RV Water Tank Calculator
Size RV fresh and gray water from crew, days, and daily rate.
What it does
Water sets how long an RV can stay off grid. Fresh runs the taps, gray catches the drains, and the smaller of the two ends the trip.
The tool above sizes both tanks from crew, days, and daily rate. Two people for 3 days at 5 gal each need 30 gal fresh and about 24 gal gray, or 10 gal a day. Metric crews get 114 L fresh and 91 L gray.
Daily rate is the lever. Careful campers with navy showers and metered taps hold near 5 gal, while long showers can triple that and fill gray by night two.
Gray trails fresh by design. About four fifths of fresh reaches the drains, with the rest lost to cooking, spills, and drinking. A gray tank near 80 % of fresh keeps the pair in balance.
Gauges and backup close the plan. Tank sensors read wrong on slopes, so a 5 gal jug reserve and a mid trip dump stop turn a guess into a calm exit.
How to use
- Pick imperial (gal) or metric (L).
- Enter crew size.
- Enter days off grid.
- Enter daily rate: 5 gal or 19 L suits careful use.
- Read fresh first, then gray.
- Add 5 gal in jugs as backup.
Formula
- people
- Crew drinking, cooking, washing.
- days
- Nights off grid.
- rate
- Water per person per day.
Rate is gal per person per day imperial and liters metric. Gray at 80 % assumes most fresh reaches drains, with some lost to cooking and spills.
Worked example
Example: 2 people, 3 days
Imperial defaults: 2 people, 3 days, 5 gal each per day. These are also the default values, so the page opens on this answer.
- Crew: 2 people.
- Days: 3 off grid.
- Rate: 5 gal per person per day.
- Fresh: 2 x 3 x 5 = 30 gal.
- Gray: 30 x 0.8 = 24 gal. Daily: 2 x 5 = 10 gal.
- Tool shows 30 gal fresh, 24 gal gray, 10 gal per day.
Reference table
| Crew | Days | Fresh | Gray |
|---|---|---|---|
| 2 | 2 | 20 gal | 16 gal |
| 2 | 3 | 30 gal | 24 gal |
| 4 | 2 | 40 gal | 32 gal |
| 4 | 3 | 60 gal | 48 gal |
Limits
- Estimate only. Showers swing use most.
- Ignores black tank, which follows toilet use.
- Assumes level gauges. Slopes misread.
- Drinking safety needs clean fill and hose care.
Common mistakes
- Planning long showers on a small gray tank.
- Trusting tank lights on a slope.
- Forgetting backup jugs for drinking.
- Filling fresh at a suspect spigot.
- Dumping gray where banned.
Try it yourself
Quick checks. Answers sit tucked under each question.
4 people, 2 days, 5 gal?
Fresh is 4 x 2 x 5 = 40 gal. Gray is 40 x 0.8 = 32 gal. Daily is 4 x 5 = 20 gal.
2 people, 5 days, 19 L?
Fresh is 2 x 5 x 19 = 190 L. Gray is 190 x 0.8 = 152 L. Daily is 2 x 19 = 38 L.
Questions
How much fresh water?
Multiply people by days by daily rate. Two people for 3 days at 5 gal need 30 gal fresh and about 24 gal gray.
How big a gray tank?
Plan 80 % of fresh as gray. Here 30 x 0.8 = 24 gal.
What daily rate?
Near 5 gal per person per day for careful use: navy showers, paper plates, and metered taps. Full hookups use far more.
Do showers matter most?
Yes. Showers dominate. Cut shower minutes and the whole trip fits a smaller tank.
Backup water?
Carry 5 extra gallons in jugs for drinking and a dry camp exit. Tank gauges lie when parked off level.
Is tank water drinkable?
No. Drinking safety still needs clean fill and sanitized hose.