Dream Hammock has an online calculator at http://www.dream-hammock.com/HammockCalculator.html. You can also check out this graphic they have at http://www.dream-hammock.com/ComfortRating.html on material:
It's all kind of specific to their products, but it should give you at least a general feeling for materials and their weight carrying capabilities. They also have Rope and Down calculators for anyone interested.
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