I’ve found Google great for simple problems like this:
20 pounds per cubic foot in kilograms per cubic meter= 320.36926 kilograms per cubic meter
But wolframalpha also lets you combine unit conversions with calculations:
(2 * 175 grams) per square meter * (2*3 meters squared) + ((20 pounds per cubic foot in kilograms per cubic meter) * 3 square meters * 5 millimeters)
Or just click on this link: http://www.wolframalpha.com/input/?i=(2+*+175+grams)+per+square+meter+*+(2*3+meters+squared)+%2B+((20+pounds+per+cubic+foot+in+kilograms+per+cubic+meter)+*+3+square+meters+*+5+millimeters)
What’s the above tell me? It’s the theoretical weight of the kayak I’m currently building – about 7kg. In reality when hand laying fibreglass you get a much higher weight of epoxy rather than the one to one ratio that’s possible under vacuum application and which I’m using above.