Rich LaSalle/Getty Images
Calculating payload capacity requires you to know your vehicle's GVWR (gross vehicle weight rating) and curb weight.

So how are you supposed to know how much payload you can afford to carry in your car or truck? To calculate your vehicle's payload capacity, you only need to know two things: your vehicle's gross vehicle weight rating (GVWR) and its curb weight.

Gross vehicle weight rating is simply the maximum allowable weight of an entire vehicle when it's packed up and ready to go -- that includes the weight of the vehicle itself, all of the necessary fluids for operation (gasoline, diesel, oil and so on) and any cargo, passengers and tools. A vehicle's GVWR is calculated by its manufacturer, and you can typically find it in your owner's manual or on the vehicle's doorframe, near the door latch. You might also find your vehicle's GVWR on Web sites that list these numbers.

Curb weight is simply how much the vehicle weighs on its own, without any cargo or passengers. This measurement includes a full tank of gas and any other fluids that keep a car running.

So, a vehicle's payload capacity -- the amount of stuff it can safely carry after you've filled up the tank with gas and topped off all of the fluids -- is just a matter of subtraction.

Gross vehicle weight rating - curb weight = payload capacity

For example, if you have a car with a GVWR of 3,500 pounds (1,588 kilograms), and its curb weight is 3,000 pounds (1,361 kilograms), you would subtract 3,000 pounds from 3,500 pounds.

3,500 pounds (GVWR) - 3,000 pounds (curb weight) = 500 pounds (227 kilograms) of payload capacity

Do you need to take into consideration whether your vehicle's a car, truck or SUV? Read on to find out.