Water Heater Size Calculator
Water Heater Size is evaluated from Number of People in Household, Peak Simultaneous Hot Water Outlets and Cold Water Inlet Temperature. The calculation reports Recommended Tank Size, Required First Hour Rating and Tankless Flow Rate Needed.
Results
About the Water Heater Size Calculator
The calculator uses a multi formula configuration. Each reported value is read as a direct evaluation of the stored rules with the declared field formats and units.
Formula basis:
First Hour Rating (FHR) = peak morning demand
FHR = (number of people x 12 gallons/person) + (simultaneous fixtures x 8 gallons/fixture)
Tankless GPM = number of simultaneous hot water outlets x 2.0 GPM avg per outlet
BTU/hr = flow rate (GPM) x 8.34 lb/gal x 60 min/hr x ΔT ( degF temperature rise)
ΔT = desired hot water temperature - cold water inlet temperature
Typical ΔT: 65 degF in southern US (55 degF inlet → 120 degF), 75 degF in northern US (45 degF inlet → 120 degF)
Interpret the outputs in the order shown by the result fields. Optional inputs affect only the outputs that depend on those variables.
Formula & How It Works
The calculation applies the following relations exactly as recorded in the metadata: First Hour Rating (FHR) = peak morning demand FHR = (number of people x 12 gallons/person) + (simultaneous fixtures x 8 gallons/fixture) Tankless GPM = number of simultaneous hot water outlets x 2.0 GPM avg per outlet BTU/hr = flow rate (GPM) x 8.34 lb/gal x 60 min/hr x ΔT ( degF temperature rise) ΔT = desired hot water temperature - cold water inlet temperature Typical ΔT: 65 degF in southern US (55 degF inlet → 120 degF), 75 degF in northern US (45 degF inlet → 120 degF) Each output field is produced by substituting the supplied inputs into the relevant relation and then applying the declared rounding or text format.
Worked Examples
Example 1: Family of 4, suburban home, 2 simultaneous outlets, 55°F inlet
Inputs
With Number of People in Household = 4, Peak Simultaneous Hot Water Outlets = 2, Cold Water Inlet Temperature = 55 and Desired Hot Water Temperature = 120 as the stated inputs, the result is Recommended Tank Size = 50 gal, Required First Hour Rating = 64 gal/hr and Tankless Flow Rate Needed = 4 GPM. Each value corresponds to the declared output fields.
Example 2: Master bath: 3 showers + kitchen, 4 simultaneous, cold climate 45°F inlet
Inputs
With Number of People in Household = 5, Peak Simultaneous Hot Water Outlets = 4, Cold Water Inlet Temperature = 45 and Desired Hot Water Temperature = 120 as the stated inputs, the result is Recommended Tank Size = 80 gal, Required First Hour Rating = 92 gal/hr and Tankless Flow Rate Needed = 8 GPM. Each value corresponds to the declared output fields.
Example 3: Studio apartment: 1 person, basic usage, 58°F inlet
Inputs
With Number of People in Household = 1, Peak Simultaneous Hot Water Outlets = 1, Cold Water Inlet Temperature = 58 and Desired Hot Water Temperature = 120 as the stated inputs, the result is Recommended Tank Size = 23 gal, Required First Hour Rating = 20 gal/hr and Tankless Flow Rate Needed = 2 GPM. Each value corresponds to the declared output fields.
Example 4: Commercial break room: 10 employees, constant use, 50°F inlet
Inputs
With Number of People in Household = 10, Peak Simultaneous Hot Water Outlets = 3, Cold Water Inlet Temperature = 50 and Desired Hot Water Temperature = 120 as the stated inputs, the result is Recommended Tank Size = 100 gal, Required First Hour Rating = 144 gal/hr and Tankless Flow Rate Needed = 6 GPM. Each value corresponds to the declared output fields.
Common Use Cases
- Determine water heater tank size for a family of four
- Size a tankless water heater for simultaneous showers
- Calculate BTU/hour needed for a gas water heater replacement