How this is calculated
Four separate limits govern a tow setup, and a trailer has to clear all four. Most calculators check one — usually the tow rating, which is the one least likely to bind on a half-ton pickup. Here is each in full.
1. Payload — the door sticker
Everything you add to the truck counts: people, luggage, the dog, the hitch hardware, and the trailer’s tongue resting on the ball.
Payload used = passengers + cab cargo + bed cargo + hitch hardware + tongue weight. The limit is the figure on the yellow door-jamb label.
2. GVWR — the truck on a scale
Same arithmetic, measured from the truck’s own weight rather than from the sticker.
Truck weight = curb weight + passengers + cab cargo + bed cargo + hitch + tongue weight, checked against GVWR.
3. GCWR — the whole rig
Tongue weight is not counted twice: it sits on the truck, so the combined figure is simply the truck plus the trailer.
Combined = curb weight + everything in the truck + loaded trailer weight, checked against GCWR.
4. Rear axle — GAWR RR
This is the one nobody models, and on a bumper-pull it is frequently the first to go. The hitch ball sits three to four feet behind the rear axle, so the axle acts as a fulcrum: tongue weight presses down behind it and levers weight off the front wheels. The rear axle therefore carries more than the tongue weight itself.
Rear axle load ≈ (curb weight × rear share) + bed cargo + 35% of the people and cab cargo + tongue weight × transfer factor. The transfer factor is taken as 1.25 on a plain ball and 0.90 with a weight-distribution hitch, which is what a correctly adjusted WDH is for — it pushes load back onto the front axle and the trailer axles.
Trailer weight and tongue weight
Loaded trailer = dry weight + your cargo + fresh water + propane + batteries. Water is 8.34 lb per gallon and propane 4.2 lb per gallon, so a full 40-gallon fresh tank is 334 lb and a pair of 30 lb propane bottles is about 59 lb. Dealers quote dry weight, which is the trailer before any of that.
Tongue weight = loaded trailer × tongue percentage. Conventional trailers sit between 10% and 15%; 13% is a fair planning default. Fifth wheels and goosenecks sit at 20% to 25%, and that load lands over the axle rather than behind it.
Reverse mode
Switch to “find my max” and each limit is rearranged to solve for trailer weight instead of checking one. The smallest of the four answers is the real ceiling, and the gauges then show the rig loaded to exactly that point — which is why one gauge reads full and the other three show slack.
Worked example — the figures this page opens with. A crew-cab half-ton: door sticker 1,750 lb, GVWR 7,050 lb, GCWR 17,100 lb, rear GAWR 3,800 lb, scale curb weight 5,420 lb with 44% on the rear axle. Four people at 160 lb, 120 lb in the cab, 250 lb in the bed, 100 lb of weight-distribution hitch.
Trailer: 6,200 lb dry, 900 lb of gear, 40 gallons of water (334 lb), 14 gallons of propane (59 lb), 60 lb of battery. Loaded trailer = 7,552 lb. At 13%, tongue weight = 982 lb.
In the truck before the trailer: 640 + 120 + 250 + 100 = 1,110 lb. Add the tongue and payload used is 2,092 lb against a 1,750 lb sticker — over by 342 lb. On the scale, 5,420 + 1,110 + 982 = 7,512 lb against a 7,050 lb GVWR — over by 462 lb.
Combined weight is 5,420 + 1,110 + 7,552 = 14,082 lb against 17,100 lb, so GCWR has 3,018 lb to spare — the number the salesman will quote. The rear axle carries 2,385 + 250 + 266 + 884 = 3,784 lb against 3,800 lb: legal by 16 lb, which is inside the error of every estimate on this page.
Run it backwards and the ceiling is a 4,000 lb loaded trailer, set by GVWR. The window sticker says 13,200 lb.
Why the brochure number isn’t your limit
Since the 2015 model year, essentially every light-duty tow rating sold in the United States has been tested to SAE J2807. The standard is a genuine improvement — before it, manufacturers set their own test conditions and the numbers were not comparable between brands. What it does not do is describe your truck on your holiday.
J2807 loads the test truck with a driver and one passenger at 150 lb each, plus the hitch hardware. That is the whole payload. No children, no dog, no cooler, no generator, no firewood, no bikes on the tailgate. The rating that comes out the other end is the trailer weight that truck could pull up the Davis Dam grade and stop from 60 mph in that configuration.
