What Size Water Heater Does Your Home Need?
On this page
- Sizing a Tank Heater by First Hour Rating (FHR) vs. Peak-Hour Demand
- How to Estimate Your Home’s Peak Simultaneous Hot-Water Use
- Sizing a Tankless Heater by GPM and Temperature Rise
- Why Incoming Cold-Water Temperature Changes Tankless Sizing by Region
- Common Sizing Mistakes That Lead to Running Out of Hot Water
- FAQ
- Sources
- Related posts:
The “40 or 50 gallon?” question is the wrong place to start. Two heater types size by two completely different yardsticks, and a tank’s gallon number is one of the least useful figures on the box. A storage tank is sized by how much hot water it can deliver in the busiest hour of your day, not by how much it holds. A tankless unit is sized by how many gallons per minute it can heat across the temperature gap between your incoming cold water and the temperature you want at the tap. Get those two metrics right and you stop running out of hot water mid-shower. Get them wrong and a brand-new heater can feel undersized on day one.
This guide walks through the right yardstick for each type, how to estimate your home’s peak demand, and the regional factor that quietly changes tankless sizing. It does not cover which type to buy, what a replacement costs, or how the units work inside; those live in their own guides.
Sizing a Tank Heater by First Hour Rating (FHR) vs. Peak-Hour Demand
Size a storage tank by its First Hour Rating, not its tank capacity. The First Hour Rating (FHR) is the number of gallons of hot water the unit can supply in one hour starting with a full tank, according to the U.S. Department of Energy. It combines tank volume with how fast the burner or element reheats incoming water, which is why two 50-gallon tanks can have different FHRs. The Department of Energy notes the FHR is printed on the yellow EnergyGuide label in the top-left corner, listed as “Capacity (first hour rating).”
The number you match it against is your peak hour demand: the most hot water your household uses during its single busiest hour. For most homes that hour is a morning stretch of back-to-back showers, or an evening of dishwashing and laundry overlapping. The Department of Energy’s guidance is to choose a model with an FHR that meets your peak hour demand, aiming to land within roughly a couple of gallons of that number rather than far above or below it.
Oversizing wastes energy keeping extra water hot around the clock. Undersizing is what leaves the last person with a cold rinse. The point of FHR is that it ignores how big the tank looks and measures what you actually care about: gallons of hot water available when everyone needs them at once.
How to Estimate Your Home’s Peak Simultaneous Hot-Water Use
Build your peak hour number by adding up the hot-water uses that realistically overlap in your busiest hour. The Department of Energy publishes average gallons per use you can total on a simple worksheet:
| Hot-water use | Average gallons per use (DOE) |
|---|---|
| Shower | 20 |
| Shaving | 2 |
| Hand dishwashing or food prep | 3 |
| Automatic dishwasher | 7 |
| Clothes washer (top-loading) | 25 |
| Clothes washer (horizontal-axis) | 15 |
To use it, pick your busiest hour, then count how many times each use happens in that window. Two showers, one round of shaving, and the dishwasher running would total roughly 49 gallons. Add a top-loading laundry cycle in the same hour and you are near 74. The Department of Energy’s own example worksheet lands at a total peak hour demand of 66 gallons, and a home with that pattern would look for a tank rated at an FHR of about 66 gallons.
A few habits change the math. Newer horizontal-axis washers use less hot water than top-loaders. A large soaking tub or a high-flow showerhead pushes a single shower well past the 20-gallon average. And a household that staggers showers across two hours instead of one has a lower peak than the same number of people all rushing out the door at 7 a.m. Size for how your home actually behaves on its heaviest morning, not its quietest one.
Sizing a Tankless Heater by GPM and Temperature Rise
A tankless heater is sized by flow rate in gallons per minute (GPM) at the temperature rise you need, not by any gallon capacity. It has no tank, so the question is not “how much does it hold” but “how many gallons per minute can it heat right now, by enough degrees.” The Department of Energy explains that tankless units are rated by the maximum temperature rise possible at a given flow rate.
Temperature rise is the gap the heater has to close: your desired output temperature minus your incoming cold-water temperature. The Department of Energy says that unless you know your incoming temperature, you can assume it is around 50°F, and that most households want hot water at about 120°F. That works out to a roughly 70°F rise in a typical case.
Here is the part that surprises people. The same unit delivers fewer hot gallons per minute as the required rise grows. The Department of Energy notes that a 70°F temperature rise is generally possible at about 5 GPM through a gas-fired demand unit and about 2 GPM through an electric one, and that tankless heaters typically deliver hot water at 2 to 5 GPM overall. So a tankless heater is sized to a specific job: enough GPM at your rise to run the fixtures you expect to use at the same time.
To estimate the GPM you need, add up the flow of fixtures that might run together. A modern WaterSense-labeled showerhead is capped at 2.0 GPM (rated at 80 psi) and a bathroom faucet at 1.5 GPM at 60 psi, per the EPA; older fixtures can run higher. A shower plus a kitchen tap can ask for several GPM at once, which is exactly why the Department of Energy warns that running a shower and a dishwasher at the same time can stretch a single tankless unit to its limit. For homes with heavy simultaneous demand, the Department of Energy notes the options are to install more than one unit or to add a dedicated heater for a high-use appliance. Sizing this is a job to confirm with a licensed plumber against the specific model’s GPM-at-rise chart.
