What size water heater? How long will a shower last?


Do a google search and if you don’t pull your hair out trying to get a simple answer then you are from another planet. This is terribly complex (for a non-engineer like me)  but I am going to make it simple by assuming many things for you.

The easy and conservative answer is this:

A 50 gallon gas DHW will allow two 10 +- minute shower at 110 deg F with a 2.5 gpm shower head (with my huge list of assumptions mentioned below). Depending on the recovery rate another person may have to wait at least 40 minutes  or so to take another 20 minute shower.  So what does that mean. It means take a 7 minute shower so you can do dishes, run a load of laundry and still take another shower an hour later. It means buy a 75 gallon if you have a couple of kids. It means buy a 100 gallon quick recovery (high BTU output) commercial DHW heater if you don’t want to plan and sequence usage or if you have a “thing” about long hot showers or never want to run out (also investigate on-demand/tankless heaters).

 Here is the partial  list of factors:

Inlet cold water temperature (avg)50 deg F (colder in Alaska, warmer at Equator)
Hot water heater (nat. or propane) temperature140 deg F (over that US code requires mix valve)
Hot water heater size (adjust with the formula)50 gallon (189 liters)
Distance of furthest showerLess than 50 ft
Draw Down (30 deg temp drop)70 % or 35+- gallons on a 50 gal tank
Shower head rating2.5 gpm (use 3.5 if you remove flow constrictor)
Temperature of a hot shower (min)110 deg F at the head (cools quickly as it leaves)
Ratio of Hot to Cold water (see formula)Approx. 70% hot 30% cold
First hour rating (FHR)Approx. 50 gallons
We are ignoring mixing valve calcsToo complicated for me, too many variables.


  1. A couple (2) can use a 40 gallon heater by adjusting use pattern but a family of 4 will have trouble so if you have a 40 I will just say it is undersized (don’t argue).
  2. If you take more than a 10 minute shower I hope you are doing it for therapeutic reasons. If you keep your 2.5 water saving shower heads (I hate em…!) then you get a 14 min shower. I take a small drill bit to my 2.5 heads to increase the flow. Do I feel bad…? NO!
  3. Quick recovery DHW heaters and boilers for floor heat connected to domestic hot water change everything. The BTU input can shorten recovery time considerably.
  4. On-demand heaters change everything. This information is for gas heater with a storage tank. Do not get a 11 liter per minute on demand heater and expect to take long showers while any other appliances are being used (dishwasher, clothes washer etc). On demand heaters are great energy savers but get a good plumber so you don’t have sudden “cold plugs” and insure you have the flow rate and pressure needed. Low flow shower heads are notorious for causing cold surges as the heater shuts down (happens more when pipe size and pressure add to the problem). Adding a pressure pump and expansion tank are probably required equipment.
  5. If you live in a warmer climate with mild winters, you will have quicker recovery and a better FHR.

Skip the Algebra if it isn’t your thing……Formula: X is gpm hot, Y is gpm cold, 140 deg F hot, 50 deg F cold. 110 deg F is desired “hot shower” temp. If you like it scalding then you will have a few minutes less of hot shower. Just increase the 110 deg F desired temp. This is the “Ratio Formula” for hot to cold.

140 X + 50 Y = 110 (X+Y)  Hot and Cold in, 110 hot shower out.


30X-60Y=0 or 30X=60Y so……X=2Y  

You need twice the hot as cold to maintain the 110 deg F. (I know, it sounds wrong, other sources often say a shower uses 70% hot to 30% cold). So we are close to the 2 to 1 ratio. Without an assumed ratio you cannot calculate but you can get a bucket and measure how many gallons at a given temperature (use a gauge) you get before the “draw down” drops you to below your desired shower temperature.

So using slightly conservative numbers,  a 50 gallon hot water heater will give you 34+ gallons of hot water. Add another 16+ gallons of cold (ratio above) to the approx.. 34 gallons of hot water to get about 50 gallons of shower water at 110 deg F.  50 divided by 2.5 gpm gives you a 20 minute shower (or 2 @ 10 minutes). I think this number is the best you can expect from a 50. With mineral build up, higher gpm because you drilled the flow constrictor, slower FHR and seasonal changes you will be closer to 14-15 minute shower max.

If you like it really hot hot, reduce to 14 (2 @ 7) minutes. If you can reduce to 10 minutes then you can get 4 people showered by waiting 40-60 minutes after the first 2 showers. All bets are off if you wash dishes or clothes during shower time.

Good luck and don’t hassle me over my assumption and bad math. Just send me the better math formulas or make your case for longer shower numbers. Better yet, just say the size you like for a couple, family of 4, etc living in say Santa Fe, NM or San Miguel de Allende, Mexico (cold winters, warm summers).

Ron Smith (cranky ex-general contractor).