Originally Posted by MAD777
Actually you don't. It's insulation ability is determined not by the actual loft that is restricted by baffle height, but the effective loft that the volume of down would create if unrestricted. At least until you start doubling the compression.
If you sew two pieces of fabric together to yield a 6"X40" chamber after allowing for the seam, that chamber would then be filled with down. Were the chamber to be completely filled, it will transform from a flat 6" wide two dimensional object into a cylinder. As the cylinder expands, the 6" dimension will contract until it becomes a circle. Two 6" pieces combine to make a 12" circumference. That 12" circumference will then yield a tube roughly 3.8" across.
Taking the radius of 1.9 squared X pi X 40, that will give you a Volume of 453 cubic inches ( roughly 1/2 ounce of 900 fill down) with no compression.
Now if I only took the length and width into consideration (in this case 40"X6"), how would I derive the amount of down required to fill the chamber without first calculating the volume?
And that is not even taking a diff cut baffle into consideration.