FREE RV TRIP TOOL

RV Water Budget Calculator: Fresh, Grey & Black Tanks

Will freshwater run out before a waste tank fills? Plan all three tanks together, using your own daily water use and the water already on board.

1. Your trip

PeopleDays without a refill or dumpVolume units

Switching units converts your volumes. Gallons here always means US liquid gallons, not Imperial gallons.

2. Water drawn from the fresh tank

Enter daily water use per person in US gal. Defaults are illustrative planning inputs, not measured averages or drinking-water advice. If you use bottled drinking water, enter zero for that activity here and budget the bottles separately.

Drinking per person / dayCooking per person / dayShowering per person / dayDishes and washing per person / dayToilet flushing per person / day

3. Tank capacity and current amount

Enter volumes in US gal. Use your RV specification for capacity. Current fresh means water available now; current grey and black mean waste already in those tanks.

Fresh capacityFresh amount nowGrey capacityGrey amount nowBlack capacityBlack amount nowFreshwater reserve to keep unusedAdditional black-tank volume per person / day

The additional black input covers estimated bodily waste and other additions, beyond flushing water. Its 0.25 US gal default is an adjustable example assumption. Change it for your setup.

Adjust where used water goes

For each activity, enter the percentage reaching each waste tank. The total must be 100% or less. The remainder leaves this modeled water system: for example, water drunk or evaporated. Drinking water is not automatically assigned to grey water. Extra bodily waste is entered separately above. Use your plumbing layout, particularly if a bathroom sink feeds the black tank.

DrinkingTo grey tank (%)To black tank (%)

CookingTo grey tank (%)To black tank (%)

ShoweringTo grey tank (%)To black tank (%)

Dishes and washingTo grey tank (%)To black tank (%)

Toilet flushingTo grey tank (%)To black tank (%)

How the water budget works

The planner treats freshwater, grey water and black water as separate balances. It assumes your daily rates stay constant and that your crew uses onboard water without refilling, dumping or connecting to city water during the period. It is useful for comparing an overnight stop with a longer dry-camping stay. It does not determine what is safe to drink or how much a person should drink.

Fresh use per day = people × the sum of all five per-person daily use inputs.
Grey addition per day = people × the sum of each use input multiplied by its grey percentage.
Black addition per day = people × (each use input multiplied by its black percentage, plus the additional black-tank input).
Fresh days = (current fresh − reserve) ÷ daily fresh use.
Waste-tank days = (capacity − current amount) ÷ daily addition, calculated separately for grey and black.

The first limit is the lowest of those three day estimates. A tank with zero modeled daily flow has no time limit in this model. A full waste tank with a positive inflow has zero time remaining. The tool reports fractional days and the number of complete days before a limit. Projected end amounts can exceed a capacity, or become negative, to make an over-budget plan visible; they are never operating instructions.

A three-day example

Two people each use 1 US gal for drinking, 0.25 for cooking, 2 for showers, 0.5 for washing and 0.5 for flushing each day. That is 8.5 US gal drawn daily, or 25.5 over three days. With 40 US gal onboard and a 5 US gal reserve, freshwater reaches the reserve after about 4.12 days.

Using the example allocations, grey receives 5.25 US gal daily: all showering and washing water plus half the cooking water. A 30 US gal grey tank starting at 2 US gal reaches capacity after about 5.33 days. Black receives 1.5 US gal daily, including the illustrative bodily-waste allowance; a 25 US gal black tank starting at 2 US gal lasts about 15.33 days. Freshwater is the first constraint. At the end of three days the arithmetic amounts are 14.5 fresh, 17.75 grey and 6.5 black US gal.

Make the estimate useful for your RV

Measure a few representative days where possible, then adjust one input at a time. Showers may vary with flow, duration and whether you stop the water while soaping. Cooking and washing can also vary greatly. An onboard sensor that reports thirds is a rough reading, so preserve a margin rather than treating the last fraction of a gallon as dependable. A grey tank can become the constraint even while freshwater remains.

For multiple grey tanks, calculate the tank that receives each activity separately; this simplified tool uses one combined grey capacity. Do not combine capacities if the tanks cannot share their unused space. Water used outside the onboard system should not be entered as fresh-tank consumption. The additional black allowance is a volume assumption, not a medical measurement. The model also excludes leaks, line priming, tank-flush water, fresh-tank sanitizing, spills, ice melt, weather effects and changes in crew habits. Set conservative inputs and check the actual gauges along the trip.

Units, saving and next steps

The conversion is 1 US liquid gallon = 3.785411784 liters. Calculations retain numeric precision; result displays round to two decimal places. The share button puts all calculator inputs in the page address. It shares the plan, not your saved dump stops or notes. Reset restores the worked example. Print uses your browser's print or save-as-PDF dialog.

After checking your limiting tank, find a suitable dump station and add eligible public stops to your dump-stop checklist. Confirm access and opening conditions before travel. Use the planning guides for further context.

Methodology and sources

The balances and example inputs are this site's planning model, last reviewed October 19, 2026. They are not manufacturer-certified operating durations. NIST Handbook 130 (2025 archived edition), section 2.6.1 supplies the gallon/liter equivalence. Jayco's overview of RV water systems explains the distinct fresh, grey and black systems. Your own RV owner's manual and tank specifications take precedence for your vehicle.