Water Heater Size Calculator: Tank First Hour Rating or Tankless GPM
For a household whose busiest hour has two showers, one shave, one hand dishwashing or food prep, one dishwasher load and one front-loading washer load, the US Department of Energy worksheet gives 67 gal peak-hour demand, so look for a storage tank with a First Hour Rating (FHR) of at least 67 gallons. For a tankless unit, size by flow and temperature rise instead: 5.5 GPM (two showers at 2.0 GPM and a kitchen faucet) heated 70°F, from 50°F groundwater to 120°F, needs about 234,800 BTU/hr gas input, or 56.4 kW electric. This is sizing guidance from a published worksheet and a heat-balance formula, not a design; a plumber or the manufacturer’s sizing chart has the last word.
Figures based on the sources listed belowUpdated How we calculate
67 gal peak-hour demand.
67 gal peak-hour demand
- Method: storage tank, DOE peak-hour worksheet
- Peak-hour demand: 67 gal (Showers 2 × 20 = 40, Shaving 1 × 2 = 2, Hand dishwashing or food prep 1 × 3 = 3, Automatic dishwasher 1 × 7 = 7, Clothes washer 1 × 15 = 15)
- First Hour Rating to look for: at least 67 gal; older DOE wording allowed a rating within 1 or 2 gal of it (65 gal and up)
- Your model’s First Hour Rating: enter the First Hour Rating from a model’s EnergyGuide label to compare it
Shopping list
- Water heater expansion tank
- Water heater drain pan
- Braided water heater connectors
- Pipe insulation foam
- Plumbing thermometer
Your project
- Type of water heater: Storage tank (size by First Hour Rating)
- Showers in the busiest hour: 2
- Shaving in the busiest hour: 1
- Hand dishwashing or food prep in the busiest hour: 1
- Automatic dishwasher loads in the busiest hour: 1
- Clothes washer loads in the busiest hour: 1
- Clothes washer type: Front-loading, horizontal axis (15 gallons per load)
- Hot water per shower: 20 gal
- Hot water per dishwasher load: 7 gal
- First Hour Rating of a model you are considering (optional): 0 gal
Also pick up
- Water heater expansion tank
- Water heater drain pan
- Braided water heater connectors
- Pipe insulation foam
- Plumbing thermometer
Reopen this calculation: /calculators/water-heater-size/?s=eyJyb29tIjp7Imxlbmd0aEZ0IjowLCJ3aWR0aEZ0IjowfSwiZGVkdWN0aW9ucyI6W10sIndhc3RlUGN0IjowLjEsInBhcmFtcyI6eyJtb2RlIjowLCJzaG93ZXJzIjoyLCJzaGF2aW5nIjoxLCJoYW5kV2FzaGluZyI6MSwiZGlzaHdhc2hlciI6MSwid2FzaGVyTG9hZHMiOjEsIndhc2hlclR5cGUiOjEsImdhbFBlclNob3dlciI6MjAsImdhbFBlckRpc2h3YXNoZXIiOjcsImNhbmRpZGF0ZUZociI6MCwidGxTaG93ZXJzIjoyLCJzaG93ZXJHcG0iOjIsImJhdGhTaW5rcyI6MCwia2l0Y2hlblNpbmtzIjoxLCJ0bERpc2h3YXNoZXIiOjAsIm91dGxldEYiOjEyMCwiaW5sZXRGIjo1MCwiZnVlbCI6MCwiZWZmaWNpZW5jeVBjdCI6ODJ9fQ
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Gallons per use in the DOE peak-hour worksheet
| Use | Gallons per use | Note |
|---|---|---|
| Shower | 20 | a 10-minute shower at about 2 gallons per minute |
| Shaving | 2 | |
| Hand dishwashing or food preparation | 3 | one line on the worksheet |
| Automatic dishwasher | 7 | per load |
| Clothes washer, top-loading | 25 | per load |
| Clothes washer, front-loading (horizontal axis) | 15 | per load |
Tankless gas input and electric power by flow and temperature rise
| Flow | 40°F rise | 50°F rise | 60°F rise | 70°F rise | 80°F rise |
|---|---|---|---|---|---|
| 2 GPM | 48,800 / 11.7 | 61,000 / 14.6 | 73,200 / 17.6 | 85,400 / 20.5 | 97,600 / 23.4 |
| 3 GPM | 73,200 / 17.6 | 91,500 / 22 | 109,800 / 26.4 | 128,000 / 30.7 | 146,300 / 35.1 |
| 4 GPM | 97,600 / 23.4 | 122,000 / 29.3 | 146,300 / 35.1 | 170,700 / 41 | 195,100 / 46.9 |
| 5 GPM | 122,000 / 29.3 | 152,400 / 36.6 | 182,900 / 43.9 | 213,400 / 51.2 | 243,900 / 58.6 |
| 6 GPM | 146,300 / 35.1 | 182,900 / 43.9 | 219,500 / 52.7 | 256,100 / 61.5 | 292,700 / 70.3 |
| 8 GPM | 195,100 / 46.9 | 243,900 / 58.6 | 292,700 / 70.3 | 341,500 / 82 | 390,200 / 93.7 |
Temperature rise by incoming water temperature
| Incoming water | Temperature rise | Gas input (BTU/hr) | Electric (kW) |
|---|---|---|---|
| 40°F | 80°F | 268,300 | 64.4 |
| 45°F | 75°F | 251,500 | 60.4 |
| 50°F | 70°F | 234,800 | 56.4 |
| 55°F | 65°F | 218,000 | 52.3 |
| 60°F | 60°F | 201,200 | 48.3 |
| 70°F | 50°F | 167,700 | 40.3 |
Fixture flow rates used for the tankless method