Two more things get lost by the time it reaches a window sticker. The advertised maximum usually belongs to the lightest version of that model — regular cab, two-wheel drive, the tow package and little else — because a lighter truck has more payload to spend on tongue weight. And the rating describes the drivetrain’s capability, not the chassis’s. The engine and transmission really can pull 13,200 lb. The door sticker is what says whether you may.
The full explanation is here, including why three-quarter-ton trucks behave differently.
Why payload and GVWR can disagree
In theory the door sticker payload is GVWR minus curb weight, so the first two gauges should always agree. In practice they often don’t, and the difference is worth knowing.
The sticker was printed at the factory against the truck as it left the line. Everything bolted on since — bed liner, running boards, tonneau, bull bar, tool box, fifth-wheel rails, a second battery — is now part of the curb weight but was never subtracted from the sticker. A truck that weighs 120 lb more than the factory assumed has 120 lb less real payload than its own label claims.
If you enter a curb weight measured on a scale, the GVWR gauge is the more honest of the two and it will read tighter. If you enter the brochure curb weight, the two agree exactly. The tool flags the gap whenever it appears.
What this does not account for
- Front axle rating. A heavy tongue on a plain ball can unload the front axle enough to hurt steering and braking long before anything is technically over. A WDH is the fix; a scale ticket is the proof.
- Tire load ratings. Axle ratings assume the tyres fitted at the factory. Different tyres, or under-inflated ones, can be the real limit.
- Hitch receiver and ball mount ratings. These carry their own weight-carrying and weight-distributing maximums, stamped on the receiver.
- Trailer axle ratings and GVWR. This page checks whether the truck can take the trailer, not whether the trailer can take what you put in it.
- Where the load sits. The rear-axle figure is a planning estimate that assumes a typical crew-cab layout. Weight ahead of the axle behaves differently from weight behind it, and this cannot see your truck.
- Brakes, licensing and speed limits. Trailer brake requirements, licence class thresholds by combined weight and towing speed limits are set by state and are not modelled here.
- Grade, altitude and heat. Being inside every limit is not the same as being comfortable at 6% and 100°F with a headwind.
Common questions
The salesman says my truck tows 13,200 lb. Is he lying?
Not exactly — he is quoting a real number that answers a different question. The tow rating describes what the drivetrain can pull with a test payload of two adults. Your payload is what decides which trailer you can hitch to it, and on half-tons payload runs out first almost every time. Ask him to show you the door sticker instead.
Does tongue weight really count against payload?
Every pound of it. The tongue rests on the truck’s hitch, so the truck carries it exactly as it carries a passenger. On a 7,500 lb trailer at 13% that is roughly 975 lb of payload gone before anyone gets in.
Will a weight-distribution hitch give me more payload?
No. It redistributes load between the axles — that is what brings the rear axle back inside its rating and restores steering feel — but the tongue weight still sits on the truck. Payload, GVWR and GCWR are unchanged. It solves an axle problem, not a payload problem, and it is not optional on most trailers over about 5,000 lb.
What about airbags, helper springs or a bigger tyre?
They level the truck and improve how it drives. They do not raise GVWR, GAWR or payload, because those are ratings of the frame, axles, brakes and wheels — not of the springs. A level truck that is 400 lb over is still 400 lb over.
Should I use dry weight or GVWR for the trailer?
Neither, if you can avoid it. Dry weight is the trailer before water, propane, batteries or a single fork, and no trailer travels that way. Trailer GVWR is the most it could ever weigh, which is pessimistic unless you genuinely load to the limit. Build it up from dry weight the way this page does, then weigh it once and use the ticket forever.
My tongue weight percentage — where did 13% come from?
It is the middle of the 10–15% range that keeps a conventional trailer stable. Below about 10% is where sway starts and it is genuinely dangerous; above 15% you are eating payload for no benefit. It is a planning figure, not a measurement — how you load the trailer moves it, and a full fresh-water tank at the nose moves it a lot.
What happens if I tow over the limits anyway?
Mechanically, the parts that complain first are brakes and tyres, which is also the worst possible pair. Legally, an overweight rig can be cited at a weigh station, and in a collision an insurer or an opposing lawyer will find the numbers before you do. Nothing about being over is a cliff edge — it is a steady erosion of margin at exactly the moment you need margin.
How do I use a CAT scale?
Pull on with the steer axle on pad one, the drive axle on pad two and the trailer axles on pad three, and buy a ticket. Then unhitch, park the trailer, and re-weigh the truck alone. The difference between the two truck totals is your true tongue weight. Plate IV above prints the card with your own target numbers on it.