Every dry-camping question comes down to one tank and one rate. The tank is smaller than the sticker says once you allow for the part you cannot actually use, and the rate depends entirely on how many people are in the rig. Round down, because a tank that is 90% full on day four is a day-four problem.
A 60-gallon gray tank at 85% usable, with two people showering and washing at 12 gallons each, is two days. Not three.
The example
Three rigs on the same loop, with the tank that will fill first in each.
| A | B | C | D | E | F | G | |
|---|---|---|---|---|---|---|---|
| 1 | Rig | Tank gal | Usable % | People | Gal/person/day | Usable gal | Days |
| 2 | Travel trailer | 40 | 85% | 2 | 3.5 | 34.0 | 4 |
| 3 | Fifth wheel | 60 | 85% | 2 | 12 | 51.0 | 2 |
| 4 | Class A | 90 | 85% | 4 | 12 | 76.5 | 1 |
The formula
Usable volume first, because it is the number people get wrong:
How it works
Four inputs, each worth its own cell:
B2*C2discounts the nominal tank. Sensors read full well before the tank is, and nobody fills a black tank to the neck on purpose. Eighty-five percent is a workable default.D2*E2is the daily rate. Gallons per person per day is the number to argue about: roughly 3–4 for a black tank on normal use, 10–15 for gray if anyone is showering in the rig.ROUNDDOWN(...,0)gives whole days. Two-point-one days is two days, because the overflow happens on day three and there is no partial-day version of that.- Run the formula for gray and black separately. Whichever gives the smaller number is the one that sets your dump schedule — the other tank is irrelevant.
Park managers can flip this around: multiply sites by average daily gallons to size the dump station queue on a Sunday checkout.
Try it: interactive demo
Enter the tank size, how much of it you count as usable, and the daily use.
Variations
Hours, not days, for a short stay
Weekend rallies and event parking run in hours. Multiply by 24 and round down to the hour instead.
Gallons per day you can afford
Solve backwards when the stay length is fixed: how much can each person use and still make it to checkout?
Pitfalls & errors
Gray fills far faster than black in almost every rig. If you only track one tank, track gray — a single ten-minute shower can be twenty gallons.
Tank sensors are notoriously unreliable once any residue builds up. The calculation is often more trustworthy than the panel, which is exactly why it is worth keeping.
Do not use the nominal tank size straight. Dividing by daily use without the usable factor overstates capacity by 15% and puts the answer a whole day late on a small tank.
Practice workbook
Frequently asked questions
What is a realistic gallons-per-person-per-day figure?
Should the usable percentage be the same for gray and black?
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