Water Heater Size Calculator

Storage class {{ storageClassBadge }} Gas class {{ gasClassBadge }} Electric class {{ electricClassBadge }}

Temperature path preview unavailable

Peak-use profile:

Profiles are editable planning assumptions, not household surveys.

Storage technology:

Compare the result with a product's listed FHR, not tank gallons alone.

Worksheet volume unit:

The canonical storage calculation normalizes values to gallons.

Worksheet flow unit:

The canonical tankless calculation normalizes values to gallons per minute.

Peak-use worksheet:

Columns: use, peak count, volume/use, simultaneous count, flow, priority.

Use fixture labels or manufacturer data where available.

Incoming cold water:

Unit changes convert the displayed value instead of reinterpreting it.

Delivered hot water:

120°F (about 49°C) is the default planning setpoint; always verify delivery temperature.

Sizing reserve:

Use enough reserve to cover uncertainty without substituting for product tables.

Planning headroom{{ formatPercent(reserve_pct) }}

Gas thermal efficiency:

The default 84% is a planning assumption, not a model rating.

Electric conversion efficiency:

The default 98% reflects a near-resistance planning assumption.

Tankless flow adjustment:

The neutral default is 0%; negative values reduce and positive values increase simultaneous flow.

Display precision:

Use more decimals to audit close capacity comparisons.

The chart renderer is unavailable. The exact fit ratios remain in the sizing ledger.

Measure	Canonical value	Decision use	Copy
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Check	Status	Next action	Copy
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Hot-water shortages usually happen during a short burst, not because the household's full-day total is unusually high. Morning showers, handwashing, laundry, and a dishwasher can overlap within one hour. Sizing therefore begins with the busiest realistic period and the fixtures that may run at the same time.

Storage and tankless heaters answer that peak in different ways. A storage heater draws from already heated water while its burner, element, or heat pump begins recovery. Its useful comparison is first-hour rating (FHR), the amount of hot water the model can deliver in a starting hour under its rating conditions. Tankless equipment has little stored reserve, so flow at the required temperature rise is the central rating.

Peak-hour volume: All hot-water draws expected during the busiest hour, even when they do not occur simultaneously.
Simultaneous flow: The combined flow of fixtures expected to run together.
Temperature rise: Delivered hot-water temperature minus the coldest expected inlet temperature.
Reserve: Extra planning capacity for uncertain timing, guests, and modest growth in demand.

Climate can change a tankless decision even when fixture use stays the same. Heating 3 gallons per minute from 40°F to 120°F requires much more input than heating the same flow from 70°F to 120°F. Storage selection is less directly tied to that instantaneous heat-rate equation, but recovery, efficiency, and the published FHR still matter more than tank gallons alone.

A planning result is only the beginning of product selection. Manufacturer flow-at-rise tables, electrical service, fuel supply, venting, condensate, space, local code, and installation conditions can rule out a nominal size that looks adequate on paper. Hot-water temperatures can also cause severe scalds, so setpoints and anti-scald protection need deliberate professional review.

How to Use This Tool:

Start with simultaneous hot-water demand, then confirm the incoming-water and heater assumptions.

Choose a Peak-use profile as a starting point, then edit the worksheet so each row reflects your household rather than the preset.
For storage sizing, enter the expected count and hot-water volume per use during the busiest hour. For tankless sizing, enter only the fixture count and flow that can overlap.
Set the coldest realistic Incoming cold water and the intended Delivered hot water temperature. Changing °F or °C converts the same physical temperature.
Add a Sizing reserve for uncertainty. Keep gas and electric efficiency values tied to the type of unit being compared, and use the tankless adjustment only for a documented derating or correction.
Compare the recommended FHR, flow, and input demand with a product's published performance at the same temperature rise.

Interpreting Results:

For a storage heater, compare Recommended FHR with the model's listed FHR, not merely its tank volume. A selected example class with a fit ratio below 1 does not meet the modeled target; review a larger, staged, or commercial option.

For tankless equipment, the flow target and heat input must both clear. The same nominal gallons per minute can correspond to different capacity at different inlet temperatures. Check the manufacturer's flow table at the calculated rise, then confirm that gas or electrical infrastructure can supply the required input.

Technical Details:

The worksheet separates volume demand from overlapping flow. Each row contributes peak count multiplied by volume per use to the storage total, and simultaneous count multiplied by fixture flow to the tankless total. Litres are converted to U.S. gallons before calculation.

Formula Core

Storage demand is the busiest-hour volume multiplied by the reserve factor.

{FHR}_{recommended} = (\sum_{i}^{n} c_{i} v_{i}) (1 + \frac{r}{100})

Here c is the peak-hour count, v is gallons per use, and r is reserve percent. The selected storage example is the first class whose listed FHR is at least the recommendation; if none fits, the largest example remains visible with a failing fit ratio.

Tankless flow begins with simultaneous fixture flow, then applies reserve and the optional adjustment. The heat-rate identity uses U.S. gallons per minute and degrees Fahrenheit.

Q_{thermal} = G_{adjusted} \times 500 \times (T_{out} - T_{in})

Q_{input} = \frac{Q_{thermal}}{η}

Gas input is reported in BTU/h. Electric input divides the thermal BTU/h by 3,412.141633 BTU/h per kW and by electric efficiency. The example classes are planning brackets, not claims about a specific product.

Rule Core

Storage examples are selected by first-hour rating, while gas and electric tankless examples are selected by required input. The first class at or above the requirement wins; if the requirement exceeds the ladder, the largest example remains visible with a fit ratio below 1.

Storage water heater planning classes
Technology	Tank volume / FHR examples in gallons
Gas storage	40 / 54, 50 / 70, 75 / 90, 100 / 115
Electric storage	40 / 49, 50 / 62, 65 / 72, 80 / 85, 100 / 100
Heat pump	50 / 62, 65 / 74, 80 / 90, 120 / 120

The gas input ladder is 120,000, 160,000, 180,000, and 199,000 BTU/h. The electric ladder is 18, 24, 27, 36, and 54 kW. These brackets support comparison only; product flow-at-rise and installation data remain controlling.

Water heater calculation boundaries
Input or rule	Supported behavior
Worksheet	1 to 40 nonblank rows and no more than 10,000 characters.
Temperature	Inlet 0°F to 90°F; outlet 100°F to 160°F; outlet must be warmer than inlet.
Reserve	0% to 40%, applied to storage volume and simultaneous flow before class selection.
Tankless adjustment	−50% to +25%, applied only to adjusted tankless flow.
Class selection	First listed class meeting the requirement; otherwise the largest example with a fit warning.

With the default family-morning worksheet, 69 peak-hour gallons and 2.8 simultaneous gpm become 79.35 gallons FHR and 3.22 gpm after a 15% reserve. A 70°F rise requires 112,700 BTU/h of delivered heat, about 134,167 BTU/h of gas input at 84% efficiency or 33.7 kW at 98% electric efficiency.

Safety and Installation Notes:

The result does not verify a product, installation, or code path. Have a qualified installer check fuel piping, combustion air, venting, condensate disposal, electrical service, breakers, conductors, water pressure, freeze protection, and required mixing or anti-scald controls. Use the manufacturer's current performance table and instructions as the final sizing authority.

References:

Federal water-heater criteria and first-hour rating definition, U.S. Department of Energy.
Measure Guideline: Transitioning to a Tankless Water Heater, U.S. Department of Energy, 2012.
Avoiding Tap Water Scalds, U.S. Consumer Product Safety Commission.