Kayak load calculator
Kayak makers list a maximum capacity: the most weight the boat can float, not the weight it paddles well with. A common guideline is to keep your total load at or under about 70% of the maximum for good handling.
This is a guideline, not a safety rating. Source for the 70% rule of thumb: Brooklyn Kayak Company, "Clarifying kayak weight capacity"; REI's kayak buying guide also says to count you, your gear and anything you carry. Capacity plates required on small boats by the Coast Guard don't apply to canoes and kayaks, so use the maker's number.