Great question — answering as the UphillTech team:
Add the area of the ceiling by the area of the walls and then factor in some extra for waste. So if you have 8' ceilings, those walls would be 1120 sqft themselves. Add the 490 sqft for the ceiling would give 1610 sqft. Then add in a factor for waste. The less experience you have, and the more angles and corners you have to cut, the more waste you'll have. I think 5% waste is on the low end for our projects, but we have lots of closets, halls, and inexperienced volunteers doing the cutting. That would give you 36-38 sheets of 12x4 (1610 / 48 = almost 34, with 5% waste you get 36, 10% waste gives you 38). Before you order, since this is a basement, if there's any mold or mildew risk, get...
Full answer by BMitch: https://diy.stackexchange.com/a/8525 (CC BY-SA 3.0).
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