12,800+ estimates calculated this month
People in household
Full + half baths
Ground water temp: ~57°F
What runs at the same time during your busiest hour? Typically morning showers plus kitchen use.
Recommended Tank Size
Usage & Demand
Estimated Costs
How to Use This Calculator
Choose tank or tankless
Select whether you want a traditional storage tank water heater or a tankless (on-demand) unit. Tank heaters store hot water and are sized by gallon capacity. Tankless heaters are sized by GPM flow rate. Also select gas or electric fuel type.
Enter household size and bathrooms
Enter the number of people living in the home and the total number of bathrooms. A household of 4 uses roughly 80 gallons of hot water per day. More occupants means higher demand and a larger unit.
Select your climate region
Your climate determines the incoming ground water temperature. Cold climates (37-47°F inlet) require the heater to work harder to reach 120°F, which reduces tankless effective output and increases tank recovery time.
Add your peak hour fixtures
List the fixtures that run simultaneously during your busiest hour. Most homes peak in the morning when showers, sinks, and the dishwasher overlap. The calculator totals the GPM flow and gallon demand to size the unit correctly.
Water Heater Sizing Method
Tank Sizing:
First Hour Demand (FHD) = Sum of (GPM x minutes) for each peak-hour fixture
Select tank where First Hour Rating (FHR) >= FHD
Tankless Sizing:
Peak GPM = Sum of simultaneous fixture flow rates
Temp Rise = 120°F - Inlet Water Temp
Effective GPM = Rated GPM x (77 / Temp Rise)
Select unit where Effective GPM >= Peak GPMWhere:
- FHD
- = First Hour Demand - total gallons of hot water used during the busiest hour of the day
- FHR
- = First Hour Rating - gallons a tank can deliver in the first hour from full heat
- GPM
- = Gallons Per Minute - flow rate at each fixture
- Temp Rise
- = Degrees the water must be heated: 120°F target minus inlet temp
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Frequently Asked Questions
What size water heater do I need for a family of 4?
A family of 4 typically needs a 50-gallon gas tank or a 65-gallon electric tank. Gas tanks recover faster (about 40 gallons per hour vs. 21 for electric), so they can use a smaller tank for the same demand. If two people shower back-to-back while the kitchen sink runs, you need roughly 50-60 gallons in the first hour. A 50-gallon gas unit with an 80-gallon First Hour Rating covers that comfortably.
How do I calculate first hour demand?
First Hour Demand (FHD) is the total hot water you use during your busiest hour. Add up the gallons used by each fixture running during that peak period. A shower at 2 GPM for 8 minutes uses 16 gallons. A kitchen sink at 1.5 GPM for 5 minutes uses 7.5 gallons. Two showers plus a kitchen sink equals 39.5 gallons in the first hour. Match your FHD to a tank with a First Hour Rating (FHR) that meets or exceeds it.
What size tankless water heater do I need?
Tankless sizing is based on peak simultaneous GPM flow, not storage capacity. Add up the GPM of every fixture that could run at the same time. Two showers (2 GPM each) plus a kitchen sink (1.5 GPM) equals 5.5 GPM. In cold climates, derate the unit by 15-40% because the heater works harder to raise cold inlet water to 120°F. A unit rated at 8 GPM in warm climates may only deliver 5-6 GPM in Minnesota.
Does climate affect water heater sizing?
Yes. Inlet water temperature is the single biggest variable in tankless sizing. In Florida, ground water enters at about 72°F, requiring only a 48°F rise to reach 120°F. In Minnesota, ground water is around 37°F, requiring an 83°F rise. That nearly doubles the energy needed per gallon. Tank heaters are less affected because they store and maintain temperature, but recovery time is slower in cold climates.
What is the difference between FHR and recovery rate?
First Hour Rating (FHR) is how many gallons of hot water a tank can deliver in the first hour of use, starting from a fully heated tank. It combines the stored hot water with what the unit can heat during that hour. Recovery rate is how many gallons per hour the unit heats from cold once the tank is depleted. Gas units recover at roughly 40 gallons per hour. Electric units recover at about 21 gallons per hour for a standard 4,500-watt element.
How much does a new water heater cost installed?
A 50-gallon gas tank water heater runs $700-$1,600 installed. A 50-gallon electric tank runs $600-$1,200 installed. Tankless gas units cost $1,500-$3,700 installed, though the higher upfront cost is offset by 20-30% lower annual energy costs and a longer 20-year lifespan vs. 10-12 years for tanks. Installation costs vary based on whether you need new gas lines, venting, or electrical upgrades.
Tank vs tankless - which is better?
It depends on your household. Tank heaters cost less upfront, are simpler to install, and work fine for most families. Tankless heaters never run out of hot water, last nearly twice as long, and save $100-$150 per year on energy. But they cost 2-3x more to install and may need gas line or electrical upgrades. For homes with 5+ people or high simultaneous demand, tankless usually makes more sense. For 1-3 people, a tank is often the better value.
How do I pair water heater sizing with a plumbing estimate?
Once you know the right water heater size, use the construction cost estimator to build a complete plumbing estimate including the unit, labor, permits, and any gas or electrical work. EstimationPro generates full plumbing estimates from photos and notes - including water heater replacement - so you can quote the job in minutes instead of spending the evening on spreadsheets.
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