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

{{ error }}

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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Hot-water sizing is a peak-demand problem before it is a tank-size problem. A home can use a modest amount of hot water across a full day and still run short during the one hour when showers, sinks, laundry, and a dishwasher overlap. Another home may use more water overall but spread it out enough that a smaller heater feels adequate.

The first distinction is between stored hot water and on-demand heating. A storage water heater starts with a heated tank and recovers as water is drawn, so the useful rating is first-hour rating, or FHR. A tankless water heater has little stored reserve, so the useful rating is delivered flow at a stated temperature rise. Those ratings answer related questions, but they are not interchangeable.

Peak-hour demand: The hot-water volume expected during the busiest one-hour period.
First-hour rating: A storage-heater delivery rating that includes stored water plus recovery during the first hour.
Temperature rise: The gap between incoming cold water and the delivered hot-water setpoint.
Simultaneous flow: The combined fixture flow a tankless unit must heat at the same time.

Fixture timing changes the answer. Two 20 gallon showers during the same morning hour create a storage load even if they happen one after the other. For a tankless heater, two showers running together are the harder case because the unit must heat both flows immediately. A tub fill, body spray, or high-flow fixture can dominate either decision if it happens during the peak event.

Temperature is the other major swing factor. A 120 degrees F setpoint with 70 degrees F incoming water needs a 50 degrees F rise. The same fixtures with 40 degrees F incoming water need an 80 degrees F rise, which can push a demand heater into a larger gas input class or beyond a practical electric service load. Higher storage setpoints can also change usable mixed water, but they raise scald risk unless mixing and anti-scald controls are part of the installation.

Common water heater sizing mistakes
Mistake	Why it matters	Better check
Using old tank gallons as the only clue	Tank volume alone misses burner, element, or heat-pump recovery.	Compare first-hour rating with the busiest-hour draw.
Counting every fixture as simultaneous	Demand sizing can become unrealistic and expensive.	Count fixtures that can truly overlap in the design case.
Using warm-season inlet water	Tankless output falls when cold-season inlet water is colder.	Use the coldest realistic water entering the heater.
Ignoring installation constraints	Gas, venting, wiring, clearance, and drain requirements can rule out a model.	Confirm product literature, local code, and installer requirements.

A capacity estimate narrows the shopping range, but it cannot approve the final installation. Gas-line sizing, combustion air, vent material, condensate handling, water hardness, breaker space, wiring, pressure relief discharge, seismic strapping, drain pans, expansion control, and local permit rules can decide whether a heater that looks large enough on paper is suitable for the home.

How to Use This Tool:

Start with a peak-use worksheet, then read the storage and tankless targets as separate model-comparison numbers.

Choose Peak-use profile to load a realistic starting worksheet, then treat the sample as a draft rather than a final household survey.
Select Unit basis. US entry uses gallons, GPM, and degrees F. Metric entry uses liters, L/min, and degrees C, then converts to the same sizing math.
Set Storage target to compare the first-hour result with gas storage, electric storage, or heat pump water heater example classes.
Edit Peak-use worksheet. Each row needs a use name, peak-hour count, volume per use, simultaneous count, fixture flow, and priority. Use Sample to reload the selected profile or Normalize to clean parsed rows back into CSV form.
Enter Incoming cold water for the coldest season and Hot water setpoint for delivered hot water. The setpoint must be higher than incoming water.
Adjust Sizing reserve for uncertain fixture data, future use, seasonal swings, and product-rating headroom. Advanced controls let you change tankless simultaneity, gas efficiency, electric efficiency, and visible rounding.
Check Sizing Ledger, Peak-Use Worksheet, Install Checks, Capacity Fit Map, and JSON. A ready result shows an FHR target, a tankless flow target at the calculated rise, and no worksheet error.

Interpreting Results:

The summary gives two shopping targets: recommended first-hour rating for storage-style heaters and tankless flow at the calculated temperature rise. Do not compare the gallon FHR number directly with the GPM number. A storage tank meets a one-hour draw pattern, while a tankless unit must keep up with overlapping flow.

Sizing Ledger is the main audit trail. It shows peak-hour demand before reserve, recommended FHR after reserve, the chosen storage class, simultaneous fixture flow, temperature rise, gas input estimate, and electric power estimate. A surprising result usually comes from one worksheet row, a high reserve, cold inlet water, or simultaneous counts that are too broad for the intended design case.

Water heater result cues
Result cue	Use it for	Do not assume
Recommended FHR	Finding a storage heater whose EnergyGuide label or product literature meets the busiest-hour target.	That tank volume alone proves recovery performance.
Tankless flow target	Comparing manufacturer GPM-at-rise tables for gas or electric demand heaters.	That headline BTU/h or kW rating guarantees whole-home output.
Install Checks	Spotting storage shortfall, gas cascade review, electric service review, high setpoint risk, and light worksheet detail.	That a green fit badge replaces code review or product manuals.
Capacity Fit Map	Seeing which example storage, gas tankless, and electric tankless classes clear the target.	That listed classes are exact product recommendations.

The JSON view mirrors the current scenario for copying or downloading. It is useful for saving assumptions, but the numbers remain only as good as the fixture volumes, overlap counts, inlet temperature, reserve, and efficiency settings behind them.

Technical Details:

Storage sizing sums hot-water volume over the design hour. Each worksheet row contributes peak-hour count multiplied by gallons per use. Reserve is applied after the row total, so it protects the entire storage estimate rather than only one fixture.

Tankless sizing uses the same worksheet rows differently. Simultaneous count is multiplied by fixture flow to get raw GPM, then a simultaneity factor and reserve adjust the target. Temperature rise is the setpoint minus incoming cold-water temperature. Flow, rise, and efficiency drive the gas input and electric kW estimates.

