SizingKitPeak hour, not household size
Water Heater Size Calculator
A free tool that sizes a storage heater on its first-hour rating and a tankless one on gallons per minute at a temperature rise — the two figures manufacturers actually publish, and neither of them tank gallons against the number of people in the house. It builds your busiest hour from real fixture flows and the durations you enter, converts each mixed gallon to tank gallons through the mixing balance, and prints the incoming water temperature it assumed, because that one figure moves a tankless answer more than anything else on the page. No signup, and it runs entirely in the browser.
- 100% free
- No signup
- Storage and tankless
- IPC 604.4 flow rates
- Rise in °F and °C
Default 55 °F — A representative mains water temperature for the continental United States. Shallow groundwater sits within a degree or two of the mean annual air temperature at the location, which is the fastest way to estimate it for a given place. Runs from about 35–45 °F in the northern tier in winter to 75–80 °F in the Gulf South in summer, and the seasonal swing at one address can be 25 °F. It matters more than any other input to a water heater answer, because it sets the temperature rise: a heater sized on 55 °F inlet water delivers about a third less at a 40 °F winter inlet. Use the local winter figure for sizing, not the annual average.
Default 140 °F — ASHRAE Guideline 12, Managing the Risk of Legionellosis Associated with Building Water Systems, which recommends storing domestic hot water at 140 °F (60 °C) or above and maintaining distribution above 124 °F, because Legionella multiplies between roughly 77 °F and 113 °F and is killed rapidly above 140 °F.
Default 120 °F — IPC 424.3 and 424.5: shower and bathtub valves shall be balanced-pressure, thermostatic or combination type, with the handle stops adjusted to a maximum mixed-water setting of 120 °F. ASSE 1016 and ASSE 1017 are the referenced valve standards. That is the ceiling rather than the setting most people shower at; lower it and the draw falls with it.
At these three temperatures, 76% of every gallon leaving a mixing valve comes from the tank and the rest comes off the cold side.
Build the worst hour
Not the day — the hour. Put in what plausibly overlaps in the busiest sixty minutes of the week, with the durations your household actually runs.
Showers
IPC Table 604.4 caps a showerhead at 2.5 gpm at 80 psi; a WaterSense head is 2. A head made before the 1992 Act passes 4 to 5, and nothing about the fitting tells you which you have — time a bucket.
Tub fills
mixed
A tub is a volume rather than a duration: fill it once with a bucket, or take the capacity off the tub's own spec sheet. A standard 60 in alcove tub is filled to somewhere between 25 and 45 gallons depending on how deep the bather likes it, which is too wide a range for this site to pick a number inside.
Sink and lavatory use
IPC Table 604.4 caps a sink or private lavatory faucet at 2.2 gpm at 60 psi. Hand washing and rinsing are short, and this row is usually the smallest term in the hour — it is here because leaving it out is how a peak hour comes out optimistic.
Dishwasher cycles
drawn hot
Gallons of hot water per cycle, from the appliance's own manual or its ENERGY STAR listing. This site does not supply a figure: a machine with an internal booster heater draws cold and heats it itself, and one without draws several gallons at tank temperature, so the range across models spans the whole term.
Clothes washer cycles
drawn hot
Gallons of hot water per cycle, from the machine's manual. A front loader on a warm wash may draw under 5 gallons and an old top loader on hot may draw 20 — and a household that washes in cold draws none at all, which is why this is a field and not an assumption.
Default 0.7 — The fraction of a storage tank's nominal volume that can be drawn as usable hot water before the outlet temperature falls below a useful level, as incoming cold water mixes with what is left. Used in the arithmetic behind a first-hour rating. Ranges from about 60% to 80% depending on the tank's height, the dip-tube design and how far the outlet temperature is allowed to fall. It is the reason a 50 gallon tank does not deliver 50 gallons. Where the manufacturer publishes a first-hour rating, use that instead of estimating from this.
Both are on the heater's own spec sheet — recovery efficiency for a gas unit, and 0.98 for a resistance element, which puts nearly all of its input into the water. Fill them and the plate adds what this unit recovers in an hour and what tank that leaves you needing. Leave them and the first-hour rating still stands, which is the number you shop with.