| Fixture | Flow | Source | In this calculator |
|---|---|---|---|
| Showerhead, WaterSense | 2.0 GPM or less | EPA WaterSense | used as an option |
| Showerhead, federal maximum | 2.5 GPM | US federal standard (Energy Policy Act of 1992) | used as an option |
| Bathroom faucet, WaterSense | 1.5 GPM | EPA WaterSense (federal standard 2.2 GPM) | used: 1.5 GPM |
| Kitchen faucet | 1.0 to 2.5 GPM | US DOE Building America, typical end-use flow | estimate, used: 1.5 GPM |
| Dishwasher | 1.0 to 3.0 GPM | US DOE Building America, typical end-use flow | estimate, used: 1.5 GPM |
| Bathtub | 2.0 to 4.0 GPM | US DOE Building America, typical end-use flow | not in the calculator |
Methodology & assumptions
Two methods, one for each kind of water heater. STORAGE TANK. The US Department of Energy’s “Sizing a New Water Heater” worksheet: find the hour in which your household uses the most hot water, count the uses in that hour, multiply each by its gallons per use and add them up. That is the peak-hour demand. Gallons per use are 20 for a shower, 2 for shaving, 3 for hand dishwashing or food preparation (one line on the worksheet), 7 for an automatic dishwasher load, and 25 for a top-loading or 15 for a front-loading clothes washer load. TWO VERSIONS OF THE WORKSHEET. The published copies of this worksheet do not agree. The widely reproduced older version lists shower 10, shaving 2, hand dishwashing or food prep 4, automatic dishwasher 6 and clothes washer 7 gallons per use. The current DOE page and DOE-hosted guidance, as found in search results, list shower 20, shaving 2, hand dishwashing or food prep 3, dishwasher 7 and clothes washer 25 (top-loading) or 15 (front-loading). This calculator uses the second set by default, because a 10-minute shower at the 2.0 GPM WaterSense flow is 20 gallons, and undersizing a tank means running out of hot water. The gallons per shower and per dishwasher load are editable, so you can enter the older values (the default household then comes to 47 gal peak-hour demand, not 67 gal peak-hour demand) or your own. The values could not be checked on energy.gov itself when this page was written. The tank is then chosen by its First Hour Rating, the number of gallons of hot water the heater can deliver in one hour starting with a full tank of hot water, printed on the yellow EnergyGuide label as “Capacity (first hour rating)”. The current DOE wording is that the FHR should at least match the peak-hour demand. Older copies of the DOE page and many contractor guides say to pick a model within 1 or 2 gallons of the peak-hour demand; this tool shows both, the target (at least 67 gal here) and the lowest rating the older rule would accept (65 gal), and says which one a model you enter falls under. A tank’s size in gallons is not its FHR: a gas tank recovers faster than an electric one and so has a higher FHR for the same size, which is why the label, not the gallon rating, is the number to compare. Because the worksheet is built on averages, a household that runs a long bath or several loads at once in its busiest hour should count them. TANKLESS. A tankless heater has no stored water, so what limits it is how many gallons per minute (GPM) it can heat by how many degrees. Flow = the sum of the fixtures that run at the same time (showerhead flow from its label, 1.5 GPM for a bathroom faucet, and estimates of 1.5 GPM for a kitchen faucet and a dishwasher). Temperature rise = the hot water temperature you want (120°F by default) minus the temperature of the incoming water in the coldest month, which you enter: it depends on where you live and follows shallow groundwater temperature (the Building America maps of groundwater temperature give it by region; the PNNL gas tankless guide uses about 52°F for Seattle). DERIVATION. Water weighs 8.34 lb per gallon and one BTU raises one pound of water one °F, so heating a flow of 1 gallon per minute by 1°F takes 60 gal/h × 8.34 lb/gal × 1 BTU/lb·°F = 500.4 BTU/h, which is rounded to 500. Heat needed (BTU/h) = GPM × 500 × temperature rise. A gas unit loses some of its fuel heat up the flue, so its burner input is that figure divided by its thermal efficiency: input (BTU/hr) = GPM × 500 × ΔT ÷ efficiency, and this tool uses 82% unless you