4 occupants, 2 baths, 2.5 GPM, 70°F rise
- Simultaneous showers ≈ 2
- Peak GPM ≈ 5
- First-hour demand ≈ 5 × 60 × (70/70) = 300 gal
- Suggested tank ≈ 60–80 gal
construction calculator
Water Heater Size Calculator
Estimate peak hot water demand and recommended tank size using occupants, baths, flow rate, and temp rise.
Results
- Peak hot water flow (GPM)
- 5.00
- Estimated first-hour demand (gallons)
- 300.00
- Suggested tank size (gallons)
- 50.00
How to use this calculator
- Enter bedrooms/bathrooms and occupants to estimate simultaneous showers.
- Enter shower flow rate and desired temperature rise.
- Review peak GPM, first-hour demand, and a suggested tank size.
Inputs explained
- Bedrooms/Bathrooms
- Used to estimate how many showers may run at once.
- Occupants
- Total people using hot water; we limit simultaneous showers by bathroom count.
- Shower flow (GPM)
- Flow per showerhead; low-flow heads run ~1.8 GPM, standard ~2.5 GPM.
- Desired temp rise
- Difference between inlet cold and target hot outlet temperature.
How it works
Peak GPM ≈ simultaneous showers (bounded by bathrooms/occupants) × shower flow.
First-hour demand ≈ peak GPM × 60 scaled to your temperature rise baseline.
Suggested tank size uses rough bins: ~50 gal for typical households, ~60–80 gal for higher peak demand.
Formula
Simultaneous showers ≈ min(Occupants, Bathrooms) Peak GPM ≈ Simultaneous showers × Shower flow First-hour demand ≈ Peak GPM × 60 × (Temp rise ÷ 70) Suggested tank ≈ 50–80 gal depending on first-hour demand
When to use it
- Sizing a replacement tank for a family home.
- Checking if you need 50 vs 60–80 gallons based on household size.
- Planning for peak shower demand when guests visit.
Tips & cautions
- If you run laundry/dishwasher during showers, add a buffer or consider a larger tank/on-demand system.
- Colder climates need larger temp rise—consider upsizing if inlet water is very cold.
- Low-flow showerheads reduce peak GPM, letting smaller tanks keep up more easily.
- Simplified: actual first-hour ratings depend on recovery rate and fuel type (gas vs electric).
- Does not size on-demand tankless heaters; those use direct GPM at temp rise charts.
- Assumes showers drive peak load; heavy tub fills or simultaneous appliances can raise demand.
Worked examples
3 occupants, 2 baths, 1.8 GPM, 60°F rise
- Simultaneous showers ≈ 2
- Peak GPM ≈ 3.6
- First-hour demand ≈ 216 gal
- Suggested tank ≈ 50–60 gal
Deep dive
This water heater size calculator estimates peak hot water demand from occupants, bathrooms, shower flow, and temperature rise to suggest a tank size.
Use it to decide between 50, 60, or 80-gallon tanks, then confirm with manufacturer first-hour ratings for your chosen model.
FAQs
- Gas vs electric?
- Recovery rates differ. This suggests a size; verify the model’s first-hour rating for your fuel type.
- Tank vs tankless?
- This tool suggests tank sizes. For tankless, use manufacturer GPM vs temp rise charts instead.
- Very cold inlet water?
- Increase temp rise or consider upsizing the tank or using higher recovery equipment.
- Large soaking tubs?
- Add buffer—tubs can demand high gallons quickly. Consider upsizing if you fill tubs often.
- Low-flow fixtures?
- Lower flow reduces peak GPM, letting a smaller tank keep up more easily.
Related calculators
construction
HVAC BTU Calculator
Estimate cooling BTU and tonnage based on room size, ceiling height, insulation, sun exposure, and occupancy.
construction
Propane Usage Calculator
Estimate propane gallons and cost from appliance BTU load, daily hours, and price per gallon.
finance
AC Cost Calculator
Estimate daily and monthly energy cost for an air conditioner by BTU, SEER, and usage.
Planning tool only. Confirm final sizing with manufacturer first-hour ratings and a licensed plumber, especially for local code requirements.