Water Heater Sizing Calculator Formula
Understand the math behind the water heater sizing calculator. Each variable explained with a worked example.
Formulas Used
Recommended Tank Size
tank_gallons = ceil(daily_demand / 10) * 10BTU Input Needed
btu_needed = recovery_hours > 0 ? daily_demand * 8.33 * temp_rise / recovery_hours : 0Daily Hot Water Demand
daily_demand_out = daily_demandVariables
| Variable | Description | Default |
|---|---|---|
occupants | Number of Occupants(people) | 4 |
gal_per_person | Gallons per Person per Day(gal) | 20 |
temp_rise | Temperature Rise(degF) | 70 |
recovery_hours | Recovery Time(hr) | 1 |
daily_demand | Derived value= occupants * gal_per_person | calculated |
How It Works
Water Heater Sizing
Daily Demand = Occupants x Gallons per Person
BTU/hr = Demand x 8.33 x Temp Rise / Recovery Hours
The constant 8.33 is the weight of one gallon of water in pounds. Tank size is rounded up to the nearest standard 10-gallon increment.
Worked Example
A household of 4 using 20 gallons each, 70 degF rise, 1-hour recovery.
- 01Daily demand = 4 x 20 = 80 gal
- 02Tank size = round up to 80 gal
- 03BTU = 80 x 8.33 x 70 / 1 = 46,648 BTU/hr
Frequently Asked Questions
What temperature rise should I use?
In most regions, incoming water is 50 to 60 degF and the target is 120 degF, giving a rise of 60 to 70 degF. Use 70 degF as a safe default.
Tank vs tankless - how does sizing differ?
Tankless heaters are sized by flow rate (GPM) and temperature rise. This calculator is for tank-style heaters where storage capacity is the key variable.
Learn More
Guide
Plumbing Pipe Sizing Guide: Water Supply and Drain Lines
Learn how to size plumbing pipes for water supply, drainage, and venting. Covers fixture units, pipe materials, code requirements, and pressure considerations.
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