Question 1

How do I choose between sizing a Storage Tank versus a Tankless (on-demand) water heater?

Accepted Answer

Pick whichever type you're actually installing or comparing — this calculator sizes both, but with different logic because they fail differently when undersized. A storage tank runs out of hot water once its First Hour Rating is exceeded (everyone after the third shower gets cold water) until it recovers; a tankless unit never runs out, but if simultaneous demand exceeds its rated flow, every fixture running at once gets a lower flow rate or a smaller temperature rise (lukewarm water at every tap, not just the last one). If you haven't decided yet, run both modes with your actual household details and compare the practical trade-offs (upfront cost and installation complexity vs. running out of hot water) before committing.

Question 2

What is First Hour Rating (FHR), and why does this calculator target it instead of just "tank size"?

Accepted Answer

First Hour Rating is the actual gallons of hot water a storage heater can deliver in one hour starting from a full tank — printed on every unit's EnergyGuide label. It matters more than raw tank volume because FHR already blends tank size with recovery rate (how fast the heater reheats water as it's drawn): a gas heater recovers much faster than electric, so a 40 gallon gas tank can have a similar FHR to a 50-plus gallon electric tank. Shopping by FHR (matching or slightly exceeding your calculated peak-hour demand) is more accurate than shopping by tank gallons alone, which is why A.O. Smith's own sizing guidance — the source for this calculator's household-size table — frames it this way.

Question 3

What's the difference between the Quick and Detailed storage sizing methods, and which should I use?

Accepted Answer

Quick sizing looks up a tank-size range directly from your household's occupant count, using A.O. Smith's published guidance (e.g. a 4-person household → 50-75 gallons) — fast, but a single manufacturer's general guidance, not tailored to your specific fixtures or usage pattern. Detailed sizing has you tally the actual gallons of hot water your household draws during its busiest hour, fixture by fixture, producing a precise target First Hour Rating. Use Detailed if your household has an unusual pattern (several people showering back-to-back every morning, a large soaking tub, a top-load washer running during peak hours) — Quick sizing can meaningfully under- or over-estimate those cases.

Question 4

Where does the household-size quick-sizing table come from, and what if I have 6 or more occupants?

Accepted Answer

It's A.O. Smith's own published sizing guidance (a primary water heater manufacturer, not an independently cross-checked third-party figure) — disclosed as single-sourced rather than treated as a universal standard. Their published table stops at 5 occupants (75-100 gallons), so this calculator doesn't guess an extrapolated range for 6 or more people — it flags that case as beyond the quick-sizing table and directs you to the Detailed peak-hour method instead, which scales correctly to any household size since it's built from your actual fixture list.

Question 5

How reliable are the gallons-per-use figures used in Detailed storage sizing?

Accepted Answer

Cross-checked across 3 independent sources (A.O. Smith, energybot.com, and a Midwest Mechanical hot-water-demand guide) with reasonable convergence: shower and bath both land at 20 gallons per use, dishwasher at 7 gallons (two sources agree exactly), and clothes washer at 25 gallons top-load / 15 gallons front-load (a distinction one source made explicitly and the others' ranges support). Some sources further split "total water" from "hot water only" by applying a hot/cold mixing ratio at the tap (e.g. a shower being roughly 70% hot, 30% cold) — this calculator uses the simpler, more commonly published "gallons of hot water per use" figures directly, consistent with how most consumer sizing guides and the manufacturer source present them.

Question 6

Why does a front-load washer use less hot water per cycle than a top-load washer?

Accepted Answer

Front-load (high-efficiency) washers use significantly less total water per cycle — roughly 15 gallons versus 20-25 for a top-load machine — because they wash by tumbling clothes through a shallow pool of water rather than fully submerging them in a large tub. Less total water per cycle means less hot water drawn for the same wash temperature, which is why this calculator lists them as separate fixture options with different gallons-per-use values rather than one generic "clothes washer" entry.

Question 7

Why does Tankless sizing use a different method (GPM + BTU) than the WSFU method on your Pipe Sizing by Fixture Units calculator?

Accepted Answer

Because they answer different questions. WSFU (Water Supply Fixture Units, used on this site's Pipe Sizing by Fixture Units calculator) statistically diversifies demand across many fixtures sharing one supply main, converting a fixture-unit total into an estimated peak flow rate — appropriate when not every fixture in a building runs simultaneously. A tankless water heater has to supply real, instantaneous flow the moment multiple fixtures actually do run at once, so this calculator instead sums the specific fixtures you expect running simultaneously at their real flow rates, with no statistical diversity discount. This is the method every tankless manufacturer (Rinnai, Noritz, Rheem) and third-party sizing tool surveyed during development actually uses for consumer-facing sizing — a code-compliance WSFU-based tankless calculation exists too (some jurisdictions' permit worksheets use it), but that's a different, code-driven exercise already served by the WSFU calculator, not duplicated here.

