Why the smallest limit governs
For combined weight, subtract the loaded vehicle weight before adding trailer tongue load from GCWR. Separately check manufacturer towing, hitch limits and the GVWR allowance for tongue weight. Total trailer weight already includes its tongue share, so do not add that share again in the GCWR calculation.
trailer limit = min(GCWR − vehicle, manufacturer tow rating, hitch trailer rating, (GVWR − vehicle) ÷ tongue fraction, hitch tongue rating ÷ tongue fraction)
Example with several constraints
With 14,000 lb GCWR and a 6,000 lb loaded vehicle, the combined allowance is 8,000 lb. A 7,000 lb GVWR and 10% tongue share allow 10,000 lb through the vehicle-weight constraint. A hitch rated for 7,500 lb trailer weight and 750 lb tongue weight instead limits the result to 7,500 lb.
Other checks before towing
The tongue percentage is your input assumption, not a recommendation from the tool. Check actual axle weights, tires, trailer brakes, vehicle configuration, manufacturer instructions and load distribution. The result is only an upper bound from the listed constraints; it does not establish trailer compatibility.