Water Heater Size Calculator
Choose the perfect water heater capacity. Enter household size and bathroom count to find the ideal tank size or tankless flow rate.
Tank Water Heater Sizing
Tank water heaters store a fixed volume of hot water, delivering it on demand until depleted. Sizing depends on the First Hour Rating (FHR), which is how many gallons the heater delivers in one hour starting with a full tank.
The FHR must meet or exceed your peak hour demand. A family of four using morning showers, dishwasher, and laundry might use 60-70 gallons in one hour. A 50-gallon tank with a high recovery rate (fast reheating) can deliver 70+ gallon FHR, while a 40-gallon tank with slow recovery only delivers 55 gallons FHR.
Standard residential sizes are 30, 40, 50, 65, and 75 gallons. Smaller homes (1-2 people) use 30-40 gallons. Average homes (3-4 people) use 50 gallons. Large families (5+ people) or homes with multiple bathrooms need 65-75 gallons.
Tankless Water Heater Sizing
Tankless water heaters heat water on demand, providing endless hot water as long as flow stays within capacity. Sizing depends on the flow rate (GPM) and temperature rise needed.
Flow rate is the sum of all fixtures that might run simultaneously. A typical shower uses 2.5 GPM, a sink 1.0 GPM, a dishwasher 1.5 GPM. A house where two showers and a sink might run at once needs 6 GPM minimum capacity.
Temperature rise is the difference between incoming cold water and desired hot water temperature. Cold groundwater in northern climates averages 40-50°F, requiring a 70-80°F rise to reach 120°F output. Southern climates with 60-70°F groundwater only need a 50-60°F rise, allowing smaller units.
Comparing Tank and Tankless
Tank heaters cost less and install easily but waste energy on standby losses and eventually run out of hot water during heavy use. A 50-gallon tank delivers about 70 gallons in the first hour, then struggles to keep up if demand continues.
Tankless heaters cost 2-3 times more upfront and may require electrical or gas line upgrades. They never run out of hot water, save 20-30% on energy bills, and last 20+ years versus 10-15 for tank models. However, they deliver limited flow, so large homes may need multiple units.
Hybrid heat pump water heaters combine a small tank with heat pump efficiency, using 60% less energy than standard tanks. They cost $1,200-$2,500 but qualify for tax credits and rebates. Recovery is slower than gas tanks, so size one step larger than you would a standard gas unit.
Frequently Asked Questions
What size water heater for a family of 4?
A family of 4 needs a 50-gallon tank water heater or a tankless unit rated for 6-8 GPM. Increase to 65-75 gallons or 8-10 GPM if you have 3+ bathrooms or simultaneous high demand.
How many gallons per person for a water heater?
Plan for 10-15 gallons per person for tank heaters. A couple needs 30-40 gallons, while a family of 5 needs 65-75 gallons. Adjust upward for multiple bathrooms or heavy hot water use.
Should I get a tank or tankless water heater?
Tank heaters cost less upfront ($800-$1,500 installed) but waste energy on standby heat loss. Tankless units cost more ($2,000-$4,500 installed) but save 20-30% on energy and never run out of hot water if sized correctly.
What GPM do I need for a tankless water heater?
Add up simultaneous demand: showers (2.5 GPM each), dishwasher (1.5 GPM), washing machine (2 GPM). A home running two showers and a dishwasher simultaneously needs 6.5 GPM minimum. Add 20% for safety.
Can a water heater be too big?
Yes. Oversized tank heaters waste energy heating unused water. Tankless units too large for the demand cycle on and off, reducing efficiency and lifespan. Size within 10-20% of calculated need.