How many days will your water last?

How this works

The calculator adds up a day of water use, then divides each tank by what goes into or out of it.

• Showers a day = people × showers per week ÷ 7 × minutes the water runs × shower head flow.
• Toilet a day = people × flushes × water per flush. A composting toilet uses none.
• From the fresh tank a day = drinking and cooking (unless you refill it separately) + showers + hand washing and teeth + dishes + toilet.
• Into the grey tank a day = showers (if they drain into the tank) + hand washing and teeth + dishes. Drinking water and toilet water never go into the grey tank.
• Days = tank size ÷ daily amount, rounded down. The smaller number is when you need to stop and refill or dump.

The starting numbers are an example, not an average: two people, a 30-gallon fresh tank, a 20-gallon grey tank, 1 gallon each for drinking and cooking, 2 gallons for dishes, three 2-minute navy showers a week each with a 1.8 gpm head, half a gallon each for hand washing and teeth, and a cassette toilet. That works out to a refill or dump about every 3 days, with the grey tank filling first.

Pasta water or coffee poured down the sink also goes into the grey tank. If you do that a lot, add it to “Dishes”.

Ways to make your water last longer

• Take navy showers.Turn the water off while you soap up. Two minutes of running water instead of eight is a quarter of the water.
• Fit a low-flow head with a pause button.A WaterSense head uses 2.0 gpm or less. A pause valve makes navy showers easy.
• Wipe dishes before washing.Scrape and wipe with a paper towel or spatula, then wash in a basin, not under a running tap.
• Use a cup for teeth.Fill a cup, then turn the tap off. Hand sanitiser between washes helps too.
• Refill drinking water when you can.Filling jugs at stores or stations keeps the main tank for washing.
• Shower at gyms or campgrounds.Set showers to 0 and see how far your tanks stretch.
• Measure your own flow.Time how long your tap or shower takes to fill a 1-gallon jug. 30 seconds means 2 gpm.
• Empty grey water where it’s allowed.Rules differ from place to place. When in doubt, use a dump station.

Sources

• US EPA WaterSense: showerheads (labelled heads use no more than 2.0 gpm; standard heads use 2.5 gpm)
• Oxygenics BodySpa RV spec sheet (max flow 1.8 gpm)
• Water Research Foundation, Residential End Uses of Water 2016, summarised by the Alliance for Water Efficiency (average shower 7.8 minutes at 2.1 gpm)
• Ready.gov: emergency water (at least 1 gallon per person per day; about ¾ gallon of fluid for an active adult; more in heat)
• Navy shower: how it works and typical running time
• Dometic 320 gravity RV toilet (one pint per flush)
• Thetford Porta Potti 565E specifications (4 gal flush tank, about 56 flushes), via Campervan HQ

Product figures are examples to help you fill in the form. Check your own shower head, pump and toilet.

Why does my grey tank fill before my fresh water runs out?

Drinking water leaves in your body, not down the drain, so the fresh tank always loses more than the grey tank gains. But grey tanks are often smaller than fresh tanks, so they can still fill first.

Does this work for an RV, bus or skoolie?

Yes. Enter your tank sizes. If your RV toilet uses a lot of water, set the water per flush from your toilet’s manual.