change it. Electric resistance elements turn nearly all the electricity into heat, so there is no efficiency term. One kilowatt-hour is 3,412 BTU, so one kW is 3,412 BTU/h, and GPM × 500 × ΔT ÷ 3,412 = GPM × ΔT ÷ 6.82. With water at 8.33 lb/gal in place of 8.34 the divisor is 6.83, the figure usually quoted and the one used here: kW = GPM × ΔT ÷ 6.83. Check: 5.5 GPM at 70°F is 192,500 BTU/h of heat, which at 3412 BTU per kWh is 56.4 kW, within 1% of the 56.4 kW from the short formula. The current drawn at 240 V is shown for electric units because it decides the breaker and the wire, but a tankless electric unit often needs one or more 40 to 60 A circuits and may be more than a house’s spare panel capacity; check with an electrician. For gas, the gas line, venting and burner turn-down also matter. Real units are rated at a stated temperature rise (for example “5 GPM at 70°F rise”), which falls as the incoming water gets colder, so compare the result with the maker’s flow-versus-rise chart for your incoming temperature, not just the headline rating. Energy.gov’s rule of thumb is that a 70°F rise is typically possible at about 5 GPM for a gas unit and 2 GPM for an electric one. The shower’s hot flow is lower than its total flow because it mixes cold water in; counting its full flow errs on the safe side. What this is not: a plumbing design. Pipe sizing, recirculation, hot-water-to-the-furthest-tap delay, code and permit requirements are outside it.
- US Department of Energy, Energy Saver, Sizing a New Water Heater: find the busiest hour, use the worksheet of gallons per use to get the peak-hour demand, and choose a storage model whose First Hour Rating matches it; gallons per use as printed on the current worksheet (shower 20, shaving 2, hand dishwashing or food prep 3, automatic dishwasher 7, top-loading clothes washer 25 and horizontal-axis 15). Older copies list shower 10, hand dishwashing 4, dishwasher 6 and washer 7 and say to pick a model within 1 or 2 gallons of the demand
- US Department of Energy, Better Buildings Solution Center, Decision Guidance for Heat Pump Water Heaters (Level 3): the same worksheet with 20, 2, 3, 7 and 25 or 15 gallons per use, and the advice to choose a first hour rating at or above the peak-hour demand
- US Department of Energy, Energy Saver, Tankless or Demand-Type Water Heaters: tankless units generally deliver 2 to 5 gallons per minute, gas more than electric, and a 70°F rise is typically possible at about 5 GPM for gas and 2 GPM for electric
- US Department of Energy, Building America, Tankless Water Heaters: typical end-use flow rates (bath sink 0.5 to 1.5 GPM, bathtub 2.0 to 4.0, showerhead 1.5 to 2.5, kitchen sink 1.0 to 2.5, dishwasher 1.0 to 3.0) and the minimum flow of about 0.5 GPM a tankless unit needs to fire
- Pacific Northwest National Laboratory, Building America Solution Center, Gas-Fired Tankless Water Heaters: size from the winter cold water temperature and the setpoint, with a map of the average temperature of shallow groundwater by region of the United States (about 52°F for Seattle)
- US EPA WaterSense, Showerheads (frequently asked questions and supporting statement): the WaterSense label means 2.0 GPM or less, 20% below the 2.5 GPM federal standard; WaterSense bathroom faucets are 1.5 GPM, 20% below the 2.2 GPM federal standard
- US Energy Information Administration, British thermal units (Btu): a Btu is the heat needed to raise one pound of liquid water one degree Fahrenheit, and 1 kilowatthour = 3,412 Btu
- Unverified, labelled estimates on this page: the energy.gov pages could not be opened directly from the tool used to build this page, so the DOE gallons per use come from search results quoting the DOE worksheet and DOE-hosted guidance and should be checked against the live page. The weight of water, 8.34 lb/gal, is a standard engineering figure not confirmed on a page read here. The 82% gas efficiency, the 1.5 GPM kitchen faucet and dishwasher flows, and the 120°F setpoint are typical figures, not from a data sheet; use the data sheet of the unit you are considering
How to size a water heater
- Choose the type. Pick storage tank to size by First Hour Rating, or tankless to size by flow and temperature rise.
