Water heater sizing
What size water heater do I need?
3-step calculator Based on DOE 10 CFR 430 FHR standards Reviewed by the WhatSize editorial team
Last reviewed June 2026 • Sources: 10 CFR 430, ENERGY STAR, AHRI Directory
Quick answer
Water heater size is measured in gallons (tank) or GPM (tankless). A 1-2 person household needs 30-40 gal or 6-8 GPM; a 3-4 person household needs 40-50 gal or 8-10 GPM; a family of 5+ needs 50-80 gal or 10+ GPM. The 3-step wizard below applies the DOE 10 CFR 430 First Hour Rating methodology to your household and usage pattern.
How many people in your household?
We'll use your household size and shower count to determine the right capacity.
What's your hot water usage?
Select how many people shower in the morning peak period.
Based on DOE 10 CFR 430, Subpart C (First Hour Rating)
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Got a result that doesn't match your situation?
Adjust your inputs above and re-run the calculator. Most users find their spec falls within 1-2 sizes of the recommendation.
Formula
Peak Demand = ∑(Fixtures × Gal/Use) → Tank Size = smallest FHR ≥ Peak Demand × 1.10
→ Tankless GPM = based on simultaneous shower count
→ DOE 10 CFR 430 First Hour Rating methodology
About this calculation
This is a simplified sizing tool based on published industry standards, not a full professional audit.
Accuracy depends on your input — subjective factors like dampness level or insulation quality are estimates. Our formulas include a safety buffer that may result in oversizing for some situations (slightly higher cost, but safer than undersizing).
Local variables (climate zone, altitude, insulation levels, building codes) can affect real-world sizing needs. Standards are re-checked quarterly — see our Editorial Policy.
Common scenarios for water heater sizing
If your situation matches one of these, jump straight to a detailed guide:
Frequently asked questions
What size water heater do I need for my family?
Use DOE First Hour Rating (FHR). 1-2 people: 30-40 gallon tank or 6 GPM tankless. 3-4 people: 40-50 gallon tank or 8-10 GPM tankless. 5+ people: 50-80 gallon tank or 10+ GPM tankless. The wizard above calculates your FHR need.
Tank or tankless water heater?
Tank: lower upfront ($800-1,500), 10-15 year lifespan, simple install, no flow rate limit within tank capacity. Tankless: higher upfront ($1,500-3,500), 20+ year lifespan, endless hot water, may require a gas line upgrade or high-amp electric circuit.
What is First Hour Rating (FHR)?
FHR is the DOE 10 CFR 430 standard: how many gallons of hot water a tank can deliver in one hour starting with a full tank. It's the real-world measure of usable hot water capacity. A 50-gallon tank with a 40,000 BTU burner typically has an FHR of 80-90 gallons.
How many GPM do I need for tankless?
Add the GPM of all fixtures you might run simultaneously. Typical shower: 2.0 GPM. Kitchen sink: 1.5 GPM. Dishwasher: 1.0 GPM. Two showers plus kitchen = 5.5 GPM. Add a 20% buffer for a 6.5 GPM minimum tankless capacity.