Three tanks, one flow, and the smallest one wins
An RV plumbing system is a single stream that splits. Water leaves the fresh tank, and every drop of it goes to one of three places: down the shower or sink drain into the grey tank, down the toilet into the black tank, or out of the coach entirely as drinking water, cooking water and the contents of a kettle. That last path is the only one that does not consume tank space twice.
This is why the fresh tank almost never decides the length of a trip. Take a common layout: 40 gallons fresh, 30 grey, 25 black. Two people showering five minutes a day through a 1.5 gal/min head, plus three gallons each for sinks and dishes, plus a gallon each for drinking and cooking, draw 23 gallons a day from the fresh tank. But 21 of those gallons land in the grey tank. The fresh tank has 1.7 days in it and the grey tank has 1.4. You stop on the morning of day two, and you stop with a third of your drinking water untouched.
The black tank in that same scenario has five days in it, because a flush is half a gallon and there are only ten of them a day. Whoever specified those three tank sizes was not modelling a trip; they were fitting containers into the space under a floor.
The usable share, and why it is not 100 percent
The percentage field defaults to 90 for a reason that has nothing to do with caution. Level sensors in RV tanks are usually probes through the sidewall, and paper or residue bridging a probe makes a tank read full when it is two thirds full. The fresh tank pickup sits slightly above the floor of the tank, so the last inch is not available to the pump. And nobody drives with a grey tank at the absolute brim, because the first hard corner puts it out of the vent.
If your sensors are known-good and you are parked rather than driving, push the figure to 95. If the fresh tank gauge has been reading full since the previous owner had it, set it lower and treat the result as a floor rather than a plan.
Levers that actually move the numbers
| Change | Effect on fresh | Effect on grey |
|---|---|---|
| Low-flow shower head, 2.0 down to 1.0 gal/min | Halves the largest single draw | Halves the largest single inflow |
| Water off while soaping | Cuts running minutes, often by half | Same cut, one for one |
| Washing dishes in a basin, tipped outside where allowed | No change | Removes sink water from the tank |
| Showering at the campground bath house | Removes the draw entirely | Removes the inflow entirely |
| Bigger fresh tank | More days | None — grey still fills first |
| Carrying drinking water in jugs | Frees a gallon per person per day | None |
Read down the grey column and the shape of the problem is obvious. Four of the six changes reduce grey inflow and the two that do not are the two most people reach for first.
Where the water goes when you stop
Dumping is where a trip stops being arithmetic. Black tank contents carry pathogens, and releasing them anywhere but a sanctioned dump station contaminates soil and groundwater and is an offence. What counts as sanctioned, whether grey water may ever be released on the ground, and what a particular park allows all vary by state and by site, and none of it can be answered from a calculator. Ask at the park, and check the state rules for the state you are actually in rather than the one you started in.
Grey water is treated differently from black in many places and identically in others. The habit worth building is to assume both go to the same place until somebody with authority over that specific ground tells you otherwise.
Related pages
The fresh water in that tank is also cargo, and 40 gallons is over 330 pounds of it — the RV weight and cargo capacity calculator shows what that does to what is left on the plate. Shower length is set as much by the water heater as by the tank, which is the RV water heater and shower planner. For household plumbing on the same ideas see the emergency water storage calculator and the household wastewater flow calculator.
Questions people ask
Why does my grey tank fill before my fresh tank empties?
Because everything that goes down a drain was fresh water a moment earlier, and the grey tank is usually smaller than the fresh tank. In a typical 40/30/25 layout, shower and sink water is around ninety percent of what leaves the fresh tank, so the grey tank receives nearly the whole fresh tank while holding three quarters of its volume. The only draws that do not return to grey are drinking, cooking, and the toilet rinse. Unless you are running the fresh tank hard through the toilet, grey filling first is the expected result rather than a fault.
Can I run the grey tank valve open on full hookups?
On a sewer hookup people commonly leave grey open and keep black closed, and the reason for the difference is worth knowing. Grey water flows and carries little solid. A black tank left open drains the liquid and leaves the solids behind on the tank floor, where they dry into a mound that no amount of flushing shifts. That is the origin of most black tank sensor failures and most clog complaints. Whether a given park permits an open grey valve at all is a park rule, so ask rather than assume.
How much does a full fresh tank weigh?
Water is 8.34 pounds per gallon, so a 40 gallon tank holds 334 pounds and a 100 gallon tank holds 834. This matters more than most people expect, because that weight comes straight out of the cargo capacity printed on the vehicle plate, and on smaller trailers a full fresh tank can be a fifth or more of everything you are allowed to carry. The common advice to travel with tanks near empty and fill on arrival is a weight decision, not a water decision. Work the numbers on the RV weight and cargo capacity calculator with your own plate figures.
Do the usual per-person water figures apply to boondocking?
The defaults here are ordinary camping habits, not conservation. People who dry camp regularly get down to two or three gallons per person per day total, by showering rarely or outside the coach, washing dishes in a basin, and using jugs for drinking water so the tank is untouched by it. If that is your intent, set the shower minutes to zero or one and reduce the sink figure, then read the result as a target to hit rather than a prediction. The calculator will follow whatever habits you put in; it has no opinion about which are realistic for you.
Should I trust the tank level sensors?
Treat them as three-position indicators rather than gauges. Most RV tank probes read through the sidewall and any residue bridging a probe holds the reading high, so a black tank that reads two thirds may be nearly empty and a fresh tank that reads full may not be. The figures that do not lie are the ones you control: how long the pump ran, how long the shower ran, how many days you have been out. Running this calculator once for your own habits gives you a daily consumption number, and counting days against that number is more reliable than the panel.