- Storage tank: count the busiest hour. Find the hour your household uses the most hot water and count the showers, shaves, hand dishwashing or food prep, dishwasher loads and washer loads in it.
- Storage tank: match the First Hour Rating. Multiply the uses by the DOE gallons per use and add them for the peak-hour demand, then choose a model whose First Hour Rating on the EnergyGuide label is at least that.
- Tankless: add the flow. Add the GPM of the fixtures that run at the same time, from the showerhead’s label and typical faucet and dishwasher flows.
- Tankless: find the temperature rise. Subtract the coldest incoming water temperature from the hot water temperature you want (120°F is usual).
- Tankless: convert to BTU/hr or kW. Gas input in BTU/hr is GPM × 500 × rise ÷ efficiency; electric power in kW is GPM × rise ÷ 6.83. Check the unit’s rated flow at that rise.
Worked examples
| Project | Result |
|---|---|
| Busiest hour: 2 showers, 1 shave, 1 hand wash or food prep, 1 dishwasher load, 1 front-loading washer load | 67 gal peak-hour demand (40 + 2 + 3 + 7 + 15), so an FHR of at least 67 gal |
| The same hour with a top-loading washer | 77 gal peak-hour demand (40 + 2 + 3 + 7 + 25) |
| A couple: 1 shower, 1 shave, 1 hand wash, no dishwasher or washer in the busy hour | 25 gal peak-hour demand (20 + 2 + 3) |
| A busy family: 3 showers, 2 shaves, 1 hand wash, 1 dishwasher load, 1 top-loading washer load | 99 gal peak-hour demand (60 + 4 + 3 + 7 + 25) |
| A 65 gal First Hour Rating against the default 67 gal demand | 67 gal peak-hour demand; the model is 2 gal short, inside the older 1 to 2 gallon tolerance |
| A 70 gal First Hour Rating against the default 67 gal demand | 67 gal peak-hour demand; the model meets the target |
| Tankless, gas: 2 showers at 2.0 GPM and a kitchen faucet, 50°F to 120°F, 82% efficient | 234,800 BTU/hr gas input (5.5 GPM × 500 × 70 ÷ 0.82) |
| The same load, electric | 56.4 kW electric (5.5 × 70 ÷ 6.83) |
| Tankless, cold climate: 3 showers and a kitchen faucet, 40°F incoming | 365,900 BTU/hr gas input (7.5 GPM at 80°F rise) |
| Tankless, warm climate: the default load with 70°F incoming water | 167,700 BTU/hr gas input (50°F rise) |
| Tankless, one shower only, gas | 85,400 BTU/hr gas input (2.0 GPM at 70°F rise) |
Frequently asked questions
- What size water heater do I need?
- For a storage tank, add up the hot water you use in your busiest hour with the DOE worksheet and look for a First Hour Rating at least that high. Two showers (20 gallons each), one shave (2), one hand dishwashing or food prep (3), one dishwasher load (7) and one front-loading washer load (15) come to 67 gal peak-hour demand. For a tankless unit, add up the flow of the fixtures that run at once and the temperature rise instead: 234,800 BTU/hr gas input for 5.5 GPM at 70°F.
- Why do the DOE worksheet gallons differ between copies?
- Two versions are published. The older one lists shower 10, shaving 2, hand dishwashing or food prep 4, dishwasher 6, clothes washer 7. The current one lists shower 20, shaving 2, hand dishwashing or food prep 3, dishwasher 7, clothes washer 25 top-loading or 15 front-loading. This calculator defaults to the second: a 10-minute shower at the 2.0 GPM WaterSense flow is 20 gallons, and a tank that is too small runs out of hot water. With 10 gallons per shower the default household is 47 gal peak-hour demand instead of 67 gal peak-hour demand. Change “Hot water per shower” to use either. We could not check the values on energy.gov itself, so confirm them there.
- What is the First Hour Rating?
- The First Hour Rating (FHR) is how many gallons of hot water a storage water heater can deliver in one hour starting with a full tank of hot water. It combines the tank’s size with how fast the burner or element heats more, and it is printed on the yellow EnergyGuide label as “Capacity (first hour rating)”. Compare the FHR, not the tank’s gallon size: a 40 gallon gas tank can have a higher FHR than a 50 gallon electric one.
- How close does the First Hour Rating have to be to my peak-hour demand?
