Water is the weight problem
Ask someone what the heaviest thing in their camper is and they will say the batteries, or the generator, or the toolbox. It is the water. At 8.34 pounds per gallon, a 40 gallon fresh tank is 334 pounds, a 60 gallon tank is 500, and a 100 gallon tank on a larger coach is 834 pounds — more than five adults. Add a full 6 gallon water heater and the grey tank you have not dumped yet and the figure climbs again.
Set that against what you are actually allowed to add. Take the plate rating, subtract the unloaded weight, and the difference is everything you may put in: water, propane, people in a motorhome, food, bedding, tools, bikes and the solar you fitted last spring. On the default figures here that difference is 1,800 pounds, and a full fresh tank alone consumes 334 of it, or 18.5 percent. On a smaller trailer with a 900 pound capacity and a 40 gallon tank, water is 37 percent of the allowance before anything else goes aboard.
That arithmetic is the whole reason experienced owners travel with the fresh tank near empty and fill at the site. It is not fuel economy and it is not sloshing. It is that the tank is competing directly with the things they wanted to bring.
The unloaded weight in the brochure is not your coach
Manufacturers publish a dry weight for a base configuration. What arrives on the lot has options, and what leaves your driveway has three years of accumulation. The gap is routinely hundreds of pounds and it always runs in the same direction. Batteries, a second air conditioner, an awning, a bike rack, a generator, a solar array and its wiring, a lithium upgrade, a residential fridge — none of those are in a brochure number and all of them come out of your capacity.
The way to close the gap is a scale. Public scales at truck stops, grain elevators and landfills will weigh a loaded rig for a few dollars, and the ticket replaces every estimate on this page with a measurement. Weigh it the way you actually travel, tanks and all, and weigh each axle separately if the scale allows it. What you learn is usually not that you are far over; it is that you are closer than you thought and that the margin is in the wrong place.
Every rating caps you independently
| Where the limit is printed | What it caps |
|---|---|
| Vehicle certification label | The whole vehicle loaded, and often each axle separately |
| Axle tag or label | What that one axle may carry |
| Tyre sidewall | What that tyre carries at its stated pressure |
| Hitch, coupler and ball | Trailer weight and tongue weight through the connection |
| Tow vehicle towing and combined figures | The trailer, and the two together |
These are separate ceilings, not a single number expressed five ways, and the lowest applicable one is the one that governs. A trailer within its own rating can still sit on tyres that are not, and a tow vehicle inside its towing figure can be over on payload because the tongue weight lands there. This calculator works one vehicle against one rating. The rest of the chain is on you to check on the labels themselves, and no online figure substitutes for what is stamped on the equipment you own.
Where the capacity actually goes
Once people weigh a loaded rig, the list of surprises is fairly consistent. Tools and spares run heavier than remembered. Canned food and drink is dense and accumulates in lockers nobody opens. Books, tool boxes and cast iron are small and heavy. Two full propane bottles are around 80 pounds of gas and steel. Firewood bought at the gate is unaccounted for by definition. None of these are individually large; the total is.
The fix that costs nothing is to weigh a few loaded totes on a bathroom scale before the next trip and put the real figure in the gear field above. People who do this once usually find their estimate was low by several hundred pounds, and that number is exactly the margin they thought they had.
Related pages
For the tow vehicle side of the same load, use the towing and payload calculator and the trailer tongue weight calculator, with the trailer axle load calculator for the per-axle figure. The water in the equation is sized on the RV holding tank duration calculator, and the propane weight on the RV propane consumption calculator.
Questions people ask
Should I travel with the fresh tank full or empty?
It is a weight question and the numbers above answer it for your specific coach. Water is 8.34 pounds a gallon and comes straight out of the capacity you have to carry everything else, so if a full tank is a large share of that capacity the usual practice is to travel with a few gallons for the toilet and hands and fill at the site. If water is not available where you are going, you carry it and take the weight off something else instead. What you should not do is fill the tank without knowing what fraction of your capacity it represents, which is the field labelled fresh water as a share of capacity above.
What is the difference between dry weight and cargo carrying capacity?
Dry or unloaded weight is what the vehicle weighs empty as the maker shipped it. The rating on the plate is the most the loaded vehicle may weigh. The difference between the two is what you may add, and it is the number that matters, though makers print it under several different names and sometimes on a separate label. Two coaches with the same plate rating can have very different capacity if one is heavier empty. Read both figures off your own labels rather than a brochure, and if a scale ticket is available use the measured weight in place of the published dry weight.
Does propane weight change as I use it?
Yes, and it is worth accounting for. Propane is 4.24 pounds per gallon of liquid, so a full 20 pound bottle holds a little under five gallons of gas plus around 18 pounds of steel bottle. Two full bottles are roughly 40 pounds of gas and 36 of tare. The gas is what varies through a trip; the bottles are usually already in the dry weight. Put only the gas weight in the field above unless you added bottles the maker did not fit, in which case count the steel too.
Where does tongue weight count — on the trailer or the truck?
On the truck. The share of the loaded trailer that rests on the coupler transfers to the tow vehicle and counts against its payload, while the remainder sits on the trailer axles and is what the axle and tyre ratings have to cover. This is the most common way a tow vehicle ends up over its payload figure while everyone is looking at the towing figure instead: a heavy trailer with a high tongue percentage can put several hundred pounds onto a truck that is already carrying passengers and gear. The tongue weight calculator works the placement side of this in detail.
How accurate is a calculator like this compared to a scale?
It is only as accurate as the dry weight you put into it, and published dry weights are usually optimistic for a coach that has been owned for a while. Treat this page as a planning tool for deciding what to leave behind and for seeing the shape of the tradeoff between water and gear. Treat a scale ticket as the answer. If the two disagree, the scale is right. Weighing costs a few dollars at any public scale and takes ten minutes, and it is the only way to know what each axle carries rather than what the whole vehicle weighs.