Cost & material estimator
Water Heater Sizing Selector
Sizing by gallons is why people buy a bigger tank and still run out of hot water. The number that matters is first-hour rating, and for tankless it is flow at your own winter temperature rise.
- First-hour rating target from your peak-hour demand
- Tank capacity range, with the most likely size
- Tankless GPM at your real temperature rise
- How much a headline GPM figure derates in winter
- Whether your current tank is actually undersized
- Fixture flow rates you can check against your own
Water Heater Sizing Selector — interactive tool
Temperatures
Printed on the shower head. Low-flow is 1.8–2.0.
Winter is what matters — it can be 20°F colder than summer. Roughly 40°F far north, 75°F deep south.
120°F is the usual recommendation.
4 people · 70°F rise · 3.6 GPM peak
What size you actually need
first-hour rating for a tank — a better guide than raw capacity
- Tank capacity
- 30 gal
- Tankless flow
- 3.6 GPM
- Temp rise
- 70°F
Buy on first-hour rating, not gallons. A well-designed 40-gallon gas unit can out-deliver a poor 50-gallon one, because FHR combines stored volume with how fast the unit reheats. It is printed on the yellow EnergyGuide label.
Compare tankless against a tankHome & Garden Living · homeandgarden.living/tools/water-heater-sizing
Tankless ratings are quoted at a temperature rise you will not get in winter
A unit advertised at 7 GPM is usually quoted at a 35°F rise. Your rise is 70°F, which roughly halves the achievable flow — that same unit would deliver closer to 3.5 GPM at your conditions. Always check the manufacturer's flow chart at your own winter inlet temperature, never the headline number on the box.
You need 3.6 GPM at a 70°F rise
That is the figure to match against a tankless unit's flow chart, not its advertised maximum. If the chart does not go down to your temperature rise, ask the manufacturer rather than assuming.
Your current 40-gallon tank should cope
If you are still running out of hot water, the problem is more likely a failed dip tube, sediment reducing effective capacity, or a thermostat set low — all cheaper to fix than replacing the unit.
Final sizing follows the manufacturer, and installation is licensed work
This is a planning figure. Final selection must follow the manufacturer's published sizing data for the specific model, and installation involves gas, venting, water and electrical work that is licensed in almost every jurisdiction. The temperature-and-pressure relief discharge in particular must never be capped or omitted.
If you buy a tank
- Peak-hour demand
- 32 gal
- First-hour rating target
- 35 gal
- Capacity range
- 20–40 gal
- Most likely
- 30 gal
If you buy tankless
- Simultaneous flow
- 3.6 GPM
- At a rise of
- 70°F
- Fixtures counted
- 2
A unit rated at a 35°F rise delivers roughly 50% of that flow at your 70°F rise.
Fixture flow rates
| Fixture | Flow | Peak hour | Note |
|---|---|---|---|
| Shower | 2.1 GPM | 20 gal | The dominant draw in almost every household. Low-flow heads are 1.8-2.0 GPM. |
| Bath fill | 4 GPM | 30 gal | A full bath is the single largest one-off draw in a house. |
| Kitchen sink | 1.5 GPM | 4 gal | Small but often simultaneous with everything else. |
| Bathroom basin | 1 GPM | 2 gal | Minor draw. |
| Dishwasher | 1.5 GPM | 6 gal | Most modern machines heat their own water, which reduces this considerably. |
| Washing machine | 2 GPM | 15 gal | Only on a warm or hot cycle. Cold washes draw nothing. |
- Household
- 4
- Inlet temp
- 50°F
- Setpoint
- 120°F
- Current tank
- 40 gal
Take these numbers further
What this estimate assumes
- · Tank sizing uses the first-hour-rating approach: peak-hour demand plus 10% headroom. First-hour rating is a better guide than raw capacity and is published on the EnergyGuide label.
- · Peak-hour draws are typical US figures scaled by household size against a four-person reference.
- · Tankless sizing sums the flow of every fixture that could realistically run at once, at your own temperature rise rather than the manufacturer's test conditions.
- · Temperature rise is your thermostat setting minus your incoming cold water temperature. Winter inlet is what matters — it can be 20°F colder than summer.
- · Dishwashers that heat their own water draw far less than the figure here suggests. Cold-water laundry draws nothing.
- · This is a planning estimate. Final sizing must follow the manufacturer's data for the specific model.
A planning figure. Final sizing follows the manufacturer's data, and installation is licensed gas, water and electrical work.
Almost everyone sizes a water heater by capacity. Forty gallons, fifty, eighty. It is the number on the front of the tank and the number the salesperson quotes, and it is not the number that determines whether you run out of hot water during the morning rush.
