For a tank water heater, size by first-hour rating (FHR): a family of 4 typically needs an FHR of 60–80 gallons, which corresponds to a 50-gallon gas tank or a 65–80-gallon electric tank. For a tankless water heater, size by flow rate: the same family needs 4.0–5.5 GPM, translating to a 150,000–199,000 BTU gas tankless or a 27–36 kW electric tankless (warm climates only).
Choosing between tank and tankless — and picking the right size — comes down to your peak demand patterns, available fuel type, upfront budget, and long-term operating cost preferences. This guide sizes both types for your specific household.
Interactive Water Heater Sizing Calculator
Use our calculator to find the right water heater size and type for your household:
Tank Water Heater Sizing: First-Hour Rating Method
The DOE recommends sizing tank water heaters by first-hour rating (FHR), not tank capacity. FHR tells you how many gallons of hot water the unit can deliver in the first hour of heavy use, starting with a full tank. It combines tank capacity + recovery rate.
Step 1: Calculate Your Peak-Hour Demand
Add up hot water usage during your busiest hour (typically the morning routine):
| Activity | Average Hot Water (gallons) |
|---|---|
| Shower (10 min) | 20 |
| Low-flow shower (10 min) | 12 |
| Bath | 20–30 |
| Shaving (5 min) | 2 |
| Hand/face washing | 2 |
| Dishwasher (per load) | 6–10 |
| Hand-washing dishes | 4 |
| Clothes washer (hot cycle) | 7–12 |
| Food preparation | 3–5 |
Step 2: Match FHR to Your Peak-Hour Demand
| Peak-Hour Demand (gallons) | Minimum FHR Needed |
|---|---|
| 30–40 | 40 |
| 40–55 | 55 |
| 55–70 | 67 |
| 70–85 | 80 |
| 85–100 | 95 |
Household Size Quick Reference
| Household | Bathrooms | Peak-Hour Demand (gallons) | Gas Tank Size | Electric Tank Size |
|---|---|---|---|---|
| 1 person | 1 | 25–35 | 30–40 gallon | 40–50 gallon |
| 2 people | 1–2 | 35–50 | 40–50 gallon | 50–65 gallon |
| 3 people | 1–2 | 45–60 | 40–50 gallon | 50–65 gallon |
| 4 people | 2–3 | 55–75 | 50–65 gallon | 65–80 gallon |
| 5 people | 2–3 | 65–85 | 50–75 gallon | 80 gallon |
| 6+ people | 3+ | 80–100+ | 75 gallon (or 2 units) | 80 gallon (or 2 units) |
Why electric tanks are larger than gas: Electric water heaters have slower recovery rates — a 50-gallon electric heater recovers about 21 gallons per hour, while a 50-gallon gas heater recovers 40–50 gallons per hour. To match the same FHR, electric tanks need to be 20–30% larger to compensate with stored hot water volume.
Tankless Water Heater Sizing: Flow Rate Method
Tankless units are sized by GPM (gallons per minute) at a specific temperature rise. See our comprehensive tankless water heater sizing guide for the full calculation method. Here's the summary:
| Household | Bathrooms | Peak GPM Needed | Gas BTU (Cold Climate) | Gas BTU (Warm Climate) | Electric kW (Warm Only) |
|---|---|---|---|---|---|
| 1 person | 1 | 1.5–2.0 | 100,000 | 75,000 | 14–18 kW |
| 2 people | 1–2 | 2.5–3.5 | 140,000–160,000 | 100,000–120,000 | 22–27 kW |
| 3–4 people | 2–3 | 4.0–5.5 | 180,000–199,000 | 140,000–160,000 | 27–36 kW |
| 5–6 people | 3–4 | 5.5–7.5 | 199,000+ (or 2 units) | 180,000–199,000 | Not feasible |
Tank vs. Tankless: Complete Comparison
| Factor | Tank Water Heater | Tankless Water Heater |
|---|---|---|
| Upfront cost (installed) | $800–$2,000 | $1,500–$4,500 |
| Annual operating cost (gas) | $250–$400 | $175–$300 |
| Annual operating cost (electric) | $400–$600 | $300–$500 |
| Hot water supply | Limited by tank size | Unlimited (if sized correctly) |
| Recovery rate | 30–50 GPH (gas), 20 GPH (electric) | Continuous at rated GPM |
| Lifespan | 8–12 years | 15–20 years (gas), 8–15 years (electric) |
| Space required | 24" × 24" × 60" (typical 50-gal) | 14" × 10" × 26" (wall-mounted) |
| Maintenance | Annual anode rod check, periodic flush | Annual descaling (hard water) |
| Energy factor (UEF) | 0.58–0.70 (gas), 0.92–0.95 (electric) | 0.87–0.96 (gas), 0.96–0.99 (electric) |
| Venting | Existing flue (gas) | Direct vent or power vent (gas) |
| Wait time for hot water | Depends on pipe distance | 2–5 sec cold water sandwich |
| Best for | Budget installs, large simultaneous demand | Endless hot water, space savings, long-term economy |
Heat Pump Water Heaters: The Efficiency Leader
Heat pump water heaters (HPWH) deserve special mention. They use a heat pump to extract heat from surrounding air, achieving UEF ratings of 2.0–4.0 — meaning they produce 2–4× more hot water energy than the electricity they consume. They cost $1,500–$3,000 installed and save $200–$400/year compared to conventional electric tanks.
