Recommended Size
30 gal
Peak Demand
45 gal/hr
Temp Rise
70°F
Annual Cost
$350
30 gal
45 gal/hr
70°F
$350
First Hour Rating should exceed Peak Demand for adequate hot water supply.
Inputs
Result
Peak demand for 3 people at average usage = 45 gal. A 40-gallon gas tank has FHR of ~68 gal (40 + 40 × 0.7), which exceeds 45 gal peak demand.
Inputs
Result
Peak demand = 55 × 1.3 (high) + 5 (extra bath) = 76.5, rounded to 77 gal/hr. Tankless GPM = 77/60 = 1.28 GPM, rounded to 3.5 GPM standard size for simultaneous fixture use.
A family of 4 typically needs a 50-gallon gas or 65-gallon electric tank water heater. For tankless, you need at least 5.0 GPM. The key metric is First Hour Rating (FHR), which should exceed your peak hour demand of about 55 gallons.
| Household Size | Gas Tank | Electric Tank | Tankless GPM |
|---|---|---|---|
| 1–2 people | 30–40 gal | 40–50 gal | 2.0–3.5 GPM |
| 3 people | 40–50 gal | 50–65 gal | 3.5–5.0 GPM |
| 4 people | 50–60 gal | 65–80 gal | 5.0–7.0 GPM |
| 5+ people | 60–80 gal | 80+ gal | 7.0–10.0 GPM |
Peak hour demand is the most hot water your household uses in any single hour. Count simultaneous uses: shower (10 gal), dishwasher (6 gal), clothes washer (7 gal), and faucet (2 gal each). A family of 3 typically has 45 gal peak demand.
| Fixture / Appliance | Gallons per Use | Typical Duration |
|---|---|---|
| Shower | 10–15 gal | 8–10 minutes |
| Bath | 20–30 gal | One fill |
| Dishwasher | 6–8 gal | One cycle |
| Clothes washer (warm) | 7–10 gal | One cycle |
| Kitchen faucet | 2–4 gal | 5–10 minutes |
| Bathroom faucet | 1–2 gal | 2–5 minutes |
First Hour Rating is how many gallons of hot water a tank heater can deliver in the first hour starting with a full tank. It includes the stored hot water plus what the burner can heat during that hour. Your FHR should exceed peak demand.
Yes, tankless heaters are 24-34% more efficient for homes using 41 gallons or less daily. They have higher energy factors (0.82-0.99) vs tank heaters (0.60-0.92). Annual savings are typically $100-$150, but upfront cost is higher.
| Factor | Tank (Gas) | Tankless (Gas) |
|---|---|---|
| Upfront cost | $800–$1,500 | $2,000–$4,500 |
| Install cost | $300–$500 | $1,000–$3,000 |
| Annual energy cost | $350–$450 | $200–$325 |
| Lifespan | 8–12 years | 15–20 years |
| Energy factor (UEF) | 0.60–0.70 | 0.82–0.99 |
Size a tankless heater by GPM (gallons per minute) and temperature rise. For a 3-person household, you need about 3.5-5.0 GPM. Calculate temperature rise as desired temp minus inlet temp (e.g., 120F - 50F = 70F rise).
| Simultaneous Uses | GPM Needed | BTU Rating (70°F rise) |
|---|---|---|
| 1 shower | 2.0–2.5 GPM | 70,000–90,000 |
| 1 shower + 1 faucet | 3.5–4.0 GPM | 120,000–140,000 |
| 2 showers | 4.0–5.0 GPM | 140,000–175,000 |
| 2 showers + dishwasher | 5.5–7.0 GPM | 190,000–240,000 |
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Last Updated: Mar 9, 2026
This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.