Question 8

Where does the BTU/hr = GPM x Temperature Rise x 500 formula come from?

Accepted Answer

The constant 500 comes from water's weight (8.33 pounds per gallon) multiplied by 60 minutes per hour (8.33 x 60 = 499.8, rounded to 500 in virtually every published version of this formula) — it converts a flow rate and a temperature rise directly into the heat energy per hour needed to produce that rise. This exact formula and constant were corroborated across every independent source checked during development, making it one of the most consistently-cited formulas in residential water heating.

Question 9

Why does groundwater (incoming water) temperature matter so much, and what happens if I use the wrong value?

Accepted Answer

Temperature rise — the gap between your desired output temperature and the incoming groundwater temperature — is the other half of the BTU formula, and groundwater temperature varies enormously by region and season (roughly 40°F in the Northern US in winter versus 70°F in the Southern US). Using a warm summer-average value instead of the coldest-month figure is the single most commonly cited tankless sizing mistake: a unit sized for a 35°F rise (mild Southern winter) can deliver only about half its rated flow at a 70°F rise (Northern winter), leaving you with a trickle of lukewarm water exactly when demand is highest. Always size to your coldest expected groundwater temperature, not an annual average. The four regional presets here are broad bands, not a city-level figure — if you want a more precise number, several tankless manufacturers publish groundwater temperature maps by ZIP code or region, or you can check your local water utility's annual water quality report; enter that value manually instead of using a preset.

Question 10

Why does this calculator default the desired output temperature to 120°F?

Accepted Answer

120°F is a widely-recommended default that balances two competing risks: hot enough to be useful for washing and to limit bacterial growth (like Legionella) in the tank, but cool enough to meaningfully reduce scald risk at the tap compared to older 140°F defaults, especially in households with children or elderly residents. Adjust it if your fixtures, local code, or household needs call for a different setting — some jurisdictions and situations (dishwashers without their own internal booster heater, for instance) may call for a higher tank temperature with a tempering/mixing valve at the point of use rather than raising every tap's temperature.

Question 11

What does the Efficiency input on the Tankless mode represent, and why does it default to 95%?

Accepted Answer

Manufacturers publish a tankless unit's input BTU rating (the energy it consumes), not its output — some of that input energy is lost rather than transferred into heating the water. Dividing the raw GPM x Rise x 500 figure by an efficiency factor converts the theoretical heat-output requirement into the input rating you'd actually need to shop for. 95% is a reasonable default for a modern condensing gas tankless unit; adjust it lower (rougher estimate: 80-85%) for an older or non-condensing unit if you know your specific model's rated efficiency.

Question 12

Does pipe or fixture material change any of these numbers?

Accepted Answer

No — every figure in this calculator (gallons-per-use, GPM per fixture, the BTU formula) is about water volume, flow rate, and temperature, independent of the pipe or fixture material carrying it. Material selection matters for the actual plumbing installation (see this site's Pipe Sizing by Fixture Units and Drain Pipe Slope calculators for the supply and drainage sides of that), not for sizing the water heater itself.

Question 13

Can this calculator size a water heater for a commercial building or multi-unit property?

Accepted Answer

No — every figure here (the household-size table, the gallons-per-use and GPM defaults, the groundwater presets) is scaled for single-family residential use. Commercial and multi-unit sizing involves substantially higher and differently-shaped demand curves, simultaneous-use diversity factors specific to that building type, and often dedicated commercial-grade equipment sizing standards — treat this calculator as a residential planning tool only, and have a licensed engineer size anything beyond a single dwelling.

Question 14

How does this calculator relate to the Water Tank Capacity Calculator already on this site?

Accepted Answer

They answer different questions about water storage. Water Tank Capacity sizes a bulk water storage tank (for overhead/underground storage, rainwater harvesting, or general household reserve) from daily usage volume and backup-day requirements. This calculator instead sizes a water heater specifically, using First Hour Rating and peak-hour hot-water demand (storage tank mode) or flow rate and BTU/hr (tankless mode) — a water-heating-specific sizing question, not a general storage-volume one, even though a storage-tank water heater is physically a tank.

Question 15

What does this calculator not cover that a full water heater installation would need?

