Peak hour, not daily total
Household hot water use is not spread evenly. A four-person house might draw 60 gallons across a day and forty of those in a single hour on a weekday morning. The heater does not care about the day; it cares about that hour, because that is the only time it is asked to deliver more than it can make. Every other hour it is coasting.
So the sizing question is: in the busiest realistic hour, how many gallons of hot water leave the top of the tank? Add up the draws, do not average them. Two showers, a dishwasher started on the way out the door and four trips to a basin is a real morning, and it is roughly 45 gallons. That is the number the heater is sized against.
The manufacturer's answer to that question is the first hour rating, printed on the unit's energy label. It is a tested figure: start with a full hot tank, draw water until the outlet temperature falls, and count what came out in sixty minutes. It combines two things — what was already stored, and what the burner or element managed to make while you were drawing.
The two halves of a first hour rating
First hour rating ≈ usable stored volume + recovery in one hour
Usable stored volume is not the tank size. Cold water enters the bottom while hot leaves the top, and the two mix at the boundary, so the outlet cools off before the tank is empty. Sixty to seventy-five percent of nominal capacity is the range people work with, and the exact figure depends on the dip tube, the draw rate and how much stratification survives.
Recovery is straightforward arithmetic. It takes 8.34 BTU to raise one gallon of water one degree Fahrenheit, because a gallon weighs 8.34 pounds and water's specific heat is one BTU per pound per degree. So:
Recovery in gallons per hour = BTU per hour of input × efficiency ÷ (8.34 × temperature rise)
A 40,000 BTU gas burner at 80 percent efficiency, working against a 70 degree rise, makes 32,000 usable BTU per hour and divides it by 584 BTU per gallon: about 55 gallons an hour. A 4,500 watt electric element is 15,354 BTU per hour, which at the same rise is about 26 gallons an hour. That gap is the whole reason a 50 gallon electric heater and a 40 gallon gas heater can have similar first hour ratings.
Tankless is a flow limit, not a volume limit
A tankless unit never runs out of hot water and it never delivers more than its rated flow. Both halves of that sentence matter. The flow it can hold follows from the same energy arithmetic run per minute instead of per hour:
GPM = BTU per hour × efficiency ÷ (temperature rise × 8.34 × 60)
A 199,000 BTU unit at 82 percent efficiency against a 70 degree rise gives about 4.7 GPM. Against a 40 degree rise in August, the same unit gives 8.1 GPM. This is why tankless sizing is a winter problem: the unit that ran two showers in July runs one in January, and the failure mode is not a cold shower but a lukewarm one, because the unit modulates down rather than shutting off.
| Fixture | Typical flow | Hot fraction at a warm setting |
|---|---|---|
| Shower head, current standard | 1.8 - 2.0 GPM | roughly two thirds |
| Older shower head | 2.5 GPM or more | roughly two thirds |
| Kitchen faucet | 1.8 - 2.2 GPM | varies with the task |
| Lavatory faucet | 1.2 - 1.5 GPM | small |
| Tub filler | 4 - 7 GPM | most of it |
Flows depend on the fixture and on the supply pressure behind it. If the pressure at the fixture is the question rather than the flow, the pressure drop calculator works out what the run takes out before the water arrives.
Temperature: a tradeoff, not a setting to copy off a page
Storage temperature pulls in two directions at once and there is no number that wins both. Hotter stored water suppresses bacterial growth in the tank, including Legionella, which is a genuine hazard in stored warm water and more of one in low-use buildings and long dead-leg pipe runs. Hotter water at the outlet also scalds faster, and the time to a serious burn falls very steeply as temperature rises — the difference between a comfortable wash and a hospital visit is a small number of degrees and a few seconds, and the people most at risk are small children and the elderly, who are also the least able to move out of the way.
The way the industry resolves that conflict is to separate the two questions: store at whatever temperature the bacterial side calls for, then bring the delivered temperature down at the outlet with a thermostatic mixing valve, so the tank runs hot and the tap does not. That is plumbing work with a device selection attached to it, and the right storage temperature for your building depends on the system, its usage pattern, who lives there and what your jurisdiction requires. Take that decision to a plumber who can look at the installation rather than to a number on a web page. What this calculator needs from you is simply whatever temperature the heater is actually set to now, so the rise arithmetic is right.
What sizing cannot fix
Two complaints look like undersizing and are not. The first is a long wait for hot water at a distant fixture: that is the volume of pipe between the heater and the tap divided by the flow, and a bigger heater does not shorten it by a second. The pipe volume calculator gives you that number directly, and the answer is usually a smaller branch line or a recirculation loop. The second is water that is hot for two minutes and then not: that is a dip tube, a failed lower element, or a heavy layer of sediment, and it is a repair rather than a purchase.
Running cost is a separate question from sizing and is worked out in the water heater running cost calculator, which handles the annual energy and the tank-against-tankless comparison on the bill side. Plumbing work is regulated, and the requirements are not the same from one town to the next. There are several model codes, jurisdictions adopt different ones and then amend them, and the local authority having jurisdiction decides what applies to your building. Most permanent work needs a permit and an inspection. Anything that touches the water service, the gas line, or a backflow-prevention point is licensed work in essentially every jurisdiction. What is on this page is common working practice and arithmetic, not a requirement you can hold up to an inspector.
Questions people ask
What size water heater do I need for a family of four?
The bedroom count and the head count are proxies for the thing that actually matters, which is the peak hour. Four people who shower at staggered times through the evening put a very different load on a heater than four people who all leave the house at the same time. Work out the busiest hour honestly — the showers, whether the dishwasher and washing machine overlap with them, and the inlet temperature in the coldest month — and compare that against the first hour rating on the label. If your peak hour is 45 gallons, you want a first hour rating comfortably above 45, which several different combinations of tank size and burner input will satisfy.
Why does my tankless heater deliver less in winter?
Because it is heating colder water to the same outlet temperature, and its output is fixed in BTU rather than in gallons. Groundwater entering a northern house in February can be in the low 40s and in August the high 60s. Going from a 55 degree rise to an 80 degree rise cuts the achievable flow by about a third, which is often exactly the difference between two showers working simultaneously and not. Size a tankless unit against your coldest inlet temperature, not an annual average, and if you do not know what yours is, ask a local plumber what they see in that town in winter.
How is the first hour rating different from the tank size?
The tank size is how much water it holds; the first hour rating is how much hot water it can deliver in sixty minutes starting from full. The rating is always larger than the usable storage, because the burner keeps working during the draw. A 40 gallon gas heater with a strong burner can outperform a 50 gallon electric one on first hour rating despite holding a quarter less water, since gas recovery is typically twice the electric figure at the same rise. When you compare units, compare first hour ratings rather than gallons.
Can I just turn the thermostat up to get more hot water?
It does increase effective capacity, because you then mix in more cold at the fixture and each stored gallon goes further. It also increases scald risk at every tap in the building, immediately and permanently, and that risk is not evenly distributed across the people who live there. Turning up a thermostat is not the way to solve an undersizing problem on its own. If higher storage temperature is warranted for your system, the accompanying piece is thermostatic mixing at the outlet so the delivered temperature does not rise with it — and that pairing is a decision for a plumber who can see the installation, not something to set from a chart.
Does this account for gas line size or venting?
No, and deliberately so. A tankless unit at 199,000 BTU can demand several times the gas flow of the tank it replaces, and the existing line, the meter and the vent may all be wrong for it — but sizing gas piping, providing combustion air and building a vent are licensed trades work in every jurisdiction, because getting them wrong produces carbon monoxide and you cannot smell it. Use this page for the water side and hand the fuel and flue side to a licensed installer.