When I started out making my DIY down-filled underquilt, I knew I would need to calculate my overall material size to have a sufficient amount of outer shell material to accommodate the "lumps" inherent in the differential design, as well I would have to calculate the required amount of down to fill the baffle volume. The Red River Gorge Underquilt How-To provides some nice starter calculations for fill volume but lacks the means to calculate the outer shell area. As tends to be habitual for engineers, I fired up Excel and created a spreadsheet for calculating material requirements and down fills for underquilts based on the cross-sectional area of the baffles.
The sheet allows you to specify your finished quilt dimensions and baffle design, and will return material dimensions for construction and the amount of down (for an input fill power) required to fill it. Down fill can be adjusted via the Over/Understuff field or by directly altering the "Total Oz. of Down" field.
Two experimental features are also included:
- An approximation of the quilt's overall weight (minus suspension) based on the material sizes, fabric weights, and down, with 15% allowance for stitching and seams.
- A VERY experimental calculation of the quilt's theoretical temperature rating, based on the published design specs for some Underground and HammockGear quilts.
I've attached the sheet for the community's perusal, please give it a look and feel free to suggest any improvements to be made. If you want to un-protect the sheet to play with the formulas, the password is "hammockforums".