Firstly, welcome aboard boatdesign.net
A pontoon boat is, for the purposes of load rating, considered a multihull vessel. In Canada, the relevant standard is TP1332 (1999) section 4, paragraphs 61-72 at http://www.tc.gc.ca/marinesafety/TP/...htm#multi-hull
American standards are closely harmonized with Canadian ones. There should be an equivalent American standard but it will probably be harder to read and will produce virtually identical results.
You will find the Canadian standard to be quite readable and the calculations easy to do by hand.
Some will undoubtedly say in response here "just take the volume of the pontoons, multiply by the density of water, there you go". That's essentially how the standard works, with some added criteria. It is better to follow the accepted government standard, because (a) it is what the Coast Guard will hold you to if you're stopped by them, (b) the authors of the standard have already computed appropriate safety factors and outlined how to perform and evaluate the stability tests, which makes your job a lot easier and safer, and (c) they give you the formulae and definitions, and outline exactly how to do it so that it's nearly impossible to screw up if you passed grade 8 math.