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RV tank calculator: how many days until you need to dump?
Quick answer: Days until dump = usable tank space ÷ gallons added per day. Plan to dump the black tank at about two-thirds to three-quarters full and the gray tank when it is nearly full. Whichever tank reaches its limit first is your dump day.
The starting numbers are example values so you see a result right away. They are not measurements for any RV. Replace them with your own tank sizes and habits.
How the math works
Each day, toilet flushes add to the black tank, and showers, sinks and dishes add to the gray tank. The calculator multiplies your daily use by the number of people aboard, then divides it into the room left in each tank before you should dump. The how often to dump RV tanks guide explains why the black tank should be mostly full before you empty it.
How to get your real numbers
- Find your tank capacities in the owner's manual or your model's spec sheet.
- After your next dump, note how many days it takes each tank to fill. Tank sensors are often wrong, so days are a better guide than the gauge.
- Using campground restrooms and showers stretches your days a lot, especially for the gray tank.
When the number is small
If you are boondocking or on a long travel day, plan your dump stop before you are full. Browse dump stations by state and city, check each state's free list, or use the truck stop and 24-hour lists. The step-by-step dumping guide and dump station etiquette cover what to do once you get there.
Frequently asked questions
How many days until I need to dump my RV tanks?
Divide the usable space in each tank by how much goes into it per day. For the black tank, usable space is about two-thirds to three-quarters of its size, since that is when you should dump it. For the gray tank, it is close to its full size. The tank that runs out of room first sets your dump day. The calculator above does this math with your own numbers.
Why does the calculator stop the black tank at three-quarters full?
A black tank empties best when it is about two-thirds to three-quarters full, because the weight of the liquid pushes solids out. Waiting until it is completely full risks backups, so the calculator plans your dump before that point. You can change the target.
Where do I find my RV's tank sizes?
Your owner's manual or the manufacturer's spec sheet for your model lists fresh, gray and black tank capacities in gallons. Some RVs have two gray tanks; add them together if they drain through the same outlet.
Are the default numbers accurate for my RV?
No. The starting values are example numbers only, so the page shows a result before you type. Water use varies a lot by toilet, shower habits and whether you use campground restrooms. Track how many days your tanks really take to fill and put those numbers in.
Where can I dump when the tanks are full?
At a public dump station. This site lists 1,589 US dump stations across 49 states, including 175 tagged free.
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