Use the chart below to find your water heater size by household size and peak-hour demand. A family of 4 with average use needs a 40-50 gallon gas tank (FHR 80-90) or a 7-9 GPM tankless. All recommendations follow the DOE First Hour Rating standard (10 CFR 430). Use our free calculator for a custom recommendation in 60 seconds.
Water Heater Sizing Table: Tank Gallons & Tankless GPM by Household Size
Household
Peak Demand gallons in 1 hour
Gas Tank gallons (FHR)
Electric Tank gallons (FHR)
Tankless GPM
1 adult
~30 gal
30 gal (FHR 50)
30 gal (FHR 42)
3-5 GPM
1-2 people
~40 gal
30-40 gal (FHR 60-70)
30-40 gal (FHR 50-58)
5-6 GPM
2-3 people
~55 gal
40 gal (FHR 70-80)
40-50 gal (FHR 60-70)
6-7 GPM
3-4 people
~70 gal
40-50 gal (FHR 80-90)
50 gal (FHR 70-80)
7-9 GPM
4-5 people
~80 gal
50 gal (FHR 90-100)
50-65 gal (FHR 80-90)
9-10 GPM
5+ people
~100 gal
50-65 gal (FHR 100+)
65-80 gal (FHR 90+)
10-12 GPM
6+ / large family
~120+ gal
65-80 gal (FHR 120+)
80+ gal (FHR 100+)
12+ GPM
How to read: Find your household row. The "Peak Demand" column shows typical peak-hour hot water use. Match to a tank size whose FHR equals or exceeds that number, or a tankless GPM that meets your simultaneous draw. Electric tanks deliver 10-15% less FHR than gas at the same gallon size due to slower recovery. Color key: green = small household, amber = standard, orange = upgrade, red = large family or commercial.
Adjustments: When to Size Up
Multiple Showers at Once
If 3+ people shower in the same hour, add 10 gal per extra shower to peak demand.
Whirlpool or Soaking Tubs
A standard 60-gal tub needs +15 gal of FHR. A 80-gal soaking tub needs +25 gal.
Large Dishwasher Loads
Frequent dishwasher + laundry use adds +10 gal FHR to peak demand.
Example: Family of 4 (70 gal base) + 2 simultaneous showers (+20 gal) + dishwasher running (+10 gal) = 100 gal peak demand → needs 50 gal gas tank (FHR 100+) or 10+ GPM tankless.
Tank vs Tankless: Side-by-Side Sizing
Feature
30-40 gal Tank
40-50 gal Tank
50-65 gal Tank
Tankless (Whole-House)
Household Size
1-2 people
2-3 people
3-5 people
2-6+ people
FHR / GPM
FHR 50-70
FHR 70-90
FHR 90-110
5-12 GPM
Recovery Time (gas)
30-40 min
25-35 min
20-30 min
Instant (continuous)
First Cost (installed)
$800–$1,500
$1,000–$1,800
$1,200–$2,200
$1,500–$3,500
Lifespan
10-15 years
10-15 years
10-15 years
20-25 years
Annual Energy Cost
$200–$400
$200–$400
$250–$450
$50–$200
Best For
Apartments, small homes
Most 3-4 person homes
Large families, high use
Endless hot water, efficiency
What is First Hour Rating (FHR)?
First Hour Rating (FHR) is the DOE 10 CFR 430 standard measure of how much hot water a tank water heater can deliver in one hour of peak use. It combines tank capacity with the recovery rate (how fast the burner reheats water). A 40-gal gas tank with a 40,000 BTU burner has an FHR of 80-90 gallons — meaning it can deliver 80-90 gallons in the first hour, even though the tank only holds 40. Match FHR to your household's peak-hour demand, not to tank size. For tankless, the equivalent is the maximum GPM flow rate at your incoming water temperature.
Gas vs Electric
Gas tanks recover 30-40 gal/hr (40,000 BTU burner). Electric tanks recover 15-20 gal/hr (4,500W element). Same gallon tank = 10-15% lower FHR on electric.
Heat Pump Water Heaters
HPWH (50-80 gal) deliver FHR 60-80 with 60-70% lower operating cost. Best in basements with 8+ ft of headroom and warm ambient air.
Water Heater Sizing FAQ
How do I use this water heater sizes chart?
Find your household size in the left column. Look across to see peak-hour demand and recommended tank gallons (with First Hour Rating) or tankless GPM. Add adjustments for multiple simultaneous showers (+10 gal each), soaking tubs (+15 to +25 gal), and dishwasher/laundry (+10 gal). Match the FHR to your adjusted peak demand.
What size water heater for a family of 4?
A family of 4 typically needs a 40-50 gallon gas tank (FHR 80-90) or a 7-9 GPM tankless. With average use (2 showers + dishwasher + laundry), peak-hour demand is 60-80 gallons of hot water. A 50-gallon gas tank with a 40,000 BTU burner typically delivers FHR of 80-90 gallons — enough for 4 people.
What is First Hour Rating (FHR)?
First Hour Rating is the DOE 10 CFR 430 standard measure of how much hot water a tank can deliver in one hour of peak use. It combines tank capacity with the recovery rate. A 40-gal gas tank has FHR 80-90, meaning it can deliver 80-90 gallons in the first hour even though the tank only holds 40. Match FHR to your household's peak-hour demand.
Tank or tankless water heater?
Tank water heaters cost $800-1,500 installed, last 10-15 years, and have higher standby energy loss. Tankless water heaters cost $1,500-3,500 installed, last 20+ years, provide endless hot water, and are 22-34% more energy efficient. Tankless requires gas line upgrade (for gas models) or 200-amp electrical service (for electric). Whole-house gas tankless is the highest-efficiency option in most US homes.
Need an Exact Tank or Tankless Recommendation?
Our free calculator factors in household size, peak draw, and fuel type for a custom tank or tankless size in 60 seconds.
Storage-tank picks matched to DOE FHR by gallon, plus tankless by peak GPM. We verify specs against manufacturer data and sort by value — never by commission. Read our product policy.
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