Request a Service

Water Heater Sizing Guide for Homeowners

Call for Reliable Plumbing Services: 770-428-2291 Dupree Plumbing has the best warranty & labor guarantee in town!
Dupree Plumbing Services Neighborhoods In North, North-West, & North-East Metro Atlanta

So you need a new water heater for your Marietta, GA home, but you’re not sure which size you need. No worries: The pros at Dupree Plumbing will help you determine the right size for your home with this water heater sizing guide. Continue reading to determine your home’s needs for hot water and water heater size.

How to Size a Tank Water Heater

Step 1

Estimate when your home’s hot water demand is at the greatest. Perhaps it’s the morning when you have several showers in a row or the dinner hour when you use hot water in the kitchen for cooking and run the washing machine at the same time. Take the average amount of hot water used by each appliance during that time (shower, dishwasher, sink, etc.) and add those figures (in gallons) to determine the total amount you need in one hour.

These figures from the Department of Energy can get you started:

  • Washing machine: 7-25 gallons
  • Shower: 10 gallons
  • Dishwasher: 6 gallons
  • Kitchen sink: 1-2 gallons per minute
  • Bathroom sink: 1-2 gallons per minute

Using these figures, if your period of highest usage is the morning rush hour, comprising two showers, 10 minutes’ worth of hot water in the bathroom, and then a load of dishes after breakfast, you’d need a water heater that gives you at least 46 gallons of hot water.

Step 2

Select a water heater with a first-hour rating that matches your hot water needs. Take the number you got from step one and match it to a water heater with the same number or a bit higher for its first-hour rating. Don’t be tempted to go too high, however: If your water heater holds far more water than you need, you’ll be paying to keep water hot, but you’ll rarely use it.

Check Out Our
Money-Saving Special Offers

How to Size a Tankless Water Heater

Step 1

Tankless water heaters are rated by the maximum temperature rise possible at a given flow rate. You can size a tankless water heater by determining the flow rate and the temperature rise you’ll need. Ensure you know how many hot water devices you usually use at one time, the flow rate of your hot water devices, and the temperature rise you want.

To calculate your flow rate, know how much water you need for each hot water device you’ll be using. Add the flow rates of each device to calculate your home’s collective flow rate.

Your hot water devices’ flow rates will vary depending on their efficiency and whether you use accessories like low-flow showerheads and aerators. Your owner’s manuals will help you determine the exact flow rates, but these estimates will get you started:

  • Bathtub: 4 gallons per minute
  • Shower: 2.5-3 gallons per minute
  • Washing machine: 2 gallons per minute
  • Dishwasher: 1.5 gallons per minute
  • Kitchen sink: 1-2 gallons per minute

If you frequently have the washing machine, dishwasher, and shower all using hot water simultaneously, you would need a tankless water heater that provides at least 6-7 gallons per minute.

Step 2

To calculate your desired temperature rise, subtract the temperature of the groundwater in your area (in Marietta, it’s about 62 degrees Fahrenheit) from the desired temperature setting of your water heater (the Consumer Product Safety Commission recommends 120 degrees Fahrenheit, though your household may set it higher or lower).

Get Help from the Pros at Dupree Plumbing

Need assistance sizing your water heater? The team at Dupree Plumbing in Marietta can help you determine the right size tank-style or tankless water heater for your home’s hot water needs.

Give us a call at 770-428-2291 to speak with a plumber about your water heater needs today.

Request A



Robert B. arrived on time, diagnosed the problem and got the problem fixed promptly. Dupree Plumbing always comes through for us - fast and efficient service.

-Joyce B. Learn More 

Latest News

The Hidden Dangers of Ignoring a Plumbing Leak Posted On: April 12, 2024

Plumbing leaks are often hidden from plain sight, silently wreaking havoc and potentially causing significant damage before you can even notice them. Here, we’ll p...

Learn More