- The current DOE wording is that the rating should at least match the peak-hour demand. Older copies of the DOE page and contractor guides say to pick a model within 1 or 2 gallons of the demand. For a demand of 67 gal, a model rated 65 gal is 2 gal short (inside the older tolerance, below the current rule), and one rated 70 gal is above the target. A rating well below the demand runs out of hot water in the busy hour; one well above costs more to buy, so stay close.
- How do I find my peak-hour demand?
- Pick the hour of the day when your household uses the most hot water, often a weekday morning or an evening, and count what happens in it: each shower, shave, round of hand dishwashing or food prep, dishwasher load and washer load. Multiply by gallons per use and add. A couple with one shower, one shave and one hand wash comes to 25 gal peak-hour demand; a family with three showers, two shaves, a hand wash, a dishwasher load and a top-loading washer load comes to 99 gal peak-hour demand. A top-loading washer in the default household adds 10 gallons: 77 gal peak-hour demand.
- What size tankless water heater do I need?
- Work out two numbers: the flow in gallons per minute of the fixtures that run at the same time, and the temperature rise from your incoming water to the hot setpoint. The unit must deliver at least that flow at that rise. 5.5 GPM at 70°F needs about 234,800 BTU/hr of gas input at 82% efficiency, or 56.4 kW electric. A single shower at 2.0 GPM on its own needs 85,400 BTU/hr gas input.
- How do I calculate BTU/hr for a tankless water heater?
- BTU/hr = GPM × 500 × temperature rise ÷ efficiency. The 500 comes from water weighing 8.34 lb per gallon: 1 GPM is 60 gal/h × 8.34 lb/gal = 500.4 lb/h, and each pound takes 1 BTU per °F. For 5.5 GPM, a 70°F rise and 82% efficiency: 5.5 × 500 × 70 ÷ 0.82 = 234,800 BTU/hr. At a high-efficiency 95% the same load is 202,600 BTU/hr gas input.
- How do I convert GPM and temperature rise to kW?
- kW = GPM × temperature rise ÷ 6.83. It is the BTU formula divided by 3,412 BTU per kWh: 500 ÷ 3,412 = 1 ÷ 6.82, and 6.83 is the usual figure when water is taken as 8.33 lb/gal. For 5.5 GPM at 70°F: 5.5 × 70 ÷ 6.83 = 56.4 kW electric. That is a large load, about 235 A at 240 V, which is why whole-house electric tankless units often need an electrical service upgrade.
- What groundwater temperature should I use?
- Use the temperature of the incoming cold water in your coldest month, because that is the hardest case for the heater. It follows shallow groundwater temperature, which runs from the 40s in the north to the 70s in the south of the United States; Building America publishes a map of it by region, and the PNNL gas tankless guide gives Seattle as about 52°F. The default here, 50°F, is a placeholder. Your water utility or a plumber can tell you the winter figure. In a cold place, 40°F with three showers is 365,900 BTU/hr gas input on a 80°F rise, against 167,700 BTU/hr gas input at 70°F incoming for the default fixtures.
- How many GPM does a shower or faucet use?
- EPA WaterSense showerheads are 2.0 GPM or less, and the federal limit is 2.5 GPM. A WaterSense bathroom faucet is 1.5 GPM against a federal 2.2. DOE’s Building America guide gives typical ranges of 1.5 to 2.5 for a showerhead, 1.0 to 2.5 for a kitchen sink and 1.0 to 3.0 for a dishwasher. Switching the default example from 2.0 to 2.5 GPM showerheads raises it from 5.5 GPM to 6.5 GPM and the gas input from 234,800 to 277,400 BTU/hr.
- Gas or electric tankless: which size should I use?
- Use the line for your fuel. The gas figure is the burner’s input, so it is the number on the unit’s data plate, and it already allows for the efficiency you enter. The electric figure is the power drawn. The same load, 5.5 GPM at 70°F, is 234,800 BTU/hr by gas (68.8 kW-equivalent of input at 82%) and 56.4 kW by electricity. Energy.gov notes gas tankless units reach higher flow rates than electric ones.
- Is this the same as a plumber’s sizing?
- No. This is sizing guidance from a published worksheet and a heat-balance formula. It does not cover pipe sizes, recirculation, how long hot water takes to reach a distant tap, venting and gas line capacity, electrical service, codes or permits. Use it to know what to ask for, and have a plumber confirm the choice, especially for a tankless unit.
- How does this relate to sizing an air conditioner?
- Both turn a heat-per-hour requirement into a rating in BTU/hr: a room air conditioner removes heat at the rate shown on the unit, and a tankless water heater adds heat at the rate shown on its data plate. They are separate calculations, though; the air conditioner calculator uses room area, sun and people, and this one uses flow and temperature rise.