Hot water, not total water
How much hot water does a shower use?
An 8-minute shower at 2.5 gpm uses 20 gallons of water, but only about 15.7 gallons comes from the water heater when the tank is at 120°F and the incoming water is 50°F. The rest is cold water blended in at the valve. With a 2.0 gpm WaterSense head, about 12.6.
Updated
Calculate your shower
Hot water per shower
15.7gal
- Total water at the head
- 20.0 gal
- Hot fraction
- 78.6%
- Cold water mixed in
- 4.3 gal
Hot fraction = (shower − incoming) ÷ (heater − incoming). The rest of the flow is cold water blended in at the valve.
The hot-fraction formula
A shower valve mixes hot water from the heater with cold water to reach the temperature you set. By energy balance, the share that comes from the heater is:
hot fraction = (shower temp − incoming) ÷ (heater temp − incoming) = (105 − 50) ÷ (120 − 50) = 0.786 hot gallons = 2.5 gpm × 8 min × 0.786 = 15.7 gal
This is the same blending relationship used in the ANSI/RESNET/ICC 301 energy rating standard and in Lawrence Berkeley National Laboratory hot-water research, and it is how the calculator converts every blended fixture.
Hot gallons per shower
With 50°F incoming water, a 120°F heater and a 105°F shower:
| Showerhead | 5 min | 8 min | 10 min | 15 min |
|---|---|---|---|---|
| 1.5 gpm | 5.9 | 9.4 | 11.8 | 17.7 |
| 1.8 gpm | 7.1 | 11.3 | 14.1 | 21.2 |
| 2.0 gpm (WaterSense) | 7.9 | 12.6 | 15.7 | 23.6 |
| 2.5 gpm (federal max) | 9.8 | 15.7 | 19.6 | 29.5 |
And the same 8-minute, 2.5 gpm shower as the incoming water changes:
| Heater setpoint | 40°F in | 50°F in | 60°F in | 70°F in |
|---|---|---|---|---|
| 120°F | 16.3 | 15.7 | 15.0 | 14.0 |
| 130°F | 14.4 | 13.8 | 12.9 | 11.7 |
| 140°F | 13.0 | 12.2 | 11.3 | 10.0 |
Colder incoming water means more hot water per shower. A hotter tank means less, because less of it is needed in the blend.
Where the defaults come from
- 2.5 gpm is the federal maximum for showerheads made since 1994, measured at 80 psi (10 CFR 430.32(p)). Many heads flow less in practice.
- 2.0 gpm is the maximum for EPA WaterSense-labeled showerheads.
- 8 minutes rounds the 7.8-minute average shower reported by the Water Research Foundation’s Residential End Uses of Water study (2016).
- 105°F is the shower temperature used in the ANSI/RESNET/ICC 301 standard.
Why this matters for sizing
Counting every gallon at the showerhead as hot overstates demand. For a tank, it inflates the First Hour Rating you think you need. For tankless, it overstates the flow through the heater: two 2.5 gpm showers put 3.93 gpm through the heater at these temperatures, not 5.0. The tankless calculator shows both numbers.
Shower questions
How many gallons of hot water does a 10 minute shower use?
At 2.5 gpm, a 10-minute shower uses 25 gallons of water in total. With 50°F incoming water, a 120°F heater and a 105°F shower, about 19.6 gallons of that comes from the water heater. With a 2.0 gpm head, about 15.7.
Why do sizing worksheets say 20 gallons per shower?
AHRI's sizing worksheet allows 20 gallons of hot water per shower and says it assumes no water conservation measures. That matches a 7.7-minute shower from an older 3.3 gpm head, about 20.0 hot gallons at 50°F incoming water. DOE's 2001 worksheet used 15 gallons. With today's 2.5 gpm federal maximum, about 15.7.
Does turning the water heater up mean less hot water per shower?
Yes. A hotter tank needs less hot water in the blend to reach the same shower temperature, so each shower draws fewer gallons from the heater. That's why raising the setpoint (with anti-scald mixing valves) is sometimes used to stretch a tank. The tank also loses more heat standing, and scald risk rises above 120°F.
How long does the average shower last?
The Water Research Foundation's Residential End Uses of Water study (2016) found an average of about 7.8 minutes, which is why the calculator assumes 8.
Sources
- 10 CFR 430.32: Energy and water conservation standards (showerheads, faucets, dishwashers), eCFR.
- WaterSense labeled showerheads, U.S. EPA.
- Water heater sizing worksheet, AHRI.
- Water Heating: Energy-efficient strategies for supplying hot water in the home (DOE/GO-102001-0785, 2001), U.S. Department of Energy.
Water Heater Sizing Calculator for iPhone
Take the calculator to the job site
The app runs the same sizing engine as this site, offline. Basic tank and tankless sizing is free. Lifetime Pro, a one-time $4.99 purchase, adds tank vs. tankless comparison, custom fixtures, saved jobs with notes and PDF reports you can send to a customer.
