A few things to note while using this calculator:
This calculator estimates the load local to your area. If you need certified calculations for your structure, you'll need to work with your own engineer. This calculator is only meant to serve as a reference, not an official measurement of snow load local to a given location.
This calculator uses the rough calculation of the slope of a 10,12 and 20 foot high tunnel. It uses your location to find the ground snow load as a baseline.
Most snow load problems can be avoided by using 4 foot spacing between hoops and keeping the plastic tight. When plastic is allowed to sag, it can accumulate rain and snow. Water weighs 8.34 pounds per gallon, which can add up quickly over a large structure.
Here is the calculator: