Water Heater Size Calculator

Peak-use profile:

Start with the closest usage pattern, then adjust the worksheet to match the fixtures that can overlap.

Unit basis:

Choose the units used in the worksheet and temperature fields.

Storage target:

Choose the tank style you want the first-hour result compared against.

Peak-use worksheet:

Use manufacturer fixture flows where available; worksheet values are planning assumptions.

Incoming cold water:

Use the coldest season water temperature at the heater.

Cold-water range {{ formatTemperature(inlet_temp_f) }}

Hot water setpoint:

Set the delivered hot-water temperature used for tankless temperature-rise sizing.

Setpoint range {{ formatTemperature(output_temp_f) }}

Sizing reserve:

Apply the same reserve to storage first-hour rating and tankless flow output.

Reserve range {{ formatPercent(reserve_pct) }}

Check the worksheet

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Tankless simultaneity factor:

Neutral by default; reduces or increases only the tankless flow requirement.

Gas tankless efficiency:

Adjust if you are comparing non-condensing or high-efficiency gas units.

Electric tankless efficiency:

Electric resistance units are commonly near 98 percent efficient at the heating element.

Display precision:

Adjust decimals without changing the sizing logic.

Metric	Value	Use it for	Copy
{{ row.metric }}	{{ row.value }}	{{ row.note }}

Use	Peak hour	{{ storageUseHeader }}	Simultaneous	{{ tanklessFlowHeader }}	Priority	Copy
{{ row.use }}	{{ row.peakCount }}	{{ row.storageGallons }}	{{ row.simultaneousCount }}	{{ row.tanklessFlow }}	{{ row.priority }}

Check	Status	Action	Reason	Copy
{{ row.check }}	{{ row.status }}	{{ row.action }}	{{ row.reason }}

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Water heater sizing starts with the busiest hot-water period, not with the tank volume printed on the front of the appliance. A storage tank can deliver more than its tank volume during the first hour because it is already full of hot water and begins reheating as water is drawn. A tankless unit has no stored reserve, so it is sized by the flow it can heat at the coldest expected inlet temperature.

That difference is why storage and tankless sizing use different shopping numbers. Storage and heat pump water heaters are compared by first-hour rating, often abbreviated FHR. Tankless heaters are compared by gallons per minute or liters per minute at a stated temperature rise. The same household can have a storage target driven by morning shower volume and a tankless target driven by simultaneous showers, sinks, and appliance draws.

Peak hour demand is a worksheet estimate. Showers, tub fills, handwashing, kitchen prep, dishwashers, and clothes washers may overlap differently in each home. A large bathtub can dominate storage sizing even if it is used occasionally. Multiple simultaneous fixtures can dominate tankless sizing because the heater must raise all of that water to the setpoint at once.

Sizing also has safety and installation boundaries. Higher setpoints increase scald risk unless mixing or anti-scald controls are used. Large tankless loads can require gas-line, venting, electric-service, breaker, or conductor upgrades. Final product selection should be checked against manufacturer tables at the actual temperature rise, not only against a generic capacity class.

How to Use This Tool:

Use the worksheet as a peak-use model, then compare the result against storage and tankless product classes.

Choose Peak-use profile to load a sample worksheet for a family morning, compact home, bath-heavy home, low-flow fixtures, or cold-climate tankless check.
Select Unit basis. US labels use gallons, GPM, and degrees F; metric entry converts worksheet values while keeping the same sizing method.
Choose Storage target so the first-hour result is compared with gas tank, electric tank, or heat pump water heater classes.
Edit Peak-use worksheet. Each row uses use name, count in peak hour, volume per use, simultaneous count, fixture flow, and priority.
Set Incoming cold water to the coldest expected season and Hot water setpoint to the delivered temperature used for sizing.
Set Sizing reserve for uncertainty, future load, and product-rating headroom. Advanced controls adjust simultaneity, gas efficiency, electric efficiency, and display precision.
Read Sizing Ledger, Peak-Use Worksheet, Install Checks, Capacity Fit Map, and JSON.

If the worksheet fails, add at least one positive peak-hour use, one positive simultaneous fixture flow, and make sure the hot setpoint is higher than incoming cold water.

Interpreting Results:

Recommended FHR is the storage shopping target. Look for a water heater whose EnergyGuide label or product literature has a first-hour rating at or above that number. Tankless flow target is the flow that a tankless unit must deliver at the displayed temperature rise.

Water heater result interpretation cues
Result	Use it for	Verify before purchase
Peak-hour demand	Raw storage-tank worksheet demand before reserve.	Fixture volumes and counts should reflect a real peak hour.
Recommended FHR	Minimum first-hour rating for storage or heat pump tanks.	Use product literature when the EnergyGuide label does not show FHR for that model type.
Temperature rise	Cold inlet to hot outlet difference used for tankless sizing.	Use winter inlet temperature, not a mild-season guess.
Gas or electric tankless estimate	Power class needed for the flow target.	Check gas, venting, circuit, service, and manufacturer GPM-at-rise tables.