That number is first-hour rating: how many gallons the unit can actually deliver in one hour starting from full, which combines the stored volume with how fast the burner or element reheats. A well-designed 40-gallon gas heater can out-deliver a poor 50-gallon one, because gas recovers far faster than electric. It is printed on the yellow EnergyGuide label of every unit sold in the US, and hardly anyone looks at it.
Tankless sizing has its own trap, and it is worse. Units are advertised at a flow rate measured at a 35°F temperature rise — a comfortable summer condition. In a northern winter, incoming water can be 40°F and you want 120°F, which is an 80°F rise, and the same unit delivers less than half its headline flow. People buy on the box figure and discover the problem in January.
First-hour rating, and why it beats capacity
The first-hour rating answers the only question that matters during a busy morning: how much hot water can this thing give me before it runs out?
It combines two things. The stored volume, which is what the capacity figure describes. And the recovery rate — how quickly the unit heats incoming cold water while you are drawing hot water off the top. A gas burner recovers roughly twice as fast as an electric element of typical rating, which is why gas and electric tanks of identical capacity have very different first-hour ratings.
To size by FHR, work out your peak hour rather than your daily total. Two showers, a basin, and the dishwasher starting is a realistic weekday morning in a family house and comes to around forty-five gallons. Add ten per cent headroom, and that is the FHR to look for. Capacity then follows from it — typically the FHR divided by about 1.5 for a gas tank.
This is also the honest answer to whether your existing tank is too small. If you are running out and your FHR is well below your peak-hour demand, it is. If it is not, the problem is more likely a failed dip tube, sediment reducing effective volume, or a thermostat set low — all much cheaper to fix than replacing the unit.
| Fixture | Flow rate | Peak-hour draw | Note |
|---|---|---|---|
| Shower | 2.1 GPM | 20 gal | The dominant draw in almost every household |
| Bath fill | 4.0 GPM | 30 gal | The largest single one-off draw in a house |
| Kitchen sink | 1.5 GPM | 4 gal | Small, but often simultaneous with everything else |
| Bathroom basin | 1.0 GPM | 2 gal | Minor |
| Dishwasher | 1.5 GPM | 6 gal | Most modern machines heat their own water |
| Washing machine | 2.0 GPM | 15 gal | Only on warm or hot cycles |
Typical peak-hour hot water draw by fixture. Add up what could run in your busiest hour.
The tankless number on the box is not the number you get
Tankless units are rated in gallons per minute, and that rating is always quoted at a specified temperature rise. Manufacturers typically use 35°F, because it produces a flattering figure.
Your actual rise is your thermostat setting minus your incoming cold water temperature, and it is seasonal. In Minnesota in February, incoming water can be 40°F; at a 120°F setpoint that is an 80°F rise. A unit advertised at 7 GPM at 35°F delivers roughly 3 GPM at 80°F — less than half.
Three GPM is one shower and nothing else. That is how a household ends up with a tankless unit that works perfectly all summer and cannot run a shower and the kitchen tap simultaneously in January.
Every manufacturer publishes a flow chart showing output across a range of temperature rises. That chart, at your winter inlet temperature, is the only figure worth sizing on. If a retailer cannot produce it, that is a reason to go elsewhere rather than a reason to guess.
- Look up your local groundwater temperature for winter, not the annual average.
- Size on simultaneous demand, not daily volume — tankless never runs out, it just runs cold when overloaded.
- Above about 5 GPM at a large rise, a single residential unit struggles. Two in parallel or a hybrid with a buffer tank are the usual answers.
- Low-flow shower heads make a real difference here: dropping from 2.5 to 1.8 GPM is a 28% cut in the largest single demand.
Where the third option beats both
This page sizes tanks and tankless because that is what people search for. In a lot of houses the cheapest unit to run is neither: it is a heat-pump water heater.
A heat-pump unit moves heat from surrounding air into the water instead of generating it, giving it a Uniform Energy Factor above 3.0 where the best electric resistance tank manages about 0.95. Running costs are typically a third of an electric tank's, which over a fifteen-year life is a large sum.
The trade-offs are real and worth stating. It needs a reasonable volume of surrounding air — a cupboard will not do — and it cools and dehumidifies the space it sits in, which is a bonus in a humid basement and a nuisance in a heated utility room. It recovers more slowly than a gas tank, so first-hour rating matters more. And it costs more up front.
Size it exactly as you would a conventional tank, using first-hour rating. If you are replacing electric resistance and have a garage or basement, it deserves a quote even though it is not what you came here for.
Getting more from the tank you already have
Before replacing anything, three things are worth checking, because each is cheap and each produces the same symptom as an undersized tank.
Sediment. Minerals settle out of the water and build a layer on the bottom of the tank, which insulates the burner from the water and reduces usable volume. Flushing a tank annually takes half an hour and roughly doubles its working life. On a tank that has never been flushed, expect the first attempt to produce something alarming.
