🔥 Water Heater Sizing Calculator
Find the Right Tank Size Based on FHR
Peak Hour Demand (Optional - Advanced)
💧 Water Heater Sizing Fundamentals
Proper sizing ensures you have enough hot water during peak usage without wasting energy on an oversized tank.
Quick Sizing Guide (Rule of Thumb)
| Occupants | Gas Tank | Electric Tank | Tankless (GPM) |
|---|---|---|---|
| 1-2 | 30-40 gal | 40-50 gal | 5-7 GPM |
| 2-3 | 40-50 gal | 50-60 gal | 6-8 GPM |
| 3-4 | 50 gal | 60-80 gal | 7-9 GPM |
| 5+ | 60-75 gal | 80+ gal | 9+ GPM |
First Hour Rating (FHR) Method
The FHR tells you how much hot water the heater can deliver in the first hour. It's calculated as:
FHR = Tank Capacity × 0.7 + Recovery Rate
Recovery Rates: Gas ≈ 40 gal/hr, Electric ≈ 14 gal/hr
Typical Hot Water Usage
- Shower: 15-25 gallons (10-15 min at 2 GPM)
- Bath: 30-40 gallons
- Dishwasher: 6-10 gallons
- Washing Machine (hot): 15-30 gallons
- Hand washing/shaving: 2-4 gallons
"The biggest complaint I get about water heaters is 'we run out of hot water.' Usually, the tank isn't undersized—it's the RECOVERY RATE that's the problem. A 40-gallon gas heater can outperform a 60-gallon electric because gas recovers at 40 gal/hr vs electric's 14 gal/hr. If you're replacing an electric with electric, go up TWO sizes (e.g., 50 to 80 gal), or switch to gas/tankless."
⚠️ Common Mistakes
1. Only Looking at Tank Size: A 50-gallon electric heater does NOT equal a 50-gallon gas heater! The FHR (First Hour Rating) is what matters, not just capacity.
2. Undersizing Tankless: A tankless unit needs enough GPM for ALL simultaneous uses. Two showers (4 GPM) + dishwasher (1.5 GPM) = 5.5 GPM minimum. Most "whole house" tankless units are 6-10 GPM.
3. Ignoring Temperature Rise: In cold climates, incoming water can be 40°F. To heat it to 120°F requires more BTUs than in warm climates where inlet is 60°F. Always account for this when sizing tankless heaters.