How to Size a Water Heater: Complete Guide with Calculator
Learn how to properly size a water heater for your home. Step-by-step guide with water heater sizing calculator, formulas, and capacity requirements.
Selecting the right size water heater is crucial for comfort, energy efficiency, and cost savings. A properly sized water heater provides adequate hot water during peak demand periods while avoiding the energy waste and higher costs associated with oversized units. This guide covers everything you need to know about sizing both tank and tankless water heaters, including how to calculate first hour rating, understand recovery rates, and account for your specific household needs. Whether you're replacing an existing unit or designing a new system, proper sizing ensures years of reliable hot water service.
Step-by-Step Guide
1. Determine Household Size
Count the number of people in your household. Base sizing on peak usage, typically during morning hours when multiple showers and appliances run simultaneously.
2. Calculate Daily Hot Water Demand
Estimate daily usage: 12-17 gallons per person for low-medium usage, 20-25 gallons for high usage. Add 4-5 gallons for dishwasher and washing machine if present.
3. Calculate First Hour Rating (FHR)
FHR represents the amount of hot water the heater can supply in the first hour of peak demand. Typically 25-30% of daily usage. Use our Water Heater Sizing Calculator for accurate FHR.
4. Select Fuel Type
Choose between electric, gas, propane, or hybrid. Gas and propane have faster recovery rates (40 GPH vs 23 GPH for electric), allowing smaller tank sizes.
5. Determine Tank Size
Match FHR to tank capacity and recovery rate. For electric: 50-80 gallon tanks common. For gas: 40-75 gallon tanks. Use our calculator to find exact size needed.
Water Heater Sizing Guide by Household Size
Recommended water heater sizes based on household size, number of bathrooms, and fuel type. These are general guidelines - use our calculator for precise sizing based on your specific usage patterns.
| Household Size | Bathrooms | Electric (gal) | Gas (gal) | Peak Demand (GPM) |
|---|---|---|---|---|
| 1-2 people | 1 | 30-40 | 30-40 | 3-5 |
| 2-3 people | 1-2 | 40-50 | 40 | 5-7 |
| 3-4 people | 2 | 50-66 | 40-50 | 7-9 |
| 4-5 people | 2-3 | 66-80 | 50-66 | 9-12 |
| 5-6 people | 3+ | 80+ | 66-75 | 12-15 |
| 6+ people | 3+ | 80+ | 75+ | 15+ |