Why Incoming Cold-Water Temperature Changes Tankless Sizing by Region
Where you live changes tankless sizing because incoming groundwater is colder in the North than in the South. A tankless unit’s output depends on temperature rise, and rise depends on how cold the water is when it enters the heater. The Department of Energy’s default assumption is an incoming temperature of around 50°F, but it explicitly tells you to use your local figure if you know it, because that starting point is not the same everywhere.
The pattern is straightforward. A home in a cold northern climate may have incoming water well below the 50°F default in winter, which means a larger required rise to reach 120°F, which means fewer hot GPM from the same unit. A home in a warm southern region starts with warmer incoming water, a smaller rise, and more usable GPM from the identical heater. This is why the same tankless model can comfortably run two showers in one state and struggle with one shower plus a faucet in another.
The practical takeaway: do not size a tankless unit off a generic GPM number from a spec sheet. Use the coldest realistic incoming temperature for your area to set your required rise, then read the manufacturer’s GPM-at-that-rise figure. Your local water utility can usually tell you a representative incoming temperature. Storage tanks are less sensitive to this because they reheat a held volume rather than heating water on the fly, though very cold incoming water does slow a tank’s recovery too.
Common Sizing Mistakes That Lead to Running Out of Hot Water
The most common sizing mistake is choosing a tank by its gallon number instead of its First Hour Rating. A 50-gallon tank with a slow recovery can deliver less hot water in a busy hour than a 40-gallon tank with a fast one. Always compare FHR, the figure the Department of Energy points to on the EnergyGuide label, not tank size.
A few other traps show up again and again:
- Sizing for average use instead of peak use. A heater that handles a quiet afternoon can still fail at 7 a.m. Size for the busiest overlapping hour.
- Sizing a tankless unit at a single fixture. A unit that easily runs one shower may stall when a second hot tap or the dishwasher opens. Add up simultaneous GPM, not just one fixture.
- Ignoring incoming water temperature. Using a warm-climate GPM rating in a cold climate overstates what the unit will actually deliver in winter. Size the rise to your coldest realistic incoming temperature.
- Forgetting that fixtures change over time. A new high-flow rain showerhead or a larger tub can push your peak demand past what your heater was sized for years ago.
- Counting on a held-water cushion that isn’t there. A tankless heater has no reserve, so it cannot “borrow” against a stored tank during a spike the way a storage unit can.
If you already run out of hot water often, that is itself a sizing signal worth acting on. Selecting and installing a water heater is licensed-plumber work, and gas, venting, and electrical connections must meet local code, which varies by jurisdiction. Bring your peak demand or your GPM-at-rise target to the conversation so the unit is sized to your home rather than to a generic chart.
FAQ
Does the number of bedrooms or people tell me what size to buy?
Not reliably on its own. Household size hints at demand, but the accurate method is peak hour demand for a tank (matched to First Hour Rating) or simultaneous GPM at your temperature rise for a tankless unit. Two homes with the same headcount can have very different busiest-hour patterns.
What is the difference between tank capacity and First Hour Rating?
Tank capacity is how many gallons the tank holds. First Hour Rating is how many gallons of hot water it can deliver in one hour starting full, combining tank size with recovery speed. The Department of Energy recommends sizing by First Hour Rating because it reflects real delivery, not just storage.
Why does a tankless unit seem weaker in winter?
Colder incoming water means a bigger temperature rise to reach your target, and a tankless heater delivers fewer gallons per minute as the required rise grows. The same unit produces less hot water on a cold morning than on a warm one.
Can one tankless heater run two showers at once?
It depends on the model’s GPM at your temperature rise. Two WaterSense showerheads at 2.0 GPM each need about 4 GPM of hot water; whether a single unit can supply that at your rise depends on its rating and your incoming water temperature.
Is bigger always safer?
No. An oversized tank wastes energy keeping extra water hot, and an oversized tankless unit costs more without a benefit if your real demand is modest. The goal is matching the unit to your peak, not maximizing it.
This article is general information, not professional advice. Selecting, sizing, and installing a water heater involves gas, venting, electrical, and code-required work that varies by jurisdiction; have a licensed plumber confirm the right unit and installation for your home.
Sources
Sizing a New Water Heater, U.S. Department of Energy: https://www.energy.gov/energysaver/sizing-new-water-heater
Tankless or Demand-Type Water Heaters, U.S. Department of Energy: https://www.energy.gov/energysaver/tankless-or-demand-type-water-heaters
Lower Water Heating Temperature (DIY Savings Project), U.S. Department of Energy: https://www.energy.gov/energysaver/do-it-yourself-savings-project-lower-water-heating-temperature
WaterSense Showerheads, U.S. Environmental Protection Agency: https://www.epa.gov/watersense/showerheads
WaterSense Bathroom Faucets, U.S. Environmental Protection Agency: https://www.epa.gov/watersense/bathroom-faucets