Gray Water Tank Calculator: When is the gray tank in my RV full?
Many RVs have a smaller gray than fresh water tank – then the gray tank is full before the fresh water runs out. This calculator shows after how many days you have to dump and which tank becomes the problem first.
Result
Gray tank is full in
2.2 days
11 gal of gray water per day
Gray water level after one day11 / 24 gal
- Free volume in the gray tank
- 24 gal
- Fresh water lasts
- 2 days
- Gray water until the fresh tank is empty
- 22 gal
Show calculation
- 1Gray water per day: 13 gal × 85% = 11 gal
- 2Free volume: 24 gal × (1 − 0%) = 24 gal
- 3Full after: 24 gal ÷ 11 gal/day = 2.2 days
- 4Fresh water lasts: 26 gal ÷ 13 gal/day = 2 days
How it's calculated
Gray water [gal/day] = fresh water usage × gray water share
Full after [days] = free volume ÷ gray water per dayThe gray water share is usually 80–90%: what's drunk, cooked and the toilet flush (cassette) don't end up in the gray tank. If you use the outdoor shower or dispose of dishwater outside (only where allowed!), your share is lower.
Worked example
Camper van with a 24 gal (90 L) fresh and a 16 gal (60 L) gray tank, two people using 12 gal (45 L) a day; the gray tank is still 20% full.
Inputs
- Gray water tank: 15.9 gal
- Current gray water fill level: 20 %
- Fresh water usage per day (all persons): 11.9 gal
- Share that ends up in the gray tank: 85 %
- Fresh water tank: 23.8 gal
- Fresh water fill level: 100 %
Result
1.3 days
Gray tank is full in
- Free volume in the gray tank
- 13 gal
- Fresh water lasts
- 2 days
- Gray water until the fresh tank is empty
- 20 gal
Calculation
- Gray water per day: 12 gal × 85% = 10.1 gal
- Free volume: 16 gal × (1 − 20%) = 13 gal
- Full after: 13 gal ÷ 10.1 gal/day = 1.3 days
- Fresh water lasts: 24 gal ÷ 12 gal/day = 2 days
The variables explained
- Gray water tank
- Volume per the manufacturer; the sensors often report “full” earlier.
- Fresh water usage (per day)
- Total usage of all persons.
- Gray water share (%)
- Share of usage that ends up in the gray tank.
Common mistakes
- Dumping gray water on the ground – that's prohibited almost everywhere and harms the environment.
- Forgetting the full tank in winter: frozen gray water can't be dumped and can damage the tank.
- Trusting the tank sensor blindly: deposits distort the reading – the calculator helps with estimating.
Assumptions and limits
- Constant usage, blanket gray water share.
- Usable volume = rated volume.