The dip tube. This is the plastic tube that carries incoming cold water to the bottom of the tank so it does not mix with the hot water at the top. When it cracks or breaks, cold water enters at the top and mixes straight into the outlet, and the tank appears to have lost most of its capacity overnight. It is an inexpensive part.
The thermostat. Many tanks leave the factory set at 120°F, which is the recommended setting for scald safety and is correct for most households. If you genuinely need more capacity, raising it increases usable hot water because you mix in more cold at the tap — but above 120°F scald risk rises sharply, and a thermostatic mixing valve is the right way to do it rather than simply turning the dial up.
What this tool assumes
Every number this tool produces rests on these. Change your inputs and they change with you.
- Tank sizing uses the first-hour-rating approach: peak-hour demand plus 10% headroom. FHR is published on the EnergyGuide label.
- Peak-hour draws are typical US figures scaled by household size against a four-person reference.
- Tankless sizing sums the flow of every fixture that could realistically run at once, at your own temperature rise rather than the manufacturer's test conditions.
- Winter inlet temperature is what matters and can be 20°F below summer.
- Dishwashers that heat their own water draw far less than the table suggests. Cold-water laundry draws nothing.
- A planning estimate. Final sizing must follow the manufacturer's published data for the specific model.
Mistakes that cause rework
Each of these costs money or a whole weekend to undo.
Sizing by gallons instead of first-hour rating
Capacity ignores recovery rate. A gas 40-gallon tank frequently delivers more in the first hour than an electric 50-gallon one.
Taking the tankless GPM on the box at face value
It is quoted at a 35°F rise. At a northern winter rise of 80°F the same unit delivers less than half. Always use the manufacturer's flow chart at your own inlet temperature.
Replacing a tank that has simply never been flushed
Sediment reduces usable volume and insulates the burner. Flushing costs nothing and often restores the capacity you thought you had lost.
Ignoring a broken dip tube
It makes a healthy tank behave like a much smaller one, and the part is inexpensive. Worth ruling out before spending four figures.
Turning the thermostat up to get more hot water
It works, and it raises scald risk sharply. Above 120°F, fit a thermostatic mixing valve rather than relying on people being careful at the tap.
Safety & honest limits
- The temperature-and-pressure relief valve and its discharge pipe must never be capped, plugged or omitted. It is the device that stops a water heater becoming an explosive.
- Water above 120°F causes scalding quickly, and far faster for children and older adults. At 140°F a serious burn takes seconds.
- Gas water heaters need adequate combustion air and correct venting. Poor venting causes carbon monoxide, which is why a CO alarm belongs near any fuel-burning appliance.
- Installation involves gas, water and electrical work that is licensed in almost every jurisdiction. This tool sizes; it does not instruct on fitting.
Frequently asked questions
What size water heater do I need for a family of 4?
Typically a 40 to 50 gallon gas tank or a 50 to 80 gallon electric one — but size on first-hour rating rather than capacity. A four-person household with two morning showers and a dishwasher has a peak-hour demand around 45 gallons, so look for a first-hour rating near 50. Electric units need more capacity for the same FHR because they recover more slowly.
What is first-hour rating?
The gallons of hot water a unit can deliver in one hour starting from full, combining stored volume with recovery rate. It is a far better guide than capacity and is printed on the yellow EnergyGuide label. A gas 40-gallon tank often beats an electric 50-gallon one on FHR because gas reheats roughly twice as fast.
What size tankless water heater do I need?
Add the flow rates of every fixture that could run at once — a shower is about 2.1 GPM, a kitchen tap 1.5 — then check the unit's output at your winter temperature rise, not its headline figure. A unit advertised at 7 GPM at a 35°F rise delivers closer to 3 GPM at the 80°F rise a northern winter demands.
Is a 40 or 50 gallon water heater better?
It depends on your peak hour and fuel type, not on household size alone. A 40-gallon gas unit with a high first-hour rating often outperforms a 50-gallon electric one. If you are consistently running out with a 40, check for sediment and a failed dip tube before assuming you need more capacity.
Why do I keep running out of hot water?
Three cheap causes come before an undersized tank. Sediment on the tank bottom reduces usable volume — flush it annually. A cracked dip tube lets cold water mix straight into the outlet, which makes a healthy tank behave like a much smaller one. And the thermostat may simply be set low. Rule those out before spending on a replacement.
What temperature should a water heater be set to?
120°F is the standard recommendation: hot enough to inhibit bacterial growth in the tank, cool enough to reduce scald risk. Above that, burns happen in seconds rather than minutes, and if you need higher storage temperature for capacity reasons, fit a thermostatic mixing valve rather than relying on care at the tap.
Last reviewed July 25, 2026 · Home & Garden Living
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