RV water & tank calculator

Three limits, one trip estimate

This calculator compares three separate budgets. It subtracts your reserve from fresh water, estimates daily consumption for your group, and compares that result with the room left in each waste tank. The smallest estimate is the modeled limit.

How long does a 40-gallon tank last?

There is no fixed answer. In the starting example, two people use a combined 10 gallons a day. Keeping 4 gallons in reserve leaves 36 usable gallons, or 3.6 days. The 30 gallons of gray tank space lasts 5 days at 6 gallons per day; the 20 gallons of black tank space lasts 10 days at 2 gallons per day. Fresh water is the first modeled limit.

Change the example to four people at the same per-person use, and fresh water lasts 1.8 days. Update the waste inflows too: the calculator deliberately asks for those separately so it does not assume every gallon ends up in one particular tank.

Use your actual water habits

The default 5 gallons per person per day is an illustrative input, not a health guideline or guaranteed camping allowance. Budget drinking, cooking, pets, dishes, handwashing, showers, and toilet flushing. Your weather, activity, equipment, and habits determine the amount you need.

On a practice trip, record how much water you refill and how long you stayed. Measuring faucet flow and timing a shower can also improve your estimates. Do not ration necessary drinking water to make the result look better.

Tank capacity is not the same as tank space

If a 30-gallon gray tank already holds 10 gallons, enter 20 gallons of space remaining. Do the same for the black tank. Sensors can be imprecise, so use conservative inputs when you do not know how much room you have.

Black tank inflow includes both flush water and waste. Gray tank inflow includes the water that actually reaches that tank. External showers or a separate toilet system change the distribution; describe the setup you have, rather than copying another camper’s numbers.

If you enter zero daily inflow, the calculator reports that tank as having no modeled inflow. That is an assumption you supplied, not proof that the tank can be ignored. If the tank starts with no space and inflow is positive, it limits the trip immediately.

Stretch supplies without guessing

• Fix leaks before departure and turn off faucets while washing or soaping.
• Use a basin to understand how much dishwashing actually consumes.
• Keep an emergency drinking-water reserve outside normal use.
• Locate a confirmed potable fill point and an approved dump point before the trip.
• Leave time to reach services before your reserve or tank limit is reached.

Plan to retain RV wastewater for approved disposal. Do not assume the backcountry guidance for a small amount of wash water applies to emptying an RV tank. Check local requirements.

Compare this result with your battery runtime. The trip ends when your first practical limit is reached, even if the other resources still look comfortable.