The Capacity Fit Map compares listed storage and tankless classes with the modeled demand. A passing bar is a planning match, not an installation approval or code check.

Technical Details:

Storage sizing sums hot-water volume used during the busiest hour, then adds reserve. Tankless sizing sums simultaneous fixture flow, applies any simultaneity factor and reserve, then calculates the heat input needed to raise that flow from inlet temperature to setpoint. The worksheet keeps these two ideas separate because they answer different product questions.

Metric entry is converted to the same internal basis used by common US product labels. Liters are converted to gallons, liters per minute to gallons per minute, and Celsius temperatures to Fahrenheit before the heat-input equations are applied. Display values are then converted back to the chosen unit basis.

Formula Core:

For water heating, the 500 factor approximates the heat needed to raise one gallon per minute of water by one degree F for one hour.

\begin{array}{lcl} peak-hour gallons & = & \sum (peak count \times gallons per use) \\ required FHR & = & peak-hour gallons \times (1 + reserve) \\ tankless flow target & = & \sum (simultaneous count \times fixture GPM) \times simultaneity \times (1 + reserve) \\ gas BTU/h & = & \frac{GPM \times 500 \times temperature rise}{efficiency} \\ electric kW & = & \frac{gas BTU/h equivalent}{3412.141633} \end{array}

Water heater worksheet columns
Worksheet column	Storage role	Tankless role
Count in peak hour	Multiplies volume per use for peak-hour demand.	Does not directly affect flow unless also listed as simultaneous.
Volume per use	Gallons or liters drawn during the peak hour.	Not used for tankless flow.
Simultaneous count	Not used for first-hour rating.	Multiplies fixture flow for tankless demand.
Fixture flow	Not used for storage volume.	GPM or L/min used in the temperature-rise calculation.

For example, two showers at 20 gallons each, one 3 gallon kitchen prep draw, and one 7 gallon dishwasher draw produce 50 gallons of peak-hour storage demand. With a 15% reserve, the recommended FHR is 57.5 gallons. If one 2.0 GPM shower and one 1.5 GPM kitchen fixture can run together, the raw simultaneous flow is 3.5 GPM before reserve and simultaneity adjustments.

Safety And Accuracy Notes:

The result is a sizing estimate for residential planning. It does not confirm code compliance, combustion air, venting, condensate handling, gas service, electrical service, breaker size, conductor size, seismic strapping, drain pan, expansion tank, mixing valve, or anti-scald requirements.

Use product manufacturer flow tables at the actual temperature rise for tankless comparisons.
Large tubs, body sprays, whirlpools, or commercial-style fixtures can exceed sample worksheet assumptions.
Heat pump water heater recovery can depend on ambient air temperature, operating mode, and backup element settings.

Worked Examples:

Family morning peak:

A worksheet with two 20 gallon showers, two small handwashing uses, kitchen prep, dishwasher, and high-efficiency clothes washer produces a storage Peak-hour demand. The Recommended FHR then adds the selected reserve and rounds interpretation toward product classes.

Cold-climate tankless check:

If incoming water is 40 degrees F and the setpoint is 120 degrees F, the Temperature rise is 80 degrees F. The same 3.5 GPM flow requires more gas BTU/h or electric kW than it would at a 50 degree rise.

Worksheet repair:

If every simultaneous count is zero, storage rows may still add volume but tankless sizing cannot produce a positive Tankless flow target. Add the fixtures that can run together before comparing tankless units.

FAQ:

Is tank size the same as first-hour rating?

No. First-hour rating includes stored hot water plus recovery during the first hour. A tank's gallon size alone is not the full storage sizing number.

Why does cold inlet water matter so much for tankless sizing?

Tankless output falls as temperature rise increases. Colder incoming water means the unit must add more heat to every gallon passing through it.

Can I use metric worksheet values?

Yes. Select metric entry, then use liters, L/min, and degrees C. The result also shows the equivalent US-label basis for comparison.

Does the result choose a final model?

No. It estimates capacity targets and shopping classes. Final selection should use product literature, installer review, and local code requirements.

Glossary:

First-hour rating: The amount of hot water a storage heater can deliver in the first hour from a fully heated tank.
Temperature rise: The difference between incoming cold water and delivered hot water setpoint.
Simultaneous flow: The hot-water flow from fixtures expected to run at the same time.
Reserve: Extra sizing headroom added for worksheet uncertainty, cold-season changes, and future load.

References:

Sizing a New Water Heater, U.S. Department of Energy.
Tankless or Demand-Type Water Heaters, U.S. Department of Energy.
Water Heater Key Product Criteria, ENERGY STAR.