I do believe you measure down by weighing it. You'll need a good digital scale you can zero out and a container for the down. Weigh the container, zero out, and put in down until you have the ounces required for your application. I have never seen down measured in cu. in. It sounds impossible.

Beware of recycled down - it's probably a lot less warm than new 800 or 900 fill power down. It probably has less loft and more feathers to work their way through the shell material. I don't know if it's worth the effort to try to harvest down from old comforters - I thought about it, but visions of chasing fluff around the room - yikes. If you've never worked with down consider that it gets EVERYWHERE. Some do it yourself instructions I've seen say to sit in your closed tent in your living room to avoid coating all your belongings, and prepare to be coated yourself. It would only be a "deal" (IMO) if you were picking up the old comforters for less than ten bucks apiece, which I guess is possible in a thrift store.... To me, it would be worth it to get premeasured portions for a little more money over which I could keep better control of the stuff - Speer sells high quality down for twenty six bucks per 3 oz bag. (scroll down to the bottom) There are other places like Thru HIker that sell down or a down quilt kit. If it were me I'd buy a kit from Thru Hiker for $150 and have a nice Momentum and down quilt.
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