Commercial Water Heater Sizing Calculator
Size a commercial water heater based on peak demand, temperature rise, and recovery requirements.
About this calculator
This calculator splits cleanly into a tank-capacity track and a heating-energy track that share only Peak Demand (GPH). Recommended Tank Size is Peak Demand times Recovery Time times a 70% usable-water factor -- Inlet Water Temp and Outlet Water Temp never enter that formula at all, so two systems with identical demand and recovery time get the same tank recommendation regardless of how cold the incoming water is.
BTU Required instead comes from the standard Q = flow x 8.33 x deltaT formula, where deltaT is Outlet Water Temp minus Inlet Water Temp, so raising the outlet target or lowering the inlet temperature both raise the BTU requirement. First Hour Rating combines a share of storage capacity with a recovery rate that is, by construction, very close to Peak Demand (GPH) itself -- the BTU-per-degree math that produces Recovery Rate largely cancels the temperature terms back out, so First Hour Rating tracks Peak Demand far more closely than it tracks either temperature input, and Recommended Heater Type is selected from four BTU tiers driven by that same demand-and-temperature-rise calculation.
Inputs
Results
Recommended Tank Size
140 gal
≈ 6 wheelbarrows
BTU Required
149,940 BTU/hr
How to Use This Calculator
- Enter Peak Demand (GPH), Inlet Water Temp (°F), and Outlet Water Temp (°F).
- Set Recovery Time (hrs).
- Review Recommended Tank Size (gal) and BTU Required (BTU/hr).
- Use Recommended Heater Type and First Hour Rating (GPH) to inform your decision.
- Use the chart to visualize the results and explore different scenarios by adjusting inputs.
How the result changes with Peak Demand (GPH)
| Peak Demand (GPH) | Recommended Tank Size | BTU Required |
|---|---|---|
| 100 | 70 gal | 74,970 BTU/hr |
| 150 | 105 gal | 112,455 BTU/hr |
| 300 | 210 gal | 224,910 BTU/hr |
| 500 | 350 gal | 374,850 BTU/hr |
What each input means
- Peak Demand (GPH)
- Peak hot water demand in gallons per hour during busiest period
- Inlet Water Temp (°F)
- Cold water inlet temperature — varies by geography and season
- Outlet Water Temp (°F)
- Required hot water delivery temperature (140°F typical for commercial)
- Recovery Time (hrs)
- Time in hours to reheat the full tank after depletion
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersPeak Demand (GPH) = 200, Inlet Water Temp (°F) = 50, Outlet Water Temp (°F) = 140, Recovery Time (hrs) = 1 = 4 input(s) provided
- Calculate Recommended Tank SizeRecommended Tank Size140 = 140
- Calculate BTU RequiredBTU Required149940 = 149940
- Calculate Recommended Heater TypeCommercial Tank (Gas/Electric) = Commercial Tank (Gas/Electric)
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why don't Inlet and Outlet Water Temp change my Recommended Tank Size?
Recommended Tank Size is calculated purely from Peak Demand (GPH) and Recovery Time (hrs), multiplied by a fixed 70% usable-water factor. Inlet Water Temp and Outlet Water Temp only feed into BTU Required and First Hour Rating, not tank capacity.
Why does raising Outlet Water Temp increase my BTU Required?
BTU Required follows the standard water-heating formula: flow multiplied by 8.33 (pounds per gallon) and by the temperature rise, where the rise is Outlet Water Temp minus Inlet Water Temp. A higher delivery target directly widens that temperature rise and raises the BTU requirement.
Why does First Hour Rating track Peak Demand so closely?
First Hour Rating combines stored capacity with a recovery rate that is derived by dividing BTU Required back by the same temperature rise that produced it, which largely cancels the temperature terms out. The result tracks Peak Demand (GPH) far more directly than it tracks either water temperature input.
Why doesn't Recovery Time affect my BTU Required?
BTU Required is based only on Peak Demand and the temperature rise between inlet and outlet — it represents the instantaneous heating load, not how quickly the tank refills. Recovery Time only affects Recommended Tank Size and, through it, First Hour Rating.
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