FIRST-HOUR RATING TO SHOP FOR
31 gal
116 L drawn at 140 °F
TANK THAT COVERS IT
44 gal
Storage alone, at 70% usable — no recovery counted, because the input rating is blank
| Showers · 40.0 gal mixed | 30.6 gal |
|---|---|
| Tub fills | — |
| Sink and lavatory use | — |
| Dishwasher cycles | — |
| Clothes washer cycles | — |
First-hour rating is the gallons of hot water a storage heater can deliver in one hour starting from a full, hot tank — the usable storage plus one hour of recovery. It is a measured figure under 10 CFR Part 430, Subpart B, Appendix E, printed on the yellow EnergyGuide label, and it is the number a storage heater should be selected on. Tank gallons is not that number, which is why a 40 gallon gas heater and a 40 gallon electric heater are not interchangeable: the gas unit recovers at roughly twice the rate, so its first-hour rating is 20 to 30 gallons higher on the same tank. Size by comparing the first-hour rating against the household's peak hour — not its daily use — and add nothing for the rest of the day.
Size a storage water heater by building the household's worst hour rather than its day: count the uses that plausibly overlap in the busiest hour — showers, a dishwasher, a clothes washer — convert each to gallons at its actual flow rate and duration, convert those mixed gallons to tank gallons at the tank and inlet temperatures, and total them. That total is the first-hour rating to shop for. The method's weakness is honest and worth printing: the answer depends entirely on the durations assumed, and an eight-minute shower and a fifteen-minute shower are a different water heater. Ask for the duration rather than assuming one.
The two hot-water temperature requirements point in opposite directions and both are real. Legionella control wants the tank at 140 °F or above; scald protection requires no more than 120 °F at a shower or tub outlet, and 140 °F water produces a full-thickness burn on adult skin in about five seconds. The resolution is not a compromise setpoint — 130 °F is both a poor disinfection temperature and still a scalding one. It is to store hot and mix down: keep the tank at 140 °F and fit a thermostatic mixing valve at the heater or at each fixture, per ASSE 1017 and ASSE 1016. Storing hotter also increases effective capacity, since each stored gallon makes more mixed water.
The flow rates the hour is built from
Ceilings on what may be sold, not measurements of what is installed.
| Fixture | Maximum | Measured at | WaterSense |
|---|---|---|---|
| Showerhead | 2.5 gpm | 80 psi | 2 |
| Lavatory faucet, private | 2.2 gpm | 60 psi | 1.5 |
| Lavatory faucet, public | 0.5 gpm | 60 psi | — |
| Sink faucet | 2.2 gpm | 60 psi | — |
| Water closet | 1.6 gal per cycle | — | 1.28 |
| Urinal | 1 gal per cycle | — | 0.5 |
IPC (International Plumbing Code) 2021, Table 604.4 — Maximum Flow Rates and Consumption for Plumbing Fixtures and Fixture Fittings, which carries the federal limits of the Energy Policy Act of 1992 as codified at 10 CFR 430.32. WaterSense column: EPA WaterSense specifications, which are voluntary and stricter.
These are ceilings on what may be sold, not measurements of what is installed. A fixture made before the 1992 Act is not bound by them and an old showerhead may pass 4–5 gpm; an aerator can be removed; and the shower figure is measured at 80 psi, so a fixture on a 40 psi system delivers less than its rating. For hot-water sizing, only part of each fixture's flow is hot — roughly two-thirds of a shower at a 105 °F use temperature from a 140 °F tank, and that fraction is computed from the three temperatures rather than assumed.
How to size a water heater on a peak hour
Set three temperatures, build one hour, and read the rating to shop for.
Set the three temperatures
Incoming mains water, storage or outlet temperature, and the temperature the water is actually used at. Use your local winter mains figure rather than an annual average — shallow groundwater sits within a degree or two of the mean annual air temperature, and a heater sized on 55 °F delivers about a third less at a 40 °F January inlet. The plate shows what fraction of each mixed gallon has to come from the tank at the three temperatures you set.