Accepted Answer

This tool sizes capacity only — First Hour Rating or tank-size range for storage, and flow rate/BTU input for tankless. It does not cover venting requirements, gas line sizing, electrical circuit sizing, combustion air requirements, seismic strapping, T&P relief valve/discharge routing, or local code approval processes, all of which vary by unit type, fuel source, and jurisdiction. Treat the output as a planning-stage sizing estimate to bring to a licensed plumber or engineer, not a complete installation design.

Question 16

Why is Heat Pump only offered for Storage Tank and not Tankless?

Accepted Answer

Genuine tankless heat-pump water heaters are a rare, largely commercial niche technology (a heat pump extracts heat gradually, which suits a storage tank's dwell time far better than instantaneous on-demand flow) rather than a standard residential product — most water heater lines don't offer one. Rather than compute a BTU/kW figure for a product category that essentially doesn't exist for homeowners, this calculator only offers Heat Pump as a fuel type under Storage Tank sizing, and silently treats a Tankless + Heat Pump combination as Electric instead.

Question 17

How is the kW and amperage figure calculated for an electric tankless unit, and how accurate is it?

Accepted Answer

The required BTU/hr is converted to kilowatts using the standard 3,412.14 BTU-per-kWh constant, then to amperage using Amps = (kW × 1,000) ÷ 240V — 240V being the standard US residential single-phase voltage for these circuits. A 125% continuous-load safety margin (consistent with NEC continuous-load practice, cited across multiple independent electrical sizing guides) is applied before rounding up to a real, standard breaker size. This is a planning-stage estimate to help you understand roughly what electrical capacity you'd need — a licensed electrician must confirm your actual panel capacity, wire gauge, and code compliance before installation.

Question 18

What does the "feasibility warning" mean, and what should I do if I see it?

Accepted Answer

It means your calculated requirement (target FHR, or tankless BTU/hr or kW) is higher than the largest common off-the-shelf model-size class this calculator lists — in practice, a single standard residential unit likely can't cover it. Rather than silently showing an oversized or fabricated number, the calculator flags this directly. Common fixes include installing two units in parallel (common for large households or high-demand tankless setups), splitting hot water service into separate zones each sized independently, or consulting a professional about commercial-grade equipment.

Question 19

What is a "model-size class," and where do those numbers come from?

Accepted Answer

It's the nearest common off-the-shelf size — 30/40/50/65/75/80/100 gallon tanks, 150,000/199,000 BTU/hr gas tankless units, or 8/11/18/20/24/27/36 kW electric tankless units — that your calculated requirement rounds up to, cross-checked against real retailer and manufacturer listings during development. It's meant to translate a precise calculated number into something you can actually search for and buy, not a claim that every brand offers exactly these sizes — always confirm the exact model lineup for the brand you're considering.

Question 20

What do "comfortable," "borderline," and "oversized" mean in the recommendation badge?

Accepted Answer

They describe how much margin the nearest model-size class gives you over your actual calculated requirement. "Comfortable" (15-50% margin) means that class covers your requirement with reasonable headroom. "Borderline" (under 15% margin) means you're close to that class's limit — worth checking whether the next size up makes sense, especially if your household or usage might grow. "Oversized" (over 50% margin) means the nearest available class is substantially bigger than this specific requirement needs, which isn't a problem, just a sign you have room to consider a smaller class if one exists below it.

Question 21

Can I skip the fixture list for Tankless sizing if I already know my required flow rate?

Accepted Answer

Yes — switch the GPM Input Mode to "Enter Required GPM Directly" and type in your known flow rate. This is useful if you've already done a fixture tally elsewhere, have a manufacturer's recommended flow rate for your situation, or are simply comparing a specific GPM figure against different temperature-rise scenarios without rebuilding a fixture list each time.

Question 22

What does the "single unit feasible" vs "two units likely needed" result mean, and how is the dual-unit figure calculated?

Accepted Answer

It's an explicit statement either way, not just silence when things are fine: "Single unit feasible" means your calculated requirement fits within the largest listed model-size class. "Two units likely needed" means it doesn't, and this calculator then checks whether two of that largest class combined (a common real installation pattern — "twinned" storage tanks or tankless units plumbed in parallel) would cover it, showing that combined capacity and margin. If even two of the largest units wouldn't be enough, it says so directly rather than fabricating a configuration that doesn't actually cover the load — at that point, a commercial-grade unit or a licensed engineer's design is the right next step.

Question 23

Why does the calculator suggest a Heat Pump storage tank instead of electric tankless at high demand?

