A Weight Distribution Hitch Calculator compares tongue weight, gross trailer weight, and stamped hitch ratings, then evaluates spring bar tension recovery using ride-height inputs.
Ride Height Squat Test (optional)
Calculate Tongue Weight Ratio and Spring Bar Tension for Your Weight Distribution Hitch
This tool checks whether a trailer’s tongue weight sits in the safe operating range for a weight-distribution hitch, compares that weight against the hitch’s rated capacity, and — if ride-height measurements are entered — calculates how much of the induced axle squat the spring bars have actually recovered. It’s built for anyone hitching a travel trailer, cargo trailer, or camper to a ball-mount weight-distribution system before towing.
Entering Your Trailer, Tongue, and Hitch Weights
Enter Gross Trailer Weight, Measured Tongue Weight, and your hitch’s Max Tongue and Max Gross ratings in pounds or kilograms, plus six optional ride-height measurements in inches or centimeters. Results show tongue weight ratio, hitch capacity utilization, and spring bar tension recovery; ratios are unit-agnostic, and absolute conversions use 1 lb = 0.453592 kg and 1 in = 2.54 cm.
- Using the trailer’s advertised or “dry” tongue weight from a spec sheet instead of a real scale reading taken with the trailer loaded the way it will actually be towed.
- Entering the hitch’s weight-carrying (WC) rating instead of its higher weight-distributing (WD) rating in the Max Tongue and Max Gross fields, which understates how much capacity the system actually has once the spring bars are engaged.
- Swapping the “bars slack” and “bars set” rear height readings in the squat test, which flips the sign of the recovery percentage and produces a result that looks like a math error but is actually two fields entered in the wrong order.
How Tongue Weight Ratio and Hitch Capacity Are Calculated
Tongue weight ratio compares the measured tongue weight to the total loaded trailer weight:
$$\text{Tongue Weight Ratio} = \frac{\text{Measured Tongue Weight}}{\text{Gross Trailer Weight}} \times 100\%$$
The 10–15% target band used to flag this ratio as safe is a widely-cited towing-industry convention, not a formal test requirement — it’s grounded in SAE J2807’s baseline assumption that 10% of trailer weight sits on the tongue for conventional towing, and is documented in trailer and hitch manufacturer guidance such as Weigh-Safe’s towing education material.
Below roughly 10%, a trailer is prone to sway; above roughly 15%, the tow vehicle’s rear axle and hitch can become overloaded even if total weight is within the trailer’s rating.
Hitch capacity utilization compares the same two inputs against the hardware’s stamped ratings:
$$\text{Tongue Capacity Used} = \frac{\text{Measured Tongue Weight}}{\text{Hitch Max Tongue Rating}} \times 100\%, \qquad \text{Gross Capacity Used} = \frac{\text{Gross Trailer Weight}}{\text{Hitch Max Gross Rating}} \times 100\%$$
This check follows SAE J684, which requires every hitch to be permanently marked with its maximum trailer GVWR and maximum vertical tongue weight allowed — the two numbers this calculator asks for as Hitch Max Gross Rating and Hitch Max Tongue Rating.
If the six ride-height fields are filled in, the tool also calculates spring bar tension recovery at the rear axle:
$$\text{Recovery \%} = \frac{(\text{Rear}_{baseline} – \text{Rear}_{barsSlack}) – (\text{Rear}_{baseline} – \text{Rear}_{barsSet})}{\text{Rear}_{baseline} – \text{Rear}_{barsSlack}} \times 100\%$$
In plain terms, this is the share of the rear squat — caused by hitching the trailer without tension — that the spring bars restore once set. A target of recovering 50–100% of that squat is, again, an industry convention rather than an SAE figure: it comes from weight-distribution hitch manufacturer installation literature (Equal-i-zer/Lippert’s setup instructions and similar guidance from other WD hitch makers), not from a published engineering standard.
A less obvious wrinkle: the front and rear axles don’t always agree. The calculator reports rear recovery percentage and front axle height delta as two separate results because a setup can show 100% rear squat recovered while the front axle is still sitting above or below its baseline height — front and rear springs rarely respond at the same rate to spring bar tension, so both numbers need to be checked, not just one.
Because tongue weight and squat readings shift with cargo placement, fresh water and propane levels, tire pressure, and road surface, treat these results as a setup guide and re-verify at a certified scale rather than a one-time check.
Every weight field must be a positive number, and Gross Trailer Weight must be greater than Measured Tongue Weight — if it isn’t, the calculation halts rather than returning a nonsensical ratio, since no conventional single-axle-behind-the-hitch trailer can carry 100% or more of its weight at the coupler.
Realistic Gross Trailer Weight inputs run from a few hundred pounds for a small utility trailer up to roughly 20,000 lbs at the top of the Class V hitch range; values outside that span will still compute but no longer reflect a real towing setup.
For the squat test, the rear “bars slack” reading must show at least a small measurable drop from baseline (more than about 0.05 in) before a recovery percentage can be calculated — if the rear doesn’t visibly squat when the trailer is hitched without bars, the tool reports the measurement as indeterminate instead of dividing by a near-zero number.
Reading the Ride-Height Squat Test Measurement Points
Trailer Hitch Class Ratings by Gross Trailer Weight and Tongue Weight
SAE J684 requires a hitch to be marked with its own maximum trailer weight and maximum tongue weight, but it does not publish a numbered class lookup table. The Class 1–5 figures below are a widely used industry convention documented by hitch manufacturers and retailers, useful for sanity-checking what to enter in the Max Tongue and Max Gross fields if the numbers aren’t stamped on your hitch.
| Hitch Class | Weight-Carrying GTW | Weight-Carrying Tongue Weight |
|---|---|---|
| Class 1 | 2,000 lbs | 200 lbs |
| Class 2 | 3,500 lbs | 350 lbs |
| Class 3 | 6,000 lbs | 600 lbs |
| Class 4 | 10,000 lbs | 1,000 lbs |
| Class 5 | 20,000 lbs | 2,000 lbs |
Classes 3 and 4 commonly step up to 10,000 lbs / 1,000 lbs and 14,000 lbs / 1,400 lbs respectively once a weight-distribution system is added — always use the rating stamped on your specific hitch rather than the class number alone.
Weight Distribution Hitch Setup Questions
What counts as a safe tongue weight percentage?
10–15% of gross trailer weight for a conventional ball-mount trailer, an industry convention grounded in SAE J2807’s 10% baseline test assumption. Below 10% risks sway; above 15% can overload the tow vehicle’s rear axle and hitch.
Should I enter my hitch’s weight-carrying or weight-distributing rating?
Whichever mode you’ll actually tow in. A weight-distribution setup can raise a hitch’s usable ratings above its weight-carrying numbers — use the figure stamped on the hitch per SAE J684’s marking requirement, not a class number.
Why does the calculator ask for six ride-height measurements?
The optional squat test compares baseline, hitched-without-bars, and hitched-with-bars heights at both axles — the only way to confirm the spring bars are actually restoring axle load rather than judging it by eye.
My tongue weight ratio looks fine, so why does the front end still feel light?
Check the front axle result separately. A trailer can land inside the 10–15% ratio yet still leave the front axle under-loaded if spring bar tension is too low; the ratio and the squat test measure different things.
Can I use this for a fifth-wheel or gooseneck trailer?
No — the 10–15% target and math here apply to conventional, ball-mount, frame-hitched trailers. Fifth-wheel and gooseneck trailers carry pin weight over the tow vehicle’s rear axle and typically run 15–25% of gross trailer weight.