A Guide to Water Heater Sizing
There’s a proven method for choosing the right size natural gas water heater for your home. It involves looking at your household’s ordinary and peak usage and how well the unit responds to your needs. Read on to learn how to pick the right one.
How water heater sizing works
Water heaters come in 30-, 40-, 50- and 75-gallon sizes but picking the right one isn’t just about its storage capacity. It’s about matching storage with heating power. Heating power is measured in British Thermal Units (BTUs), the standard unit of heat energy.
Since water heaters with higher BTUs can heat more water at a faster rate, a more powerful 40-gallon heater can outperform a basic 50-gallon model during back-to-back morning showers. Higher BTU models recover faster.
Learn more about the benefits of natural gas water heaters.
Recovery rate
Your water heater’s recovery rate matters just as much as the amount of hot water your tank can hold at any given time. The recovery rate measures how fast your unit can raise the temperature of unheated water by 90°F to replace what you’ve used. More BTUs generally means a faster recovery rate. By combining storage capacity and recovery rate, you can determine the best water heater size for your home.
Calculating the best size water heater for your home
Every family uses water differently, but these two methods help most households find the perfect water heater size.
Method 1: The household demographics audit
This analysis considers the number of family members, how you wash your dishes and clothes, and the number of bathrooms in your home. A 30-gallon water heater, which is the smallest tank water heater, is a good match for a family of two with one bathroom and a washing machine for clothing.
With that as the starting point, add 3.5 gallons for each additional bathroom and another 5 gallons for a dishwasher. If you have more people in your household, you’ll need a larger water heater. This chart provides a quick reference for what you’ll need to ensure you’ve got enough hot water for everyone.
Residential water heater sizing guide:
| Family Size | Demand | Gallon Capacity |
|---|---|---|
| 1-2 people | High Regular/Low |
40 to 50 gallons 30 |
| 2-3 people | High Regular/Low |
40 to 50 gallons 40 |
| 3-4 people | High Regular/Low |
50 to 75 gallons 40 |
| 5+ people | High Regular/Low |
75 gallons 50 gallons |
Method 2: The ‘power hour’ calculation for peak demand
To use this method, think about your home’s “Power Hour.” This is the one hour each day when your household uses the most hot water—like the morning rush of showers and laundry. You’ll want a heater with a First-Hour Rating (FHR) that matches this peak. This rating simply tells you how many gallons of hot water the unit can provide in a single hour when starting with a full, hot tank. It’s easy to calculate by following these steps:
- Determine the hour of the day when you use the most water (your “power hour”)
- Use the activity chart table to find the quantity of hot water you use for each activity during that hour.
- Add all the quantities for that hour. The total is your first-hour rating.
Choose a water heater with a first-hour rating close to the total amount of hot water you use during the hour you selected.
Peak activity usage chart*
| Activity | Hot water used |
|---|---|
| Clothes washer | 25 to 40 gallons per load |
| Dishwasher | 5 to 10 gallons per load |
| Hand dishwashing | 3 to 4 gallons |
| Tub bath | 15 to 25 gallons |
| Shower | 3 gallons per minute |
| Bathing an infant | 2 gallons |
| Shaving | 2 to 3.5 gallons |
| Shampooing | 5 gallons |
| Hand washing | 1 to 2 gallons |
| Food preparation | 3 to 6 gallons |
*Source: BlueFlame.org, Minnesota Blue Flame Gas Association
Why BTU input matters for large families
Today’s homes use water differently. High-efficiency washers use less water, but luxury rain showerheads can drain a 40-gallon heater quickly. For large families with high simultaneous demand, a high-recovery heater with 50,000-70,000 BTUs can provide more usable hot water than a larger tank with a standard burner.
Proper water heater sizing can also impact how long your water heater will last. An undersized heater needs to work constantly, which can lead to premature failure. Oversized water heaters with more capacity than needed may lead to standby heat loss and wasted energy because they heat and store water than isn’t used. The sweet spot is a water heater that handles your power hour demand with 10-15% capacity to spare.
Learn how to properly maintain your hot water heater.
Frequently asked questions about water heater considerations
Why does my 50-gallon heater run out of hot water so fast?
The issue may be your first-hour rating, not your total tank volume. You may need a higher BTU unit with a faster recovery rate instead of a larger tank.
Does a bigger tank cost more to operate?
Yes and no. Larger tanks have more standby heat loss because they keep more water hot around the clock. However, modern insulation has greatly reduced this issue. The bigger cost factor is usually whether you’re properly sized. An undersized unit that runs constantly may use more energy than a properly sized larger unit that heats more efficiently.
Can a high-BTU unit replace a larger tank?
Absolutely. A high-recovery water heater with 50,000-70,000 BTU input can deliver more usable hot water during your power hour than a standard 75-gallon unit with a basic 40,000 BTU burner. This is especially valuable in homes with space constraints or high simultaneous demand.