How To Calculate Towing Capacity

Towing a trailer can be a daunting task for someone who isn’t experienced with the process. Even the best of us still have to stop and think for a second before doing a maneuver or figuring weight balances. From fundamentals like steering from the bottom of the wheel to braking early and long, it’s easy to forget the most important part of trailer safety: weights.

Manufacturers love touting the towing capacity of their pickups and SUVs. The phrases “up to” and “maximum potential” are key, however. Just because that commercial said that your new truck can tow “up to” 15,000 pounds doesn’t mean your truck can actually tow 15,000 pounds. Each model and configuration is different. A single cab with a standard V8, six-foot bed, and rear-wheel drive will have a different towing capacity than a crew cab with a diesel, eight-foot bed, and four-wheel drive. Adding weight or changes in equipment will drastically affect actual towing capacity.

For this article, we’re assuming a bumper-style pull (hitch and ball) rather than other, heavy-duty options such as gooseneck or fifth wheel attachments. Those are a different ballgame in terms of how they affect your truck.

Calculating Towing Capacity for Your Rig

There are a few things you’ll need to know in order to calculate the actual towing capacity of your vehicle. Most of this boils down to common sense and there is a general rule of thumb that you can use to make life easier. But let’s do it the hard way first.

On the driver’s door of your vehicle will be a series of stickers. Things like the manufacturing date, tire sizes and inflation numbers, and so forth are all on those stickers. You’re looking for the gross combined vehicle weight rating (GCVWR). This is the maximum vehicle weight, including trailer, the vehicle is safely rated for. It’s required by law, so if your vehicle has a tow rating, that number will be there.

The GCVWR is the maximum total weight your vehicle can be. It includes the vehicle’s weight, the trailer’s weight, the weight of everything in the vehicle and on said trailer, etc. It’s the total, maximum load. Somewhere near that will be your vehicle’s gross vehicle weight (GVW) or gross vehicle weight rating (GVWR). Subtract that from the GCVWR and you will have the maximum weight, minus your vehicle’s weight. That’s now the weight of your trailer and all the stuff and people in your vehicle and trailer overall. You cannot exceed that number safely.

The easiest way to calculate how much you can tow is by simply adding up all of the stuff you’ll be hauling and towing and subtracting it from that total towing number. Right?

Not so fast. How much weight do you, your family, your stuff, and so on total? Before you go asking Grandma to step on the scale and get the bloody nose that’s likely to be answered with, let’s instead do some simple calculations using some rules of thumb.

If your vehicle is rated to tow 6,500 pounds (common for SUVs) and your boat weighs 4,000 pounds with its trailer, you’re good to go. Right? Yes, unless you add another 1,500 pounds in cargo to that boat. Nevermind the people in the SUV with you. Woops.

Rule of Thumb: Total Weight Calculation

Figuring out the total weight of the trailer and your cargo and passengers is easiest if you just work with a couple of simple assumptions. First, assume that your unweighed cargo (people and their stuff) is at least 10 percent of your total capacity. So if your vehicle is rated to haul 5,000 pounds in people and trailer, assume you have 500 pounds of people and their stuff. The other 4,500 pounds is how much you can tow. Adjust this as needed to account for extras you might have to carry, such as wheelchairs or that secret cooler full of after-hours beverages you snuck in.

Rule of Thumb: Total Towing Capacity Calculation

Without a tow rating, it’s likely that your vehicle should not be towing a trailer. But sometimes the tow rating is lost or difficult to find (such as on an older pickup or converted passenger van). In that case, the fastest way to figure out the total towing capability of a given vehicle is to take its GCVWR and subtract its curb weight minus another 20 percent to account for cargo capacity (unrelated to towing). We subtract cargo because most vehicles have both a cargo rating and a tow rating and they are not to be combined. Cargo capacity is also what’s used to figure safe tongue weights for your vehicle.

Curb weight is the weight of the vehicle with a full tank of fuel and all fluids required for operation. We’d also add the driver to the mix. This weight is usually listed inside the door or in the owner’s manual and will assume a driver weight of about 150 pounds. You can find it for sure on your own by weighing at a truck stop (see below). Combine this with an empty trailer for a second weight and you’ll have both the curb weight of your truck, SUV or van and a tare weight for your vehicle plus its empty trailer. Useful stuff to know.

Your tow rating will simply be the GCVWR minus your curb weight minus its cargo rating (likely about 20 percent). Assuming your trailer hitch is rated for whatever that total might be.

Trailer Hitch Size Matters

There are five classes of trailer hitch when using a ball hitch (aka “bumper pull”). They start with Class I and move their way to Class V. Each has a weight rating for both how much total weight is being pulled and how much tongue weight is allowed. Pulling weight is the total weight of the trailer plus its cargo. Tongue weight is how much weight the connecting portion of the trailer (the part going over the ball hitch attached to the vehicle) can press down on the back of the vehicle doing the towing.

