they'll be a quite a bit larger than normal; and there's the outer pair, which are how much ever loft you're adding away.

I've never done this for down, but I have for batted insulation. The basic step is figure out how much loft you want (easy with batts, you don't get a choice. Say it's an inch. That will increase the diameter of things by two inches (an inch each front and back), which will increase the circumference by 2 pi, a tad over 6 1/4 inches. There's no need to add ease: that should be part of the basic, inner pair's dimensions. you can add half that to each side of the pattern pieces. At the waist and hems, you'll need to add the whole amount, or add some to the inner piece, so the edges meet. Also probably need to add a seam allowance, or change what the planned waist is. And add the baffles, etc.

You doing side zips?

Or at least trickier than I thought. Instead of cutting two differently sized inner and outer panels, couldn't I split the difference and make them identically sized? My plan was to make sewn through baffles rather than internal ones w/3 oz of 900 fill down.

Yes, I want to do side zips as well for easy on over boots.

Splitting the difference might leave you with not enough room for full lofting and too snug a fit. You can definitely have both pieces of fabric ( inner & outer) the same size, but it has to be the size that the outer needs to be. Then the inner layer will just fold and fill in extra space – works fine. But important to get the size of the outer layer right to allow enough room for loft and ease of movement.

Ok, so if I factor 2xPI for the desired loft of 1.5" gives me 9.42" of additonal circumference to figure in. Doing so takes me on the pattern from an XS (35" hip) to a L (~48" hip). Should I cut back at the waist slightly to minimize the bunching in the waist band or leave it as is and loft away?