Build the hour that hurts
Not the day — the sixty minutes when two showers, a dishwasher and a load of laundry plausibly overlap. Enter counts and durations rather than accepting an average: an eight-minute shower and a fifteen-minute shower are a different water heater, and the honest weakness of the whole method is that the answer depends on those durations. Appliance rows ask for gallons per cycle from the manual, because a dishwasher with an internal booster draws almost nothing and one without draws several gallons.
Shop on the rating, not the tank
The total is the first-hour rating to look for on the yellow EnergyGuide label — the number the DOE test procedure measures and the number a storage heater should be selected on. Add the unit's input rating and recovery efficiency and the plate turns that into the tank size that carries the hour. For tankless, switch modes: the answer becomes gallons per minute at a rise, which is the pair every manufacturer's selection chart is drawn against.
What goes into the sizing
| Storage method | First-hour rating — usable storage plus one hour of recovery, measured under 10 CFR Part 430, Subpart B, Appendix E and printed on the EnergyGuide label. Not tank gallons: a 40 gallon gas heater and a 40 gallon electric one differ by 20 to 30 gallons of first-hour rating. |
|---|---|
| Tankless method | Flow at a temperature rise. Rise is the outlet setpoint minus the mains temperature, and the conversion of a °F rise to °C uses the difference rule, not the reading rule — 70 °F of rise is 38.9 °C, not 21.1 °C. |
| Fixture flows | IPC Table 604.4 — showerhead 2.5 gpm at 80 psi, sink and private lavatory faucet 2.2 gpm at 60 psi, public lavatory 0.5 gpm. WaterSense figures shown alongside: 2.0 and 1.5. All editable, because these are ceilings on what may be sold rather than measurements of what is fitted. |
| Mixing balance | V_hot = V_mixed × (T_use − T_cold) ÷ (T_hot − T_cold). At 55 °F in, 140 °F stored and a 105 °F shower, 59% of every gallon out of the valve is tank water; drop storage to 120 °F and it rises to 77%. |
| Inlet default | 55 °F, representative for the continental United States and editable. The real span runs from 35–45 °F in the northern tier in winter to 75–80 °F in the Gulf South in summer, and 25 °F of seasonal swing at one address is ordinary. |
| Storage temperature | 140 °F default, per ASHRAE Guideline 12, against a 120 °F cap on mixed water at a shower or tub under IPC 424.3. Both are real requirements pointing opposite ways, and the page prints how they are reconciled. |
| Usable fraction | 0.70 of nominal tank volume, editable, with the 0.60–0.80 published span shown. It is why a 50 gallon tank does not deliver 50 gallons. |
| What is deliberately absent | The first-hour-rating worksheet's gallons-per-use table. Two contradictory versions of it circulate, so the page asks for your durations and appliance figures instead of averaging them. |
Frequently asked questions
How many gallons of water heater does a family of four need?
That question has no answer, which is why this page does not ask it. Two households of four can differ by a factor of two in the hour that matters — four staggered showers and a dishwasher overnight is a different load from two simultaneous fifteen-minute showers and a hot-wash laundry cycle at seven in the morning. Sizing on household count is the shortcut every incumbent calculator takes and it is the reason so many replacements are the wrong size in both directions. Build the hour instead: it takes a minute longer and it produces a number you can defend.
What is a first-hour rating, and where do I find it?
It is the gallons of hot water a storage heater delivers in one hour starting from a full, hot tank — the usable storage plus one hour of recovery — measured under the DOE test procedure and printed on the yellow EnergyGuide label. It is the correct basis for selection because it captures the difference tank gallons hides: a gas unit recovers at roughly twice the rate of a resistance-element electric one, so on identical 40 gallon tanks the gas model's rating is 20 to 30 gallons higher. Compare the number this page returns against that label, not against the tank size on the box.
Why does my tankless heater need a bigger unit in winter?
Because the rise changed, not the flow. A tankless heater has a fixed thermal output, and what it can deliver is that output divided by the rise it has to produce — so the same machine that makes 5 gpm at a 45 °F rise makes about 2.9 gpm at a 77 °F one. In the north the mains can fall from 65 °F in August to 40 °F in February while the outlet setpoint does not move, so the required rise goes from 55 to 80 degrees and the unit gives back nearly a third of its flow. That is why tankless sizing is regional in a way storage sizing is not, and why the inlet temperature field is the most consequential one on this page.
