Water Heater Sizing Calculator
Free tool
Sizing by household size alone is why so many homes run out of hot water. This works it out the way a plumber does — from your real peak hour, fixture by fixture.
- Tank and tankless
- Uses first-hour rating
- Adjusts for your climate
They size on completely different rules — a tank on total gallons, a tankless on flow rate.
Count what actually overlaps in your peak hour — usually a weekday morning.
A tankless is sized on simultaneous flow, not on daily total.
120°F is the standard setting and what most manufacturers rate against. Going higher increases the required output sharply.
Peak hour demand of 33 gallons calls for a first-hour rating of at least 33.
Save this sizing before you shop.
What this calculator does
It sizes a water heater from your real peak hour — the busiest 60 minutes in your home — rather than from household size, which is why so many homes still run out of hot water.
How tank sizing works
- Shower — 20 gallons
- Bath — 20 gallons
- Dishwasher — 6 gallons
- Laundry (warm) — 7 gallons
- Hand washing — 4 gallons · shaving — 2
The number that matters is first-hour rating (FHR): stored hot water plus whatever the unit reheats during that hour. A 40-gallon gas heater can beat a 50-gallon electric because gas recovers far faster.
How tankless sizing works
A tankless heats water as it flows, so it is sized on simultaneous flow, not daily volume. Temperature rise is your target minus the incoming groundwater — about 40°F in Minnesota and 75°F in Florida. Reaching 120°F means an 80°F rise in the north but only 45°F in the south, and the same unit delivers roughly half the flow in a cold climate.
Why sizing wrong costs money
Undersize and you get cold showers. Oversize a tank and you pay continuously through standby heat loss, plus more upfront. Size to your peak hour with a small margin — not to the biggest unit that fits the space.
Before you buy: check the yellow EnergyGuide label for the actual FHR. Manufacturers vary within the same nominal gallon size.
Your peak hour is highlighted. First-hour rating matters more than the gallon number on the box.
First-hour ratings are typical manufacturer figures and vary by model — always check the yellow EnergyGuide label before buying.
Get your sizing plus the water heater buying checklist
We'll email the size you just calculated, along with the checklist that stops people buying the wrong unit.
- Your peak demand and recommended size
- The 8 questions to ask before you pay an installer
- Gas line, venting and breaker requirements that add cost
What size water heater do I need for a family of 4?
Most families of four land on a 50-gallon gas or 66–80 gallon electric tank — but household size alone is a poor guide. What matters is your peak hour. Four people who shower one after another need far more capacity than four who spread it across the day. Use the calculator above with your real morning routine.
What is first-hour rating, and why does it matter more than gallons?
First-hour rating (FHR) is how much hot water a heater can actually deliver in one busy hour — stored volume plus what it reheats during that hour. A 40-gallon gas heater can have a higher FHR than a 50-gallon electric, because gas reheats far faster. Match FHR to your peak hour demand and you will not run out.
Why does my location change the tankless size?
A tankless heats water as it flows, so its capacity depends on how far it has to raise the temperature. Incoming groundwater is about 40°F in Minnesota and 75°F in Florida. Reaching 120°F means a 80°F rise in the north but only 45°F in the south — the same unit delivers roughly half the flow in cold climates.
Is it bad to buy a bigger water heater than I need?
Oversizing a tank costs you money continuously through standby heat loss, and costs more upfront. Undersizing means cold showers. Size to your peak hour with a small margin — not to the biggest unit that fits the space.
How long should a water heater last?
A tank typically lasts 8–12 years, a tankless 15–20. Flushing sediment annually and replacing the anode rod every few years makes a large difference to a tank's lifespan, particularly in hard water.
