How do you calculate how much plastic you need? My greenhouse is going to be 35' x 60' arch type and 12' tall.
You need to give more information at to the shape of your GH. Does it have straight walls? Is it like a quanset hut and a true 1/2 circle? If it has straight walls how tall are they?
1eyedJack and the Dawg
Here is a link to a greenhouse area calculator. It is primarily for heater calculations, but it gives the surface area, which is the amount of plastic needed. Just subtract any parts not to be covered with plastic.
Here is a link that might be useful: greenhouse surface area calculator
Doubt whether help provided by that link, Cuestaroble. If Salvage Queen measures the arch all his problems would be solved; well not quite. I don't have an exact answer either but most references recommend that for any arch type structure you simplify the math by ignoring the height and using the width as the diameter (35 ft. in your case).
Then the width of film needed would be calculated by the formula: 1/2 * 3.14 * 35 = 55. Naturally, if you order a 56' wide sheet of film it will leave you with a few extra feet on each side. The next smaller size that I've seen available is 52' and that won't be enough unless you have raised kneeboards. I use 48' wide sheets on my 30' structure with a little to spare.
My only advise- Don't cut yourself short in the width.