What Size Water Heater Do I Need?
Updated 2026-08-16 · 6 min read
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Tank capacity is the number on the box. It's not the number that determines whether you run out of hot water.
What is a first-hour rating?
First-hour rating (FHR) is how many gallons of hot water the unit can deliver in one hour, starting with a full hot tank. It combines two things:
- Storage — the tank volume
- Recovery — how fast the burner or elements reheat incoming cold water
This is why a 40-gallon gas heater can outperform a 50-gallon electric one. Gas burners recover much faster than resistance elements, so more hot water arrives within the hour.
FHR is on the EnergyGuide label. Match FHR to your peak hour demand, not tank size to household size.
How do you work out your peak hot water hour?
Identify the busiest hour in your household — usually a weekday morning — and add up what happens in it:
| Use | Approx. gallons of hot water |
|---|---|
| Shower | 10–20 |
| Bath | 20–30 |
| Shaving (sink) | 2–4 |
| Hand/face washing | 2–4 |
| Dishwasher | 6–10 |
| Hand dishwashing | 4–8 |
| Clothes washer (warm) | 7–15 |
A worked example — four people, weekday morning:
- 3 showers × 15 gal = 45
- Shaving and washing = 5
- Dishwasher = 8
- Peak hour ≈ 58 gallons
So you want an FHR of roughly 58 or more. That might be a 50-gallon gas unit or a 60–66 gallon electric one, depending on recovery.
What size water heater does a family need?
| Household | Electric tank | Gas tank |
|---|---|---|
| 1–2 people | 30–40 gal | 30–40 gal |
| 2–3 people | 40–50 gal | 40 gal |
| 3–4 people | 50–60 gal | 40–50 gal |
| 5+ people | 66–80 gal | 50–75 gal |
Use these to sanity-check, not to decide. Two households of four can differ by a factor of two in actual peak demand depending on whether showers are staggered or simultaneous.
Can a water heater be too big?
A larger tank keeps more water hot around the clock, whether you use it or not. That's standby loss, and it runs every hour of every day.
It also costs more upfront and takes more space — which matters in a tight utility closet, particularly if you're considering a heat pump model that's taller.
There's no comfort benefit to capacity you never draw on.
What happens if a water heater is too small?
Running out mid-shower, then waiting for recovery. If that's your current experience, the fix is more FHR — which can come from more storage, faster recovery, or both.
Note that running out and not hot enough are different problems. Water that's never hot enough points at the thermostat setting or a failed element rather than sizing — see water heater temperature setting and water heater not heating: troubleshooting.
How do you size a tankless water heater?
Completely different method. Tankless is sized by flow rate at a temperature rise, not by gallons.
- Decide simultaneous fixtures. Two showers plus a sink, say.
- Add their flow rates in GPM. Two showers at 2.0 plus a sink at 1.5 = 5.5 GPM.
- Determine your temperature rise. Target temperature minus incoming groundwater temperature. Groundwater varies from around 70°F in the far south to under 40°F in northern winters.
- Check the unit's rating at that rise. This is where headline specs mislead — a unit rated 7 GPM at a 45°F rise may deliver only 4 GPM at an 80°F rise.
Cold-climate homes need considerably more tankless capacity than the same household in a warm one. See tankless vs tank water heater.
Sizing heat pump water heaters
Two adjustments from a standard electric tank:
Recovery is slower in heat-pump-only mode, because the heat pump works gradually. Many people size up one increment — choosing 65–80 gallons where they'd have taken 50 — so that storage covers peaks without falling back to resistance elements.
Hybrid mode covers peaks with the resistance elements, at resistance running cost. Sizing up trades a slightly larger tank for less time in expensive mode.
Also check physical height — heat pump models are taller — and the minimum room volume they require. See heat pump water heaters explained.
Other factors
Large tubs. A soaking tub can consume 30+ gallons at once and drives sizing on its own.
Simultaneous use patterns. Staggered showers need far less capacity than simultaneous ones. This is a behavior variable that changes the answer more than household size does.
Recirculation systems. They improve wait times but add heat loss, which effectively increases demand.
Future household changes. Water heaters last 10–15 years. If teenagers are coming, size for them.
Where to go next
- Heat pump water heaters explained
- Water heater installation cost
- cost to run a water heater
- Run the numbers: water heater replacement cost calculator
More in our heating and cooling guides.
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