Water Heater Size Calculator
Water heaters are sized by the busiest hour of the day, not by how many people live in the house. Count the showers, baths, and appliance loads that overlap in your peak hour — usually the weekday morning — and this tells you the peak-hour demand in gallons, the standard tank size whose first-hour rating covers it, and the flow rate a tankless unit would need instead.
Your project
Estimate
93gal
Peak hour demand · Match this with the first-hour rating
- Recommended tank size
- 65gal
- Tankless flow needed
- 6gpm
Estimates are guidance only and exclude tax, labor, and site conditions unless stated. Confirm with a local pro.
How it's calculated
No black boxes — here is the exact math behind your estimate, so you can check it or adapt it for your own quote.
- 1
Peak hour demand = showers × 20 + baths × 20 + hand washes × 4 + dishwasher loads × 10 + laundry loads × 7 - 2
Stored gallons needed = peak hour demand × 0.7 (a tank's first-hour rating ≈ its size ÷ 0.7) - 3
Recommended tank = smallest standard size (30, 40, 50, 65, 80 gal) at or above that target - 4
Tankless flow = greater of 3.0 gpm and showers × 2.0 gpm
Worked example
A four-person household: three back-to-back morning showers, four hand washes, one dishwasher load, and one warm load of laundry.
Inputs
- Showers
- 3
- Baths
- 0
- Hand & face washes
- 4
- Dishwasher loads
- 1
- Clothes washer loads
- 1
Result
- Peak hour demand
- 93 gal
- Recommended tank size
- 65 gal
- Tankless flow needed
- 6 gpm
Tools & materials for this project
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- Water heater expansion tank Required on closed plumbing systems whenever the heater is replaced.
- Water heater drain pan Catches drips and tank failures on any indoor install.
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Frequently asked questions
What is a first-hour rating and where do I find it?
First-hour rating (FHR) is the gallons of hot water a full tank can deliver in one hour, counting both the stored water and what the burner or elements reheat during that hour. It is printed near the top of the yellow EnergyGuide label and in the spec sheet. DOE guidance is to pick a heater whose FHR is within a gallon or two of your peak-hour demand. Gallons alone will mislead you: a 50-gallon gas tank often posts an FHR of 80–90, while a 50-gallon electric posts 60–65. Treat this calculator as a planning estimate only: the final size you can install, along with venting, combustion air, seismic strapping, the temperature and pressure relief discharge and whether a thermal expansion tank is required, is governed by your local plumbing and mechanical code, and a licensed plumber should confirm the selection before you buy.
How do I size a tankless water heater instead?
Tankless units are sized by flow at a given temperature rise, not by gallons. Add up the fixtures that run at once — about 2.0 gpm per modern showerhead, 1.5 gpm for a kitchen sink — then subtract your incoming water temperature from your 120°F target. Groundwater is 65–75°F across the deep south but 35–45°F in the northern tier in winter, so the rise ranges from 45°F to 85°F. A 199,000 BTU/h condensing unit delivers roughly 7 gpm at a 45°F rise but only about 4.5–5 gpm at 70°F. Size for your coldest month, not the annual average.
Do heat pump water heaters need to be bigger?
Usually one size up, yes. A 50-gallon heat pump water heater typically posts a 62–67 gallon first-hour rating in hybrid mode, where resistance elements assist, but in heat-pump-only mode recovery drops to roughly 10–12 gallons per hour against about 20 for a standard electric tank. Households with a tight morning peak are happier on a 65 or 80 gallon model. They also need air to work: manufacturers commonly specify about 700 cubic feet of unrestricted space or a ducted setup, and they cool and dehumidify the room they sit in, which is a bonus in a garage and a nuisance in a finished basement.
Why does the third shower always go cold?
Only about 70% of a tank is usable before the outlet temperature falls off, so a 40-gallon heater really gives you about 28 gallons — roughly a shower and a half — before it is leaning on recovery. Recovery is slow: a standard 4,500-watt electric element makes about 20 gallons an hour at a 90°F rise, while a 40,000 BTU gas burner makes about 40. Three 20-gallon showers back to back is 60 gallons in maybe 35 minutes, which no 40-gallon electric can keep up with. That is the classic symptom of sizing by household count instead of peak hour.
What size water heater does a family of four need?
Run your own peak hour, but the common landing spots are a 50-gallon gas or a 65–80 gallon electric tank. The example above — three showers, four hand washes, a dishwasher load, and a load of laundry — comes to 93 gallons in the peak hour, which needs a first-hour rating near 93 and lands on a 65-gallon tank. Moving one shower and the laundry to the evening drops the peak hour to 66 gallons and a 50-gallon tank. Staggering usage is often cheaper than buying capacity.