Accepted Answer

Above roughly 18 kW, an electric tankless unit needs a very large dedicated circuit (order of 90-150+ amps at 240V after the continuous-load margin) that many homes' electrical panels simply don't have spare capacity for without a costly service upgrade. A Heat Pump storage tank sidesteps this entirely: instead of heating water instantaneously at full household demand, it heats a stored volume gradually over many hours using a much smaller, standard circuit (similar to a standard electric resistance tank), drawing on ambient heat rather than raw electrical resistance for most of its energy. This is a suggestion to evaluate, not an automatic "better" verdict — see the fuel type buying guidance for the full tradeoff.

Question 24

How do I choose between Gas, Electric, and Heat Pump for a water heater — what are the real tradeoffs?

Accepted Answer

Gas (or propane) recovers hot water fastest, works in any climate without a large electrical circuit, and is often the lowest running cost where gas is cheap — but requires a gas line and proper venting (power-vent or direct-vent), plus combustion air in an enclosed space. Electric resistance needs no venting or gas line, is simple to install almost anywhere, but recovers slower than gas (storage) and needs a very large circuit at higher tankless demand. Heat Pump water heaters (storage only) are typically the most energy-efficient over time — commonly citing roughly 2-3x the efficiency of standard electric resistance — but respond more slowly to demand, need surrounding air space (a basement, garage, or utility room) to draw heat from, produce cooler, drier exhaust air as a byproduct (a plus in a hot space, a minus in an already-cool one), and cost more upfront. There's no universal right answer — it depends on your fuel availability, panel capacity, installation space, and climate.

Input	Value
Water Heater Type	Storage Tank
Fuel Type	Natural Gas / Propane
Occupants	4

Calculation	Substitution	Result
Tank Size	Lookup(4 occupants)	50-75 gal
Model-Size Class	Nearest class ≥ 75 gal	~75 gal (borderline)

Household Size	Recommended Tank Size
1-2 people	30-40 gal
3 people	40-50 gal
4 people	50-75 gal
5 people	75-100 gal
6+ people	Use Detailed method

Constant	Value
BTU per kWh	3,412.14
Residential Circuit Voltage	240 V
Continuous-Load Safety Margin	125%

Fuel Type	Recovery Speed	Install Constraints	Notes
Gas / Propane	Fastest	Gas line + venting (power-vent or direct-vent) + combustion air	Works in any climate; often lowest running cost where gas is cheap
Electric (Resistance)	Slower (storage); very high draw at tankless scale	No venting/gas line; simple install; large circuit for tankless	Easiest to install almost anywhere; higher running cost in most regions
Heat Pump (Storage Only)	Slowest response	Needs surrounding air space; produces cool/dry exhaust air	Typically ~2-3x more efficient than electric resistance over time; higher upfront cost

Water Heater Sizing Calculator (Storage Tank FHR & Tankless GPM/BTU)

Water Heater Type

Sizing Method

Household Size

Household Size

Design Notes

More water heater sizing calculators

Knowledge Guides — 70 Articles

Purpose of a Water Heater Sizing Calculator

How Water Heater Sizing Is Calculated

Step 1 — Storage Tank Quick Sizing (Household Size)

Step 2 — Storage Tank Detailed Sizing (Peak-Hour Tally)

Step 3 — Tankless Flow Rate & BTU/hr

Step 4 — Tankless Electric Load (kW & Amperage)

Step 5 — Model-Size Class & Feasibility

Worked Example

Step 1 — Tank Size Lookup

Reference Tables

Storage Tank — Household Size Quick Sizing

Model-Size Classes

Electric Load Conversion Constants

Fuel Type Comparison — Buying Guidance

Peak-Hour Gallons per Use

Tankless — Fixture Flow Rates

Groundwater Temperature Presets

How to Use This Calculator

Water Heater Sizing Tips & Best Practices

Common Mistakes to Avoid

Limitations

Related Calculators

FAQ

Fixture	Gallons per Use
Shower	20
Bath (Tub Fill)	20
Bathroom Sink / Shaving	3
Kitchen Sink (Hand Dishwashing)	4
Dishwasher	7
Clothes Washer (Top-Load)	25
Clothes Washer (Front-Load / HE)	15

Fixture	GPM
Shower	2.5
Bathroom Sink	1.5
Kitchen Sink	2
Dishwasher	1.5
Clothes Washer	2
Bathtub Filler	4

Region	Approx. Temp
Very Cold (~40°F)	40°F
Cold (~50°F)	50°F
Moderate (~60°F)	60°F
Warm (~70°F)	70°F