The towing number in a truck's brochure is only one of several limits. A trailer that is under the advertised capacity can still overload the truck's rear axle or exceed the combined weight the drivetrain is rated for. Safe towing means checking every limit and obeying the lowest. This calculator takes the three that matter most — the combined weight limit (GCWR), the payload limit set by tongue weight, and the manufacturer's own rating — and tells you which one governs and how heavy a trailer you can actually pull.
How to use the towing capacity calculator
- Find the ratings in the owner's manual or on the door-jamb label: GCWR, GVWR and curb weight.
- Enter the weight you will carry in the vehicle: passengers, cargo and any hitch equipment.
- Enter the manufacturer's towing rating, if you know it, and the tongue weight percentage of your trailer.
- Read the limits. The maximum loaded trailer weight is the smallest of the three, and the tongue weight is shown for that trailer.
To work out payload alone, see the payload calculator; for the weight on the hitch, the tongue weight calculator. Heavier loads also stretch stops — try the stopping distance calculator.
Towing capacity formula
Payload limit = (GVWR − curb weight − passengers and cargo) ÷ tongue weight fraction
Maximum trailer = smallest of GCWR limit, payload limit and the manufacturer rating
Tongue weight = maximum trailer × tongue weight fraction
- GCWR — gross combined weight rating: vehicle + load + trailer
- GVWR — gross vehicle weight rating: the vehicle with its load, including tongue weight
- Tongue fraction — usually 0.10–0.15 for conventional trailers
Worked example
- GCWR limit: 14,000 − 5,200 − 400 = 8,400 lb.
- Payload left: 7,000 − 5,200 − 400 = 1,400 lb; at 12% tongue weight the trailer can weigh 1,400 ÷ 0.12 = 11,667 lb.
- Manufacturer rating: 9,000 lb.
- The lowest is the GCWR limit: 8,400 lb loaded trailer, with a tongue weight of 8,400 × 0.12 = 1,008 lb.
Tongue weight by trailer weight
These figures come from multiplying trailer weight by the usual 10%, 12% and 15% tongue fractions.
| Loaded trailer weight | Tongue weight at 10% | at 12% | at 15% |
|---|---|---|---|
| 3,000 lb | 300 lb | 360 lb | 450 lb |
| 5,000 lb | 500 lb | 600 lb | 750 lb |
| 7,000 lb | 700 lb | 840 lb | 1,050 lb |
| 9,000 lb | 900 lb | 1,080 lb | 1,350 lb |
| 12,000 lb | 1,200 lb | 1,440 lb | 1,800 lb |
Why the lowest limit is the one that counts
Towing ratings exist because different parts of a vehicle fail in different ways. The frame, axles and tires carry the payload; the engine, transmission and cooling system are sized for the combined weight; brakes, hitch and suspension have their own ratings. A heavy trailer fits within the GCWR but may still put the rear axle over its limit once the tongue weight is added to a loaded truck bed. People often find that payload, not the headline towing number, is what stops them — especially with a crew-cab full of passengers and gear.
The tongue weight fraction also matters for stability. Too little tongue weight (under about 10%) lets the trailer sway; too much overloads the hitch and lifts the front wheels. Weigh a loaded trailer's tongue at a scale or with a tongue-weight scale if you can. Distribution hitches can help level the vehicle but do not change the ratings.
The calculator also treats the manufacturer's rating as a ceiling. If the hitch receiver, ball or brake controller is rated lower than the vehicle, the lowest-rated part is your real limit. When in doubt, tow lighter, and always check the numbers on your own vehicle's labels, not a brochure.
Sources
The 10–15% tongue-weight guidance for conventional trailers (15–25% for fifth-wheels and goosenecks) is quoted in Mazda's owner's manuals (trailer tongue load of 10–15% of total trailer load) and in the Blue Ox towing guide. The relationships between GCWR, curb weight and trailer weight follow the manufacturers' definitions, summarised on BoatUS's “Tow capacity math” page. Table values were computed from the formulas shown.
Frequently asked questions
What is GCWR?
Gross Combined Weight Rating is the maximum allowed weight of the loaded vehicle plus the loaded trailer together, set by the manufacturer. Curb weight, passengers, cargo and the trailer must all fit inside it.
Why does payload limit towing?
The tongue weight presses down on the hitch and counts against the vehicle’s payload, along with passengers and cargo. A truck with a high tow rating but a small payload can run out of payload first, as the towing guides listed below point out.
How much tongue weight should a trailer have?
For conventional bumper-pull trailers, 10–15% of the loaded trailer weight; fifth-wheel and gooseneck trailers carry more, typically 15–25%. Too little invites sway, too much overloads the rear axle.
Does this replace the manufacturer’s rating?
No. The result is capped by the manufacturer’s towing rating if you enter one. Hitch, brakes, tires and axle ratings can lower the real limit further, and the lowest rating always rules.
Should I tow at the maximum?
Staying 10–20% under the maximum gives margin for hills, wind and braking. See the stopping distance calculator for how weight lengthens stops.