Tow weight calculation?
 

All boat weights are dry.

So weight of boat +
Total actual weight of trailer (nothing to do with its rating).
+30-50 lbs. per battery.
+8 lbs. per gallon of water (fresh water, waste water)
+6 lbs. per gallon of fuel
+tools, fenders, ropes, lines, ANCHORS, etc. etc.
(remember the spare tire and anything in the trailer tool box adds in too)
It adds up FAST!

Remember, your passengers, coolers, ice, tools, spare tires, jacks, etc. etc. all adds into your GCWR (Gross Combined Weight Rating) of your vehicle as well. If you load up your tow vehicle with all of the aforementioned in this paragraph, you DECREASE the amount you can tow by that much as well.

Find a scale! Truck stop, grain elevator, salvage yard, gravel pit, might even find one at a landfill. Should not be more than $20. I dropped mine from the truck, but you could do the combined and then go back and weigh just the tow vehicle. It just takes all the guess work out, and all the posted numbers always seem off, dry weight does not even come close to actual trailer, fluids and gear. I think my dry weight was 4,200 LBS and the boat with fluids (Minus the drive) and trailer, no gear was 7,200 LBS.