Should the tank be at 120 °F or 140 °F?
Both requirements are real and they point in opposite directions, so the resolution is not a compromise setpoint. ASHRAE Guideline 12 wants storage at 140 °F or above because Legionella multiplies between roughly 77 °F and 113 °F and dies rapidly above 140; IPC 424.3 requires shower and tub valves set to deliver no more than 120 °F, and 140 °F water burns adult skin through in about five seconds. Splitting the difference at 130 gives you a poor disinfection temperature that still scalds. Store hot, fit a thermostatic mixing valve to ASSE 1017 at the heater or ASSE 1016 at the fixtures, and mix down — which also buys capacity, because each stored gallon then makes more mixed water.
Why does the page ask how long my showers are?
Because the answer turns on it and averaging it away would hide that. The DOE first-hour-rating worksheet's gallons-per-use figures exist in two contradictory versions in circulation, and rather than pick one this page multiplies the flow you have by the minutes you run — so an eight-minute shower at 2.5 gpm is 20 gallons of mixed water and a fifteen-minute one is 37.5. That is a whole extra person's worth of demand from one habit, and no calculator that assumes a duration can tell you which household you live in.
Do the dishwasher and the clothes washer count?
Only if they draw hot water, and only you can say how much. A machine with an internal booster heater takes cold water and heats what it needs itself, so it adds nothing to the tank load; one without draws several gallons at supply temperature. A front-loading washer on a warm cycle may use under 5 gallons of hot water and an old top loader on hot may use 20, and a household that washes in cold uses none. That range is wider than the term itself, which is why those rows ask for a figure from the appliance manual rather than offering one.
Do these gallons already account for mixing with cold?
Yes, and it is the step most hand calculations skip. Nobody showers in 140 °F water — the mixed gallons at the fixture are made of tank water and cold water in a ratio set by the three temperatures, so the tank only supplies part of each one. At 55 °F mains and 140 °F storage, a 105 °F shower is 59% tank water, meaning a 20 gallon shower draws about 12 gallons from the heater. The two appliance rows are handled the other way round, because a machine takes its water hot and does its own mixing internally.
Peak hour, first-hour rating and the rise
A water heater has one hard hour a day and is idle for most of the rest, which is why daily consumption is close to useless for sizing one. What matters is the worst sixty minutes: the morning when two showers overlap, or the evening when a bath, a dishwasher and a load of laundry land together. Build that hour out of real flows and real durations, convert each fixture's mixed gallons into tank gallons through the mixing balance, and the total is the first-hour rating to shop for. That figure — usable storage plus an hour of recovery, measured under the DOE procedure and printed on the EnergyGuide label — is the one specification that lets two heaters be compared honestly, and it is precisely the one that tank gallons obscures. Storage capacity says nothing about how fast the tank refills, and refill rate is where a gas unit doubles an electric one.
Tankless sizing is a different question wearing similar words. There is no reserve, so the machine has to make the flow in real time, and what it can make is set by its output divided by the temperature rise. That makes the answer regional: mains water at 40 °F in a Minnesota February and 78 °F on the Gulf Coast in August is a difference of 38 degrees of rise on the same 120 °F outlet, and it very nearly halves what a given unit delivers. It also makes the answer a question about simultaneity rather than about an hour — two showers at once is a bigger machine than four showers spread across a morning, which is the exact inverse of how a storage heater behaves. The mode switch on this page changes the question, not just the arithmetic.
Two constraints sit behind everything above and both are worth stating plainly. The first is the temperature conflict: legionella control wants storage at 140 °F, scald protection caps a shower outlet at 120 °F, and the resolution is to store hot and fit a thermostatic mixing valve rather than to average the two into 130. The second is that hot water still has to arrive: a heater that meets the peak hour perfectly is no use if the branch to the far bathroom holds four gallons of cooling water, which is a pipe volume question about the run rather than a capacity question about the tank. And on a private water supply the same peak hour drives the well pump sizing, because a pump that cannot keep up refills the tank slowly however good its recovery rating is.
Where your household's habits go
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
Shower durations, appliance cycles and the temperatures you set describe a household in some detail, and none of it leaves the tab or is written anywhere. Reload the page and it is gone.