Methodology & Sources

Every calculator on this site uses fixed physical constants and simple, widely accepted formulas. We do not use vehicle databases, so the results depend entirely on the ratings you enter from your own vehicle, hitch and trailer.

Weight ratings used

GVWR (Gross Vehicle Weight Rating): maximum loaded weight of a vehicle or trailer.
GCWR (Gross Combined Weight Rating): maximum weight of the tow vehicle and trailer together.
GAWR (Gross Axle Weight Rating): maximum load on one axle.
Curb weight: vehicle with standard equipment and full fluids, without people or cargo.
Payload: GVWR − curb weight.
Tongue / pin weight: the share of trailer weight carried by the hitch.

In the US, GVWR and GAWR appear on the federally required certification label, usually in the driver’s door jamb.

Industry standards, in general terms

SAE J2807

SAE J2807 is a recommended practice from SAE International that describes how manufacturers should determine a vehicle’s towing rating. It sets a standard test vehicle weight (including driver and passenger allowances and the hitch), and requires the combination to pass tests such as acceleration on level ground and up a long grade, launch on a hill, handling, braking and the structural strength of the hitch area. Many truck and SUV makers rate their vehicles to it, so ratings are more comparable between brands. Our calculators do not reproduce these tests; they apply the resulting ratings to your actual load.

SAE J684

SAE J684 covers trailer couplings, hitches and safety chains for vehicles under 10,000 lb GVWR, including strength testing. The familiar Class I–V receiver hitch classes are an industry convention built around it. Ratings vary by manufacturer, so the label on the hitch always takes priority.

Formulas

Real towing capacity = GCWR − curb weight − passengers − cargo, capped at the manufacturer’s tow rating if lower.
Payload left = (GVWR − curb weight) − passengers − cargo − tongue weight.
Gross combined weight = loaded vehicle + loaded trailer, compared with GCWR.
Tongue weight range = 10% to 15% of loaded trailer weight (bumper-pull).
Pin weight range = 15% to 25% of loaded trailer weight (fifth wheel / gooseneck).
Load per trailer tire = (loaded trailer − tongue weight) ÷ number of tires; minimum rating adds a 15% default margin.
Fuel cost = distance ÷ towing mpg × price (or distance × L/100 km ÷ 100 × price).
Axle loads (lever rule about the rear axle): tongue weight T at overhang OH changes the front axle by −T × OH ÷ wheelbase and the rear by T × (wheelbase + OH) ÷ wheelbase.
Weight distribution: the spring bar moment needed to restore a share R of the lost front-axle load is R × T × OH × L ÷ (L + OH), where L is the ball-to-trailer-axle distance; M ÷ L moves to the trailer axles.
Tongue weight by lever (bathroom scale) = (reading − beam) × support span ÷ jack-to-block distance.
Ball mount drop = receiver height − expected squat − coupler height.
Gooseneck pin weight = 20% to 25% of loaded trailer weight.
RV cargo carrying capacity = GVWR − UVW − full fresh water − full propane − 154 lb per sleeping position (motorhomes), as used on RV weight labels.
Trailer cargo capacity = trailer GVWR − empty weight; axles and tires carry the total minus tongue weight.
Load position: an item of weight w at distance x from the ball adds w × (L − x) ÷ L to the tongue.
Bulk load weight = volume × bulk density.
Ramp angle = asin(height ÷ ramp length); breakover angle = 2 × atan(2 × ground clearance ÷ wheelbase).
Tie-downs: aggregate working load limit ≥ 50% of cargo weight, at least two; a tie-down attached to the cargo counts half its WLL.
Boat ramp: water depth along a ramp grows by sin(atan(grade)) per unit of distance.
Stopping distance = speed × reaction time + speed² ÷ (2 × deceleration), with deceleration scaled by the share of weight that has working brakes.
Power on a grade = v × m × g × (sin θ + Crr × cos θ) + ½ × ρ × CdA × v³, divided by drivetrain efficiency.
Engine RPM = mph × axle ratio × gear ratio × 336.13 ÷ tire diameter (in); effective ratio = axle ratio × old diameter ÷ new diameter.
Tire diameter = rim × 25.4 + 2 × width × aspect ÷ 100 (mm).
Trailer drag = ½ × ρ × Cd × frontal area × v²; power = drag × v.
Range and trip time: range = fuel × mpg; time = distance ÷ average speed + stops.

Constants

Quantity	Value
Water (fresh, gray, black)	8.34 lb/US gal (≈ 1.0 kg/L)
Propane (liquid)	4.2 lb/US gal (≈ 0.50 kg/L)
Gasoline	6.1 lb/US gal (≈ 0.73 kg/L)
Diesel	7.1 lb/US gal (≈ 0.85 kg/L)
1 lb	0.45359237 kg (exact)
1 US gallon	3.785411784 L (exact)
1 mile	1.609344 km (exact)
US mpg ↔ L/100 km	L/100 km = 235.215 ÷ mpg
Gravity (g)	9.80665 m/s²
Air density (sea level, 15 °C)	1.225 kg/m³
Tire rolling resistance	0.012
Drivetrain efficiency	85%
Road grip used for braking	dry 0.7 g, wet 0.5 g, gravel 0.4 g, snow 0.2 g, ice 0.1 g
Reaction time (default)	1.5 s
Bulk densities	gravel 105, crushed stone 100, sand, dry 100, sand, wet 120, topsoil, dry 80, topsoil, wet 100, clay / fill dirt 110, compost 45, wood mulch 25, concrete, wet 150, firewood, seasoned (stacked) 27, firewood, green (stacked) 39, water 62.4 (lb/ft³)

Densities vary slightly with temperature and blend; these rounded values are the ones commonly used for weight planning.

Hitch class ratings used

Class	Max gross trailer weight	Max tongue weight
Class I	2,000 lb	200 lb
Class II	3,500 lb	350 lb
Class III	8,000 lb	800 lb
Class IV	10,000 lb	1,000 lb
Class V	20,000 lb	2,700 lb

Status thresholds

Towing capacity checks: OK below 80% of capacity, “at the limit” from 80% to 100%, “exceeds” above 100% (the common 80% towing rule).
GVWR, GCWR, payload and hitch class checks: OK below 90%, “at the limit” from 90% to 100%, “exceeds” above 100%.
Range checks (tongue and pin weight): “too low”, “in range” or “too high”.

Testing

The calculation code is separate from the page code and covered by automated tests for every formula, including edge cases such as zero, negative and empty values and mixed imperial/metric input.