RV Cargo Capacity Calculator: How Much Can Your Trailer Carry?

Everything you may load into a trailer is its GVWR minus its empty weight — and water counts. On a 7,000 lb trailer that weighs 5,900 lb empty, that is 1,100 lb for water, food, gear and propane together.

When the numbers come out over, the useful question is which change fixes it — and the one to check first costs nothing: tow with the fresh tank empty.

The budget is one subtraction

GVWR — gross vehicle weight rating — is the most the trailer is rated to weigh with everything in it. Take the empty weight away and what is left is your budget.

The empty figure is the one to be careful with. A scale ticket is the only number that describes your trailer as it actually sits, with whatever has been fitted to it since it left the factory. If you only have the label, use it, and treat the margin the calculator gives you as the most you have rather than the least.

What your trailer’s federal label already says

This is not our convention. An RV trailer’s cargo capacity label is set by federal rule — 49 CFR 571.110, S9.3.5 — and it has to carry two lines:

THE WEIGHT OF CARGO SHOULD NEVER EXCEED: XXX kg or XXX lbs

CAUTION: A full load of water equals XXX kg or XXX lbs of cargo @ 1 kg/L (8.3 lb/gal)

So the label itself tells you water is cargo, and puts a number on your full tank. The calculator uses 8.34 lb a gallon, which is the same figure unrounded.

Water is the biggest thing you forget to count

Water weighs 8.34 lb a gallon, so a 40 gallon fresh tank is 334 lb — on the trailer in the example above, nearly a third of everything it may carry.

It is also the easiest weight on the rig to remove. Tow with the tank empty and fill it when you arrive, and those pounds come off the budget for the whole drive. Nothing to buy, nothing to leave at home.

If you boondock and have to arrive with water, our water tank calculator will tell you how little you can get away with carrying.

The battery lever is real, and smaller

Lead-acid batteries are heavy, and this is one place where the upgrade really does save weight. The manufacturers’ own figures:

	Renogy 100Ah AGM	LiTime 100Ah lithium
Weight	63.9 lb	21.9 lb
Usable	about half	published as 100%

Weights from each manufacturer. Lead acid counted at half usable, the same assumption our lithium vs AGM comparison and battery calculator use.

Matching one lithium battery on usable capacity takes two of those AGM batteries — nearly 128 lb against 22. That frees about 106 lb.

Lithium has good reasons to exist — usable capacity, cycle life, voltage that holds up under load — and our lithium vs AGM comparison covers them. Weight is a pleasant side effect, not usually the reason.

The mattress is the other item people forget to weigh. Lippert publishes its RV mattresses from about 40 lb for an 8-inch short queen to 65 lb for a 10-inch king, so replacing one with a larger or thicker one is a real payload change — and getting the size right in the first place is its own problem, because “RV King” is published as three different sizes.

GVWR is not the only limit

A trailer can be inside its GVWR and still over something else. This calculator does not check:

• Axle ratings — the load can be inside GVWR and still sit too heavily on one axle.
• Tire load ratings — and tires have an age limit as well as a load one, which the tire age calculator reads off the sidewall.
• Tongue weight and your tow vehicle’s payload — covered by the towing payload calculator.
• Tire pressure for the load — on a trailer there is nothing to work out: Lionshead says trailer tires “should always be inflated to the maximum PSI indicated on the tire”. A motorhome is different, and our tire pressure calculator sets it from weighed axle ends.
• What the loaded trailer sits on at the site — leveling blocks carry a rating too, 40,000 lb for Tri-Lynx’s and 30,000 lb for Andersen’s leveler, and the leveling block calculator works out how many of them a sloping site needs.

It gives you a margin in pounds. It will not tell you a rig is fine to tow, because one number on one label cannot.

Where this goes wrong

1. Leaving water out of the load. The commonest one, and often the largest single item.
2. Using a brochure weight when a scale ticket exists. The trailer you own is the one that has to fit inside GVWR.
3. Forgetting propane. Count what is in the tanks with everything else — the calculator does not estimate it for you. Weigh a cylinder and take off its empty weight; our propane calculator does that sum and shows what a lighter cylinder saves.
4. Buying lithium to fix a weight problem an empty tank would fix for nothing. The tool puts both levers side by side for exactly this reason.

Questions worth answering