Formula Core:

The common hot-water shortcut uses 500 as an approximate factor for water weight and minutes per hour. Efficiencies are entered as percentages but used as decimal multipliers in the heat-input equations.

\begin{array}{lcl} peak-hour gallons & = & \sum (peak-hour count gallons per use) \\ recommended FHR & = & peak-hour gallons (1 + reserve percent / 100) \\ tankless flow target & = & \sum (simultaneous count fixture GPM) \frac{simultaneity percent}{100} (1 + reserve percent / 100) \\ gas BTU/h & = & \frac{target GPM 500 rise in degrees F}{gas efficiency} \\ electric kW & = & \frac{gas BTU/h equivalent}{3412.141633} \end{array}

For the default family morning worksheet, peak-hour storage demand is 69 gallons. A 15% reserve raises the recommended storage target to 79.4 gallons FHR. The simultaneous flow rows total 2.8 GPM; with the same reserve and a 70 degrees F rise from 50 degrees F to 120 degrees F, the tankless target is 3.2 GPM and the gas input estimate is about 134,000 BTU/h at 84% efficiency.

Worksheet columns and sizing roles
Worksheet value	Storage role	Tankless role
Peak-hour count	Multiplies volume per use.	Does not affect demand flow unless simultaneous count is also positive.
Volume per use	Adds to the one-hour hot-water draw.	Not used in instantaneous flow.
Simultaneous count	Not used for FHR.	Multiplies fixture flow for demand-heater sizing.
Fixture flow	Not used for storage volume.	Sets the GPM or L/min that must be heated at the entered rise.
Priority	Labels the row for review and export.	Labels the row for review and export.

Metric entries are converted before the core equations run. Liters convert to gallons, liters per minute convert to GPM, and Celsius temperatures convert to Fahrenheit. Display precision changes visible rounding only; parsed values, reserve, efficiencies, temperature rise, and fit ratios keep using the unrounded calculation values.

The product classes are comparison examples, not a catalog. Storage examples carry listed FHR values. Gas tankless examples use input ratings of 120k, 160k, 180k, and 199k BTU/h. Electric examples use 18, 24, 27, 36, and 54 kW. Real delivered flow still depends on the model's tested table, allowed venting, gas pressure, electric service, water conditions, and manufacturer limits.

Safety And Accuracy Notes:

This is a residential planning estimate. It does not verify fuel supply, combustion air, venting, condensate disposal, drain pan rules, pressure relief discharge, expansion control, wiring, breaker count, conductor size, seismic restraints, water treatment, warranty terms, permits, or code compliance.

Use cold-season inlet water for tankless sizing. Warm inlet assumptions can overstate delivered flow.
Compare tankless models against manufacturer flow-at-rise tables, not only input rating.
Review setpoints above 125 degrees F carefully. Hotter water increases scald risk unless mixing and anti-scald controls are used.
Heat pump water heater recovery can change with room temperature, airflow, operating mode, and backup element behavior.
Large tubs, body sprays, whirlpools, commercial fixtures, or unusually long showers can exceed the sample worksheets.
Calculations run in the browser. Copied and downloaded outputs come from the values currently entered on the page.

Worked Examples:

Family morning peak:

Two showers, two bathroom handwashing uses, kitchen prep, one dishwasher cycle, and one high-efficiency clothes washer produce 69 peak-hour gallons in the default worksheet. With 15% reserve, the storage target becomes 79.4 gallons FHR.

Cold inlet tankless check:

A 120 degrees F setpoint with 40 degrees F incoming water creates an 80 degrees F rise. If overlapping fixtures total 3.7 GPM and reserve is 20%, the tankless target is 4.4 GPM at that rise. The same flow at a 50 degrees F rise would need much less input.

Large tub case:

A single 45 gallon tub fill can push a storage target above smaller tank classes. It does not drive tankless sizing unless the fill overlaps with showers, sinks, or another active draw in the simultaneous-flow column.

Worksheet repair:

If all simultaneous counts are zero, storage demand can still calculate, but tankless sizing has no positive fixture flow. Add the fixtures that can realistically run together before comparing demand heaters.

FAQ:

Is first-hour rating the same as tank capacity?

No. Tank capacity is stored volume. First-hour rating includes stored hot water plus recovery during the first hour from a fully heated tank.

Why does winter inlet water change tankless sizing so much?

A demand heater must add enough heat immediately. Colder incoming water raises the temperature rise, so the same fixture flow requires more BTU/h or kW.

Should every fixture be counted as simultaneous?

No. Count fixtures that can realistically overlap in the peak case you want to support. Counting everything at once can oversize the unit, while ignoring common overlaps can undersize it.

Can I enter metric worksheet values?

Yes. Select metric entry and use liters, L/min, and degrees C. The calculator converts those values before applying the same sizing equations.

Does this choose the final model?

No. It estimates capacity targets and example fit classes. Final selection should use manufacturer literature, installer review, and local code requirements.

Glossary:

First-hour rating: The amount of hot water a storage water heater can deliver in the first hour from a fully heated tank.
Peak-hour demand: The estimated hot-water volume used during the busiest one-hour period.
Temperature rise: The difference between incoming cold-water temperature and delivered hot-water setpoint.
Simultaneous flow: The combined hot-water flow from fixtures expected to run at the same time.
Reserve: Extra headroom added for worksheet uncertainty, seasonal variation, product-rating differences, 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.
Residential Water Heaters Key Product Criteria, ENERGY STAR.
Avoiding Tap Water Scalds, U.S. Consumer Product Safety Commission.