Water Heater Size Calculator
Tank gallons and daily use from household size.
What it does
Water heater sizing starts from people: about 20 gallons a day each sets the load, and tank bands of 30-40, 40-50, and 50-80 gallons cover households from couples to crowds. Peak hour demand then picks the exact model.
The tool above takes the headcount from 1 to 8. It returns the tank band with daily gallons and the first hour rating tip beside them.
The default four person home lands 40-50 gallons at 80 gpd. Two people need 30-40 gallons at 40 gpd while six need 50-80 at 120 gpd.
Bands follow Ohio State Extension sizing guidance with DOE water heating references behind first hour ratings. Both stress the label rating over raw gallons, since recovery speed varies by fuel.
How to use
- Enter the people using hot water daily.
- Read the tank band first for the size class.
- Read daily gallons for the load feel.
- Compare first hour ratings within the band.
- Go up a band with big tubs or teens.
Formula
- 20 gpd
- Typical hot gallons per person daily.
- bands
- Tank classes covering headcounts.
- FHR
- First hour rating: gallons the heater delivers hourly.
Averages guide shopping, not plumbing code; local rules and simultaneous fixtures adjust real picks. Tankless units size by flow rate instead of gallons.
Worked example
Example: 4 people
Default: 4 people.
- Daily: 4 x 20 = 80 gpd.
- Band: 3-4 people maps to 40-50 gal.
- Shop: compare FHR near peak morning hour.
- Gas recovers faster, allowing smaller tanks.
- Tool shows 40-50 gal, 80 gpd, FHR tip.
Reference table
| People | Tank | Daily |
|---|---|---|
| 1 | 30-40 gal | 20 gpd |
| 2 | 30-40 gal | 40 gpd |
| 4 | 40-50 gal | 80 gpd |
| 6 | 50-80 gal | 120 gpd |
Limits
- Averages only; tubs and teens push higher.
- Tank bands, not exact models.
- Fuel recovery differences unmodeled.
- Simultaneous showers need FHR math.
Common mistakes
- Buying by tank gallons ignoring FHR.
- Sizing to averages with a soaking tub.
- Forgetting guests and teenagers.
- Oversizing far past need (standby losses).
- Skipping fuel and venting constraints.
Try it yourself
Quick checks. Answers sit tucked under each question.
2 people?
30-40 gal at 40 gpd.
6 people?
50-80 gal at 120 gpd.
Daily for 5?
5 x 20 = 100 gpd.
Questions
What size for 4 people?
40-50 gallons at about 80 gpd.
Tank or first hour rating?
FHR decides: match it to peak morning hour within the band.
Gas vs electric sizing?
Gas recovers faster, so smaller tanks serve the same peak.
What about tankless?
Size by flow (gpm) at your temperature rise, not gallons.
Big tub changes?
Yes, go up a band; tubs swallow a tank alone.
How long do tanks last?
Roughly 8 to 12 years; anode checks stretch it.