RV Hot Water Calculator: How Long Does a 6 Gallon Water Heater Last?

The calculator’s example gives about 4.5 minutes from a 6 gallon heater, or 6.6 minutes through a 1.8 gal/min head. These are model estimates. Answers online range from five minutes to ten, and each is right for someone — because the time depends on your shower head, your water temperatures and your heat source, not on the tank alone.

How long does a 6 gallon RV water heater last?

Example: a 6 US gallon tank, 130°F hot water, 55°F cold water and a 100°F shower at 2.5 gal/min.

Change the calculator inputs to update the proportions and heating estimate.

60% hot + 40% cold. Each 1 gallon of shower water uses 0.60 gallon from the hot tank.

Hot fraction = (100 − 55) ÷ (130 − 55) = 0.60.

Stored hot water alone

6 ÷ 0.60 = 10.0 gallons at shower temperature, or 4.0 minutes at 2.5 gal/min, before allowing for recovery.

Heating adds energy over time

Assumed recovery: 10.1 gal/hr = 0.168 gal/min of hot-water equivalent. Net use of the hot reserve: 1.500 − 0.168 = 1.332 gal/min. Simplified estimate: 4.5 minutes.

The physical tank stays filled as cold water replaces each draw. Heating restores energy; it does not create an extra stored tankful. This model assumes constant hot-outlet temperature and recovery. Mixing, thermostat cycling and actual temperature rise can change the result.

The entered tank temperature is an assumption to verify, not a recommendation to turn it up. Preserve scald protection and follow the heater and fixture instructions. Do not bypass a mixing valve to extend shower time.

Recovery inputs: Suburban’s SW-series specification sheet. The hot fraction is a heat-balance calculation with heat loss ignored.

Six gallons of stored hot water can supply more than six gallons at a cooler shower temperature. Cold water joins it at the fixture, so only part of the shower flow comes from the hot tank.

On the calculator’s defaults — a 130°F tank, 55°F cold water, a 100°F shower — that part is 60%: (100 − 55) ÷ (130 − 55). A 2.5 gal/min head therefore pulls 1.5 gal/min from the hot reserve. Without recovery, that is 10 gallons of mixed water and about 4 minutes. Using Suburban’s 10.1 gal/hr recovery figure as a constant hot-water equivalent adds about 0.17 gal/min to the energy budget, giving the simplified 4.5-minute result. The physical tank stays full as cold water replaces the draw; heating restores energy rather than adding water.

What Suburban publishes, and what it does not

The Suburban SW-series specification sheet gives the recovery figures used here. They do not describe every Suburban model:

Tank	Gas input	Recovery on propane	Recovery on electric
4 gallons	9,000 BTU/h	7.6 gal/hr	6.1 gal/hr
6, 10, 12 or 16 gallons	12,000 BTU/h	10.1 gal/hr	6.1 gal/hr

From Suburban’s Porcelain Steel Tank Water Heater specification sheet. The electric figure is for its 1,440 W element.

Notice the burner does not get bigger with the tank. A 16 gallon tank recovers no faster than a 6 gallon one; it simply holds more to start with. That 12,000 BTU/h is also what the propane cylinder supplies for every hour the burner fires — our propane calculator adds it to the furnace and turns both into days per cylinder.

The cited sheet leaves out the recovery temperature-rise basis and thermostat setting. The calculator uses those rates as fixed approximations rather than recalculating heater power for each inlet temperature. That is why the calculator asks for your tank temperature instead of assuming one, and why it prints the same shower at 120°F and 140°F in the tank — 3.8 minutes against 5.2 on the defaults. That spread is bigger than most of the fixes below, so the first thing worth doing is reading your own heater’s manual.

The free levers first

Before buying anything, the calculator shows what the same tank gives if you change how you use it. On its defaults:

• A shower 5°F cooler — 95°F instead of 100°F — gives about 5.2 minutes, 0.6 more.
• Running propane and electric together gives about 4.9 minutes, 0.4 more, by adding the element’s 6.1 gal/hr to the burner’s 10.1. That needs shore power and a heater that lets both run at once — Suburban’s spec sheet lists the two separately and does not say, so check your heater’s switches and manual.
• Turning the water off while you soap. The calculator cannot put a number on this one, because it depends on how long you stand there — but every paused minute is a minute of hot water still in the tank.

Lower flow can extend shower time

The federal showerhead standard uses 2.5 gal/min at 80 psi. That is a test condition, not a measurement of your RV shower. The calculator uses 2.5 as an example; enter actual flow if you can measure it.

Drop to a 1.8 gal/min head and the default shower goes from 4.5 minutes to 6.6 — 2.1 more from the same tank. That exceeds either computed free change above in this example; the benefit of pausing depends on how long you pause.

Where the hot water goes

A 4.5 minute shower at 2.5 gal/min is about 11 gallons of water, all of it heading for your grey tank. If you are boondocking, that number matters more than the hot water does — our water tank calculator works out which tank fills first.

Tankless is a different question

A tankless heater has no tank to empty, so it never runs out in the way this page describes. Its limit is how much water it can heat, by how many degrees, at once — a flow and temperature-rise figure from its own manual, which this calculator does not work out. Furrion’s tankless, for example, lets you set the output between 95°F and 124°F. If you have a tankless heater and it is winterizing time, note that it is handled differently from a tank — our winterizing calculator covers why.

The heads compared

Model	Rated flow	Flow control	Hose	Buy
Oxygenics Fury RV	1.8 GPM at 80 PSI	A "Flow Control lever" that "quickly adjusts shower pressure". Oxygenics does not describe it as a full pause or shut-off	72 in	Check price(opens in a new tab)
Oxygenics PowerFlow RV	1.8 GPM at 80 PSI	"Flow control lever allows you to reduce water to a trickle right on the shower head"	72 in	Check price(opens in a new tab)
Oxygenics BodySpa Adventure RV	2.0 GPM at 80 PSI	"Design includes a built in SmartPause® setting that preserves water in tanks"	72 in	Check price(opens in a new tab)

Where this goes wrong

1. Assuming the tank temperature. It substantially affects the answer, and the cited SW-series sheet does not publish one. Check your manual.
2. Using summer cold water in winter. Colder water in means more hot water per shower and fewer minutes.
3. Running a second tap. The calculator assumes the shower is the only thing drawing hot water.
4. Buying a bigger heater for a flow problem. The larger tanks in the cited table share the 6 gallon model’s recovery rating. Lower actual flow buys more time from the tank you already have.

Questions worth answering