RV and Trailer Calculators
RV tank days.
Enter tank sizes, people and how much water each person uses a day, and see which tank limits your stay: fresh running out or grey filling up. Runs in your browser; nothing is sent or stored.
Inputs
Result
Fill the form and press Calculate.
Days = usable tank volume / daily flow. With a 40 gal fresh tank, a 35 gal grey tank, 2 people at 5 gal each a day and a 10% reserve, fresh lasts 3.6 days and grey fills in 3.5 days, so grey is the limit.
Worked example
Fresh 40 gal, grey 35 gal, 2 people at 5 gal a day, 90% to grey, 10% reserve. Daily use: 2 x 5 = 10 gal. Usable fresh: 40 x 0.9 = 36 gal, so 36 / 10 = 3.6 days. Grey inflow: 10 x 0.9 = 9 gal a day; usable grey 31.5 gal, so 3.5 days. The grey tank is the limit: 3.5 days.
How it works
Fresh days = fresh x (1 - reserve) / (people x use per person). Grey days = grey x (1 - reserve) / (daily use x share to grey). The tool shows the smaller of the two (and the black tank if you enter it). Every figure of daily use is yours: this tool has no published per-person standard behind it, and the defaults are examples.
To stretch the days: turn the water off while soaping, use a basin, wash dishes with less water, and carry extra water in containers. Dump only at legal dump stations. RV tank gauges are often inaccurate, which is why a reserve is built in.
This tool uses plain arithmetic and your own figures; no published standard sits behind it. Planning aid only, not professional, engineering or safety advice. Your vehicle, trailer, tire, battery and boat manuals and the weight labels on your equipment come first; when in doubt, weigh it or ask a qualified professional.
FAQ
How many days will my RV fresh water last?
Divide the usable fresh capacity by the water you use per day. This calculator does that using your own per-person figure and a reserve.
Which fills first, the fresh or grey tank?
It depends on tank sizes and habits. Grey often fills first because nearly all used water drains to it while drinking and cooking water does not.
How much water does a person use camping?
It varies a lot with habits and hookups. There is no single published figure, so measure your own over a day or two and enter it.
Can I count on the full tank?
Not safely. Gauges are inaccurate and the last bit of a fresh tank may not pump out, so a 10 percent reserve is built in; change it if you like.
How do I make my tanks last longer?
Shorter showers, a basin for dishes, turning off the tap while soaping, and refilling at a fresh-water station on the way are the usual levers.