About Water Heater Sizing Calculator (FHR & Tankless GPM)
The water heater sizing calculator answers "what size water heater do I need" both ways the U.S. Department of Energy recommends. For storage and heat-pump (tank) models, size by first hour rating, not tank gallons: estimate the peak hour demand with the DOE Energy Saver "Worksheet for Estimating Peak Hour Demand/First Hour Rating" and pick a model whose FHR — printed on the EnergyGuide label as "Capacity (first hour rating)" — is at or just above that demand. The worksheet values encoded here are the DOE-published averages: shower 20 gallons, shaving 2, hand dishwashing or food prep 3, automatic dishwasher 7, and a clothes washer at 25 gallons for a top-loader or 15 for an H-axis (front-load) machine.
For tankless (demand-type) heaters, size by flow and temperature rise instead: add up the flow rates of the fixtures you expect to run at the same time — the tool uses representative values such as 2.0 gpm for a shower and 1.5 gpm for a kitchen sink, so check your actual fixture specs — and subtract the incoming groundwater temperature from the 120 °F delivery temperature to get the required rise. A typical gas tankless unit delivers about 5 gpm at a 70 °F rise; typical electric units manage only about 2 gpm, which is why whole-house electric tankless service is rare in cold-groundwater regions.
How It Works
- Pick the mode: "Tank (first hour rating)" for storage and heat-pump water heaters, or "Tankless (flow & rise)" for demand-type units.
- Tank mode: think of your busiest hour of the day — usually the morning rush — and enter how many times each worksheet use happens in that hour: showers, shaves, hand dishwashing or food prep, dishwasher loads, and clothes-washer loads (top-loader or H-axis).
- The tool multiplies each count by the DOE average gallons per usage, sums the rows into the total peak hour demand, and tells you to shop for a model with a first hour rating at or just above that number — the chart shows which uses drive the demand.
- Tankless mode: enter how many of each fixture run at the same time, then set the incoming groundwater temperature directly or from the regional presets (about 40 °F in the northern US to 70 °F in the Deep South).
- The tool sums the representative fixture flows into the required gpm and subtracts groundwater from the fixed 120 °F delivery temperature to get the required rise, then compares the pair against a typical gas unit (~5 gpm at a 70 °F rise) and notes when you should consider a larger unit, two units, or staggered use.
Worked Example
A three-person household peaks in the morning: two showers, one shave, and the dishwasher running in the same hour. From the DOE worksheet values that is 2 × 20 + 1 × 2 + 1 × 7 = 49 gallons of peak-hour demand, so they should shop for a storage or heat-pump model with a first hour rating of 49 gallons or more — the closest FHR at or above 49, not a bigger tank for its own sake. Considering tankless instead: two showers plus the kitchen sink running simultaneously need 2 × 2.0 + 1.5 = 5.5 gpm, and with 50 °F groundwater the unit must produce a 120 − 50 = 70 °F rise. That slightly exceeds a typical gas unit's ~5 gpm at a 70 °F rise, so this household would look at a larger unit, two units in parallel, or simply not running the kitchen sink hot during showers.
Formulas
- Peak hour demand (DOE worksheet)
PHD = Σ (n_use × gal_use)- First hour rating selection
FHR ≥ PHD (closest match at or above)- Tankless required flow
Q = Σ (n_fixture × gpm_fixture)- Required temperature rise
ΔT = 120 °F − T_groundwater
Standards & References
- DOE Energy Saver — Sizing a New Water Heater (energy.gov): peak-hour demand worksheet and FHR method
- FTC EnergyGuide label — "Capacity (first hour rating)" for storage water heaters
- DOE Energy Saver — tankless sizing by flow rate and temperature rise (typical gas ~5 gpm, electric ~2 gpm at a 70 °F rise)
Frequently Asked Questions
What is a first hour rating, and how is it different from tank size?
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 — tank volume plus what the burner or element reheats during that hour. It is the number DOE says to size by, printed on the EnergyGuide label as "Capacity (first hour rating)." Two 50-gallon tanks can have very different FHRs: a gas model with a strong burner may out-deliver a bigger electric tank. Match the FHR to your peak-hour demand rather than buying by tank gallons.
How do I estimate my peak hour demand?
Pick the single busiest hour of your day for hot water — typically the morning shower rush — and count every use in that hour, not the whole day. Multiply each count by the DOE worksheet averages (shower 20 gal, shave 2, hand dishwashing or food prep 3, automatic dishwasher 7, clothes washer 25 top-load / 15 H-axis) and sum. The DOE example — three showers, a shave, and hand dishwashing — works out to 65 gallons, so that household should look for an FHR of about 65 gallons or more. The worksheet values are averages without aggressive water-saving fixtures; low-flow showerheads reduce the real numbers.
What temperature rise does my tankless water heater need?
Subtract your incoming groundwater temperature from the delivery temperature — 120 °F for most homes. Groundwater runs roughly 40 °F in the northern US to 70–75 °F in the Deep South; DOE suggests assuming 50 °F if you do not know, which gives a 70 °F rise. The rise matters as much as the flow: a unit rated 7 gpm at a 35 °F rise may deliver only half that at 70 °F, so always read the manufacturer's rise-versus-flow curve at YOUR rise, not the headline gpm.
How many gpm do I need for a tankless unit?
Add the flow rates of everything you expect to run at the same time. With the representative values here, one shower (2.0 gpm) plus a bathroom sink (1.0 gpm) needs 3.0 gpm — comfortably within a typical gas unit's ~5 gpm at a 70 °F rise. Two showers plus the kitchen sink need 5.5 gpm, which slightly exceeds it. Check your actual fixture specs (US showerheads are federally capped at 2.5 gpm; many run 1.8–2.0), and consider low-flow fixtures, a larger unit, or two units where demand is high.
Is a bigger tank water heater safer than sizing to the FHR?
No — oversizing costs money twice. A tank larger than your peak-hour demand needs a bigger purchase price and then loses heat through its shell all day (standby loss), so DOE recommends the closest FHR at or above your demand, not the biggest tank that fits. If your demand only spikes rarely (guests, holidays), size for normal peaks and spread out the rare heavy hour instead of paying year-round for it.
Why does the clothes washer have two different values?
The DOE worksheet splits the clothes washer by drum type: a conventional top-loader averages 25 gallons of hot water per load, while an H-axis (horizontal-drum, front-load) machine averages 15 because it tumbles laundry through a shallow pool instead of submerging it. Use the row matching your machine — and note both values assume a hot/warm cycle; cold-wash loads draw essentially no hot water and can be left out of the peak hour.