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).
Tank size is not about how much water you store. It is about how fast you use it.
The number on a water heater (40 gallon, 50 gallon) is the storage capacity. But storage capacity alone does not tell you whether the heater can keep up with your household. What matters is the first-hour rating: how many gallons of hot water the unit delivers in its peak usage hour. A 40-gallon tank with a powerful burner might have a higher first-hour rating than a 50-gallon tank with a weak one. The calculator above sizes based on your household demand pattern, not just headcount.
How we calculated these numbers▾
Sizing follows DOE first-hour rating methodology: sum of peak-hour hot water draws at the fixture level. GPM ratings for tankless units based on temperature rise requirements (groundwater temp to 120°F setpoint) by region. Cost data from plumber association surveys and manufacturer MSRP (2025-2026). Operating costs use EIA average residential utility rates.
Understanding peak demand
Add up the hot water draws in your busiest hour. Two showers back to back (32 gallons) plus a dishwasher (6 gallons) equals 38 gallons in one hour. Your water heater's first-hour rating needs to exceed this number. If it does not, the second shower goes cold before the rinse cycle finishes. Tankless units solve this differently: they heat water on demand at a fixed flow rate (GPM). As long as total simultaneous flow stays under the unit's GPM capacity, you never run out.
What each type costs to buy and run
Composite illustration based on typical project dimensions, regional contractor pricing, and 2026 material costs. Not a specific real project.
Tank | Tankless | Heat pump WH | |
|---|---|---|---|
| Hot water supply | Limited by tank size | Unlimited (at flow rate) | Limited by tank size |
| Lifespan | 8–12 yr | 15–20 yr | 12–15 yr |
| Space needed | Floor space for tank | Wall-mounted (small) | Floor space + clearance for airflow |
| Installation complexity | Simple replacement | May need gas line/vent upgrade | Needs 240V circuit + condensate drain |
| Best for | Budget, simple replacement | Endless hot water, gas homes | Max efficiency, tax credit, electric homes |
For electric homes, heat pump water heaters are the best long-term value. For gas homes with high demand, tankless gas is the premium option.
For whole-house heat pump sizing, use the heat pump calculator. If you are considering switching from gas to electric water heating as part of an electrification project, the BTU calculator helps size the HVAC side of that transition.
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.
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