RV Tank Dump Schedule Calculator
Estimate how many days until your RV black or grey tank fills up, based on tank size, number of people and daily usage — so you know when to dump.
How it works
Usable = tank × dump-at% · Fill rate = people × gal/person/day · Days = usable ÷ fill rate
A holding tank fills at a steady rate set by how many people use it and how much each uses per day, so the days until a dump is just the usable capacity divided by that daily rate. Dumping at 85% rather than 100% leaves room for the tank flush to work and keeps the sensors from misreading, which is why the calculator subtracts a buffer. Grey tanks fill faster than you expect once showers and dishwashing are counted, so it is often the grey tank — not the black — that dictates the schedule.
Worked example
Two people using 7 gal each per day add 14 gal daily. A 40 gal tank dumped at 85% gives 34 usable gallons, so 34 ÷ 14 ≈ 2.4 days between dumps.
Frequently asked questions
How often should I dump my RV tanks?
When the tank reaches about two-thirds to three-quarters full — often every 2–4 days for a couple, sooner for a family. Dumping while there is still liquid volume helps carry solids out; letting a black tank sit nearly empty for days invites clogs.
Which fills faster, the black or grey tank?
Usually the grey tank, because dish washing and showers add up quickly. The black tank only takes toilet use and flush water, so plan your dump schedule around whichever tank the calculator shows filling first.
Should I dump when the tank is completely full?
No — dump at around 80–90%. A little air space lets the tank flush and rinse work, and keeps you from an overflow if usage runs higher than expected on the last day.
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This calculator is for educational purposes. Double-check important results before acting on them.