Heat steadily.
Right-size a tank or tankless water heater for your household. Based on peak-hour demand, fixture count, and household size.
How we calculated this
Tank water heaters are sized by the First-Hour Rating (FHR) — how many gallons of hot water the unit can deliver in the first hour, starting with a full tank. The tank size itself is usually smaller than FHR because the heater continues to heat water as you use it. A 50-gallon tank with an FHR of 65-75 gallons is typical.
Tank peak-hour demand is the total hot water you use during your busiest hour. The calculator estimates 20 gallons per shower, 3 gallons per person for sinks (hand washing, shaving), and adds 14 gallons if 3+ simultaneous uses suggest a washing machine is running too. Morning rush in a 4-person household is typically 80-100 gallons.
Standard tank sizes are 30, 40, 50, 65, 75, and 80 gallons for residential. 50-gallon is the most common size for typical 3-4 person homes. 40-gallon works for 1-2 person households. 65-80 gallon for 5+ person homes or homes with large soaking tubs.
Tankless water heaters are sized completely differently: by flow rate (GPM) and temperature rise. The heater must deliver hot water at your peak simultaneous flow, raising the incoming water temperature to 120°F. In cold climates with 35°F incoming water, the temp rise is 85°F — a huge demand. In warm climates with 70°F incoming water, only a 50°F rise is needed.
Tankless BTU calculation: BTU/hr = GPM × temp rise × 500. A family that wants 2 simultaneous showers (5 GPM) in a cold climate (85°F rise) needs 212,500 BTU/hr — more than residential tankless can provide. They'd need two units or a large commercial unit. The same family in Florida (40°F rise) only needs 100,000 BTU, which any standard tankless handles easily.
Not covered: vent sizing (gas units), electrical panel capacity (electric tankless needs large 240V circuits — a 27 kW tankless pulls 113 amps), gas line size (tankless units need ¾" gas lines, tank heaters typically use ½"), or water treatment (tankless is sensitive to hard water — sediment causes scale buildup requiring annual flushing).
Sources
- US DOE — Water Heater Sizing — Official guide to first-hour rating and tank sizing
- Rheem/Rinnai — Tankless Sizing Guides — Industry reference for flow rate and temperature rise
Frequently asked
What size water heater do I need for a family of 4?
A 50-gallon tank handles most 4-person families comfortably. Upgrade to 65-75 gallons if you have 3+ bathrooms with frequent simultaneous use, a large soaking tub, or teenagers who take long showers. For tankless, 160,000-199,000 BTU gas or 27 kW electric handles 2 simultaneous showers in cold climates.
Tank or tankless — which is better?
Tankless: lasts 20+ years, endless hot water, 30% more efficient, takes less space. Tank: cheaper upfront ($400-1,000 vs $1,500-3,500), simpler install, no flow-rate limit. Tankless pays back in 8-15 years via energy savings. For homes where gas line and venting are already set up for tank, a tank is the easier swap. New construction or cold climates: tankless is increasingly the default.
What's first-hour rating (FHR)?
FHR is the gallons of hot water a tank water heater can deliver in the first hour of use, starting with a full tank. More important than tank capacity when comparing heaters. A 50-gallon tank with a high-output burner might have an FHR of 80 gallons — better for busy mornings than a 75-gallon tank with a smaller burner (FHR 70 gallons).
How do I calculate tankless size?
Start with peak simultaneous GPM (shower = 2.5, sink = 1.5, dishwasher = 1.5). Then determine temperature rise (120°F output − your incoming water temp). Multiply: GPM × temp rise × 500 = BTU/hr needed. For a 2-shower family in cold water: 5 GPM × 85°F × 500 = 212,500 BTU/hr, which requires two units or a commercial tankless.
How long do water heaters last?
Tank water heaters: 8-12 years typical, up to 15-20 with anode rod maintenance. Tankless: 20-25 years with annual flushing. Gas tank heaters often outlast electric because the pilot/burner cycle doesn't corrode the tank lining as quickly. Hard water significantly shortens tank heater life.
How much does a new water heater cost?
40-50 gallon gas tank installed: $1,200-2,500. 40-50 gallon electric tank: $900-2,000. Gas tankless: $2,500-4,500 installed (includes venting). Electric tankless: $1,500-3,500 installed (often needs panel upgrade). Heat pump water heater: $2,500-4,500 (Energy Star rebates and tax credits often available).
Should I consider a heat pump water heater?
Yes if you have space (they need 1,000 cu ft of room air), don't mind unit cost ($1,500-3,000 vs $500 for a standard electric), and want lowest operating cost. Heat pump water heaters use 70% less electricity than standard electric. Pay back in 3-7 years in most climates. Don't install in unheated garages where the unit can freeze.
What if I run out of hot water?
Tank: heater is undersized, OR the dip tube is broken, OR the bottom heating element (electric) is failed. For sizing issues: upgrade tank or switch to tankless. Tankless: flow exceeds capacity, OR incoming water is colder than spec, OR gas line is undersized. Verify specs match usage; larger unit may be needed.