| HPWH Factor | Specification |
|---|---|
| Tank sizes available | 40, 50, 65, 80 gallon |
| UEF range | 2.0–4.0 |
| Annual operating cost | $100–$250 |
| Space needed | 750+ cubic feet of ambient air (can't be in a tight closet) |
| Noise level | 45–55 dB (like a running dishwasher) |
| Best for | Homes with electric water heating, garages, basements, utility rooms |
| IRA tax credit (2026) | Up to $2,000 |
| Temperature range | Works best in 40–120°F ambient air |
Example 1: Family of 4 in Phoenix (warm climate, gas available)
Morning routine: 2 showers (20 gallons each), dishwasher (8 gallons), hand washing (4 gallons) = 52 gallons peak hour.
Tank option: 50-gallon gas tank with FHR of 67 gallons. Cost: $1,000–$1,500 installed. Annual operating cost: $300.
Tankless option: 2 simultaneous showers (4.0 GPM) + dishwasher (1.0 GPM) = 5.0 GPM. Temperature rise: 55°F. Need: 140,000+ BTU gas tankless. Cost: $2,500–$3,500 installed. Annual operating cost: $200.
Payback period for tankless: ($3,000 − $1,250) / ($300 − $200) = 17.5 years. The tankless unit is more expensive but lasts longer (18 years vs 10 years), provides unlimited hot water, and takes less space.
Example 2: Couple in Seattle (cool climate, electric only)
Morning routine: 1 shower (12 gallons, low-flow), coffee + breakfast prep (3 gallons) = 15 gallons peak hour.
Tank option: 40-gallon electric tank with FHR of 40 gallons. Cost: $800–$1,200 installed. Annual operating cost: $350.
HPWH option: 50-gallon heat pump water heater. Cost: $2,000–$2,800 installed. Annual operating cost: $120. IRA credit: $2,000. Effective cost: $0–$800.
HPWH wins decisively — after the tax credit, it costs similar to (or less than) a standard electric tank and saves $230/year in operating costs.
Example 3: Family of 6 in Chicago (cold climate, gas available)
Morning rush: 3 showers (60 gallons), breakfast dishes (6 gallons), morning laundry (10 gallons) = 76 gallons peak hour.
Tank option: 75-gallon gas tank with FHR of 90 gallons. Cost: $1,500–$2,200 installed. Annual operating cost: $400.
Tankless option: Peak demand = 3 simultaneous showers (6.0 GPM). Temperature rise: 76°F. Need: 250,000+ BTU — exceeds single-unit capacity. Requires two gas tankless units in parallel. Cost: $4,500–$6,000 installed. Annual operating cost: $275.
The 75-gallon tank is the smarter choice here. Two tankless units cost $3,000–$4,000 more upfront, and the $125/year savings gives a 24–32 year payback — longer than the equipment lifespan. The tank also handles the extreme simultaneous demand more reliably.
Key Takeaways
- Size tank heaters by first-hour rating (FHR), not just tank capacity — a 50-gallon gas tank delivers 67+ gallons in the first hour
- Size tankless heaters by peak GPM and temperature rise — always use winter groundwater temps
- Gas tanks recover 2× faster than electric; electric tanks need to be 20–30% larger to compensate
- Heat pump water heaters are the most efficient electric option (UEF 2.0–4.0), with up to $2,000 in IRA tax credits in 2026
- Tankless is best for 1–4 person households in warm climates; large families in cold climates often get better value from large tank heaters
- The most important metric is matching your peak-hour demand — running out of hot water during morning routines is the #1 complaint
Frequently Asked Questions
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