This calculator estimates what size tank water heater a home needs. It follows A. O. Smith’s published sizing method: add up the hot water you use in your busiest hour, then look for a heater whose first-hour rating matches that number. It also shows A. O. Smith’s rough tank size for your household.
This calculator is for estimating only — have a licensed plumber confirm sizing. Fuel type, venting, gas supply and local code all affect the final choice. In Georgia, the plumbing code in force since January 1, 2026 (Table 506) also sets a minimum first-hour rating by bedrooms and baths, from 40 gallons for one bedroom and 1 to 1-1/2 baths up to 72 gallons for six bedrooms and 3 to 3-1/2 baths; tankless heaters are sized to the maker’s recommendations.
How to size a tank water heater: add up your busiest hour of hot water with A. O. Smith’s figures: about 20 gallons per shower, half the tub for a bath, 3 for washing dishes by hand, 7 per dishwasher run and 20 to 25 per washer load. Choose a heater whose first-hour rating is the same as or slightly larger than that total. In Georgia, the first-hour rating must also meet the plumbing code’s Table 506 minimum, 40 to 72 gallons depending on bedrooms and baths.
How the calculator works
Busiest-hour demand (gallons) = (showers × gallons per shower) + (baths × half the tub’s capacity) + (hand dishwashing × 3) + (dishwasher runs × 7) + (washer loads × 20 or 25).
Those per-use figures are A. O. Smith’s: about 20 gallons per shower, half the tub’s capacity for a bath, 3 gallons for washing dishes by hand, 7 for an automatic dishwasher and 20 to 25 for a clothes washer, depending on load size. A. O. Smith then says to choose a heater with a first-hour rating “the same as or slightly larger than” that total. The calculator rounds the total up to the next whole gallon.
The household range comes from A. O. Smith’s guide table:
| People in the home | Tank size (gallons) |
|---|---|
| Up to 2 | 30 to 40 |
| 3 | 40 to 50 |
| 4 | 50 to 75 |
| 5 | 75 to 100 |
Worked examples
| People | Busiest hour | Demand | First-hour rating | A. O. Smith tank guide |
|---|---|---|---|---|
| Up to 2 | 1 shower + 1 dishwasher run | 20 + 7 = 27 gal | At least 27 gal | 30 to 40 gal |
| 3 | 2 showers | 20 + 20 = 40 gal | At least 40 gal | 40 to 50 gal |
| 4 | 3 showers + 1 dishwasher run | 60 + 7 = 67 gal | At least 67 gal | 50 to 75 gal |
| 5 | 3 showers + 1 dishwasher run + 1 smaller washer load | 60 + 7 + 20 = 87 gal | At least 87 gal | 75 to 100 gal |
In Georgia, use the Table 506 minimum first-hour rating when it is higher. It runs from 40 gallons (1 bedroom, 1 to 1-1/2 baths) to 72 gallons (6 bedrooms, 3 to 3-1/2 baths), so the first row would need at least 40 and a larger home may need more on any row.
Limits of this estimate
- The per-use gallons are rough averages. Long showers use more; A. O. Smith says so.
- Count only uses that overlap in the same hour. Spread-out use needs a smaller heater than the daily total suggests.
- Compare the first-hour rating on the spec sheet, not just the tank size. A. O. Smith calls it an important measure of how much hot water a heater can provide over the first hour of use.
- It does not adjust for your incoming water temperature or climate.
- It does not choose a fuel or check venting, gas supply or wiring. That work needs a licensed plumber, and electrical work a licensed electrician.
- A. O. Smith recommends a 120°F setting and says not to set a heater above 140°F.
FAQ
What size water heater does a family of 4 need?
A. O. Smith’s household guide puts 4 people at a 50 to 75 gallon tank. The better check is your busiest hour: add about 20 gallons per shower, 7 for a dishwasher run and 20 to 25 for a washer load, then look for a first-hour rating at least that high.
What is a first-hour rating?
A. O. Smith describes the first-hour rating as a measure of how much hot water the heater can provide over the first hour of use. Match it to your busiest hour of hot water use.
Is this calculator right for a tankless water heater?
No. Tankless heaters are sized by flow rate in gallons per minute and temperature rise, not by gallons stored. Use the tankless water heater size calculator instead.
Related guides
- Best water heaters: our tank picks
- Tankless water heater size calculator
- Tank vs tankless water heater
- Best tankless water heaters by fuel and climate
- Gas vs electric water heater
- What it costs to replace a water heater
Sources
- A. O. Smith, “What Size Water Heater Is Right for You?” (hotwater.com)
- Georgia Department of Community Affairs, “Georgia State Amendments to the International Plumbing Code (2024 Edition)”, Section 506 and Table 506 (effective Jan. 1, 2026) (dca.georgia.gov)
Sources checked September 29, 2026.