Fresh Water Tank Duration Calculator: How long will the water tank last in my RV?
Two people get by about two days with a 26 gal (100 L) tank if they take a short shower every other day – without showers a good three. This calculator determines water usage per person from drinking, cooking, dishes, hygiene and showers and shows when the fresh water is empty and the gray tank is full.
Result
Fresh water lasts
1.9 days
at 14 gal per day for 2 persons
- Usage per person and day
- 6.6 gal
- Total usage per day
- 13.8 gal
- Gray tank full after
- 2.1 days
- Less than two days of duration: bring jugs as a reserve or use the campground showers.
- The calculator assumes constant usage; heat, children and dogs noticeably increase demand. Don't forget the weight of water: 26 gal (100 L) = 220 lb (100 kg) of payload.
Show calculation
- 1Showers per person: 0.5 × 4 min × 1.3 gal/min = 2.6 gal/day
- 2Per person: 1.1 gal + 1.6 gal + 0.8 gal + 0.5 gal + 2.6 gal = 6.6 gal/day
- 3Total: 2 × 6.6 gal + 0.5 gal = 13.8 gal/day
- 4Duration: 26 gal ÷ 13.8 gal/day = 1.9 days
- 5Gray water: 13.8 gal × 85% = 11.7 gal/day → tank full after 2.1 days
How it's calculated
Usage per person [gal/day] = drinking/cooking + dishes + hygiene + toilet + showers × minutes × gal/min
Total = persons × usage per person + other
Duration [days] = tank volume × fill level ÷ totalGray water: almost everything except drinking water and toilet flushing ends up in the gray tank – typically 80–90%. If the gray tank is smaller than the fresh tank, it often limits first.
| Use | frugal | normal | comfortable |
|---|---|---|---|
| Drinking, cooking | 0.8 gal (3 L) | 1.1 gal (4 L) | 1.3 gal (5 L) |
| Dishes | 1.1 gal (4 L) | 1.6 gal (6 L) | 3.2 gal (12 L) |
| Hygiene | 0.5 gal (2 L) | 0.8 gal (3 L) | 1.3 gal (5 L) |
| Shower (per shower) | 2.6 gal (10 L) | 5.3 gal (20 L) | 10.6 gal (40 L) |
| Total (shower every 2nd day) | ≈ 3.7 gal (14 L) | ≈ 6.1 gal (23 L) | ≈ 11 gal (42 L) |
Worked example
Family of four, 32 gal (120 L) fresh tank, 26 gal (100 L) gray tank: everyone showers every other day for three minutes with a low-flow head (0.8 gal/min), dishes are done frugally (1.3 gal).
Inputs
- Fresh water tank: 31.7 gal
- Fill level: 100 %
- Persons: 4 pers.
- Drinking and cooking: 1.06 gal
- Dishes: 1.32 gal
- Hygiene (teeth, hands, sponge bath): 0.79 gal
- Toilet flushing: 0.53 gal
- Showers per person and day: 0.5 ×
- Water running time per shower: 3 min
- Shower flow rate: 0.79 gal/min
- Other per day (dog, cleaning, plants …): 0.53 gal
- Gray water tank: 26.4 gal
- Share that ends up in the gray tank: 85 %
- Planned trip length (optional): 10 days
Result
1.6 days
Fresh water lasts
- Usage per person and day
- 4.9 gal
- Total usage per day
- 20.1 gal
- Gray tank full after
- 1.5 days
- Water needed for the trip
- 201 gal
Calculation
- Showers per person: 0.5 × 3 min × 0.8 gal/min = 1.2 gal/day
- Per person: 1.1 gal + 1.3 gal + 0.8 gal + 0.5 gal + 1.2 gal = 4.9 gal/day
- Total: 4 × 4.9 gal + 0.5 gal = 20.1 gal/day
- Duration: 32 gal ÷ 20.1 gal/day = 1.6 days
- Gray water: 20.1 gal × 85% = 17.1 gal/day → tank full after 1.5 days
- Trip: 10 days × 20.1 gal = 201 gal → 6 × refill
The variables explained
- Fresh water tank
- Usable volume per the manufacturer. Often only partially filled while driving (payload).
- Usage per activity
- Rules of thumb, individually very different. Adjust your own values by measuring (tank gauge).
- Showers × minutes × gal/min
- Only the time the water runs counts; with a shut-off valve while soaping, usage is halved.
- Gray water share (%)
- Share of usage that ends up in the gray tank.
Common mistakes
- Using household figures (80 gal per person per day) – in an RV it's 4–10 gal.
- Forgetting the gray tank: it's often smaller than the fresh tank and full before the water runs out.
- Washing dishes under running water – the biggest avoidable item.
- Ignoring the weight of water: 32 gal is 267 lb of payload.
Assumptions and limits
- Constant daily usage; weather, children and dogs not modeled separately.
- Toilet flushing from the fresh tank (enter 0 for a separate flush tank).
- Gray water share as a blanket value.
Frequently asked questions
How much water do you use in an RV per day?
Frugal campers get by with 2.6–4 gal (10–15 L) per person, 5–8 gal (20–30 L) is normal, with daily showers 10 gal (40 L) and more. Two people with a 26 gal (100 L) tank can thus stay 2–4 days.
How much water should I fill before driving?
As much as necessary, as little as possible: every gallon costs payload. Many drive with 5–13 gal (20–50 L) and top up at the destination – fill time calculator.