Trailer Hitch Classes

• Class I - rated for 2,000 pounds of pulling and 200 pounds on the tongue
• Class II - 3,500 pounds of pulling and 350 pounds of tongue weight
• Class III - 5,000 pounds of towing and 800 pounds on the tongue
• Class IV - 10,000 pounds of towing and 1,200 pounds of tongue weight
• Class V - 20,000 pounds of towing and 2,000 pounds on the tongue

Most vans and SUVs have a Class II hitch included with any towing add-ons. Most full-sized pickup trucks will have a Class IV and most heavy-duty trucks and vans will have a Class V. Any trailer hitch setup should also have a plug included to run the lighting and, potentially, the trailer’s independently-operated brakes. Note that most trailers with independent brakes will have both a braked and unbraked load capacity. The braked capacity means the trailer’s brakes are connected to the towing vehicle and can operate. This is usually the higher of the two ratings. Unbraked is, obviously, the capacity without those trailer brakes being engaged.

Knowing what type of hitch you have is important because it could reduce or change your vehicle’s towing capacity accordingly. Note that a higher-capacity hitch does not increase the towing capacity of the vehicle it’s attached to. But a lower-capacity hitch will reduce total towing capability.

Tongue Weight Calculations

Calculating tongue weight is another important part of towing. When you see a truck or SUV towing a trailer and doing the “squat” thing, that means the vehicle is overloaded. Most trailers will include an estimated tongue weight chart on their chassis alongside the other tire and GVW stickers. That could be a handy guide. Your trailer’s tongue weight should not exceed your vehicle’s cargo rating.

To figure out tongue weight on your own, though, requires more math and a lot of known variables. If the trailer has just one axle, it will have more tongue weight than a trailer with two axles. That’s a matter of physics, with the simple fulcrum of a single-axle trailer tilting more weight forward or rearward and the more complex fulcrum of a dual-axle trailer balancing the load more evenly. In general, a single-axle trailer will likely have a small cup meant for the smaller ball hitch found on a Class I or II hitch. A two-axle trailer will be sized for a Class III or larger ball.

Calculating tongue weight can be done with measurement or some guesswork. Measurement can work if you know a few things about your truck or SUV. Measure the height of the bumper or hitch receiver when nothing is attached. Then measure again when the loaded trailer is attached. The change in height, relative to the ground, can tell you the approximate tongue weight. Knowing how much that change in height means requires that you know the suspension setup and its tolerances. Most likely you do not know these things. So you’ll go with guesstimation. That’s much easier.

Rule of Thumb: Trailer Tongue Weights

On a single-axle trailer, about a third of the trailer’s total weight, with its cargo evenly distributed, will be on the tongue. So a 2,000 pound trailer and cargo will put about 660 pounds onto the tongue. A dual axle trailer will put about one quarter of the total weight onto the tongue. In this case, 500 pounds. This rule of thumb errs on the side of caution, but works pretty well in the real world.

Other Useful Rules of Thumb

Loading the Trailer

When loading your trailer, if possible the heaviest items should be in the center above the trailer’s axle(s). With lighter items to front and back. This even distribution rule will maximize your weight carrying capability while minimizing stress on the vehicle and its trailer and also keep tongue weights low.

Use a Truck Scale

The best way to know your total vehicle weight and trailer load weight and distribution is to use a truck scale. Found at most truck stops, scales cost a few dollars to use and allow you to use them twice for the same charge. So you can pull on with an empty trailer and get an empty (tare) weight and then load the trailer and get a loaded weight. Or you can weigh a loaded truck and trailer and then redistribute the load for better balance and weigh in again to see how you did. If possible, we highly recommend using a truck scale if you’re even moderately nervous about your load.

How Much Towing Do I Need?

You should assume that however much towing you plan to do regularly (trailer, cargo) is larger than you think it is. Add 10 percent at minimum to your expected needs. Fifteen or twenty is better. This gives you a safe buffer zone. It’s better to have too much towing capacity than too little and being right on the dot might sound important, but it’s not. More is better and safer.

Terms to Know

Terms you should be familiar with for towing include:

• Curb Weight - The total weight of a vehicle with all of the fluids and fuel required to operate. Sometimes includes a driver as well.
• Dry Weight - The total weight of a vehicle without fluids or fuel. Often used as the weight measurement for shipping vehicles.
• GVWR - Gross Vehicle Weight Rating. This is the maximum amount of weight a vehicle can hold while sitting still. Mostly important for things like rooftop campers and trailers used for storage as well as towing. This number is different, and usually higher, than the moving weight capability used for towing and hauling.
• GTW - Gross Trailer Weight. The total weight of the trailer and its cargo. This is what should be used when deciding if your vehicle can tow the trailer safely. Your vehicle’s towing capacity should not be less than GTW.
• GCVWR - Gross Combined Vehicle Weight Rating. Discussed above, this is the total weight your vehicle plus all of its cargo plus a trailer plus its cargo can be at maximum.
• GAWR - Gross Axle Weight Rating. This is the maximum amount of weight one axle can support. Most vehicles will have a GAWR listed for individual axles while trailers will often just have one rating for all axles. A pickup truck, for example, will have a higher GAWR for its rear axle than it will for the front.
• Payload - The payload capacity of a vehicle. How much weight it can carry inside it or in its cargo bed. This is also what’s used to measure against tongue weight. Tongue weight should not exceed payload capacity.
• Tongue Weight - The total tongue weight is the amount of weight the trailer and its cargo is pressing down on the connection to the towing vehicle. It should be measured against payload capacity to ensure it does not surpass the vehicle’s safety measures.