TryBuildCalc40 Gallon Water Heater Calculator (Detailed Peak-Hour Demand Example)
Check whether a 40 gallon water heater covers your peak-hour demand.
🕒 Last updated: August 1, 2026
Water Heater Type
ℹ️Storage Tank is sized by First Hour Rating; Tankless is sized by simultaneous flow rate and BTU/hr input.
ℹ️Electric and Heat Pump storage heaters recover slower than gas — the target FHR is the same, but consider sizing up one class for the same real-world performance.
Sizing Method
ℹ️Quick uses a household-size lookup table; Detailed tallies your actual fixture use during the busiest hour.
Peak-Hour Fixtures
Add every fixture used during your busiest hot-water hour.
Fixture 1
Fixture 2
Up to 20 fixture rows total.
🔧 Target First Hour Rating (FHR): 40 gallons
Fuel: Natural Gas / Propane
Peak-Hour Fixture Tally
Shower (x1): 20 gal
Bath (Tub Fill) (x1): 20 gal
Total Peak-Hour Demand: 40 gal
Shop for a storage heater with a First Hour Rating (FHR) at or slightly above this figure.
Model-Size Class: ~40 gal class
Design Notes
First Hour Rating (FHR) — printed on every storage heater's EnergyGuide label — already accounts for tank volume and recovery rate together. A gas heater typically reaches a given FHR with a smaller tank than an electric heater, since gas recovers hot water faster.
Household-size quick sizing is a single manufacturer's (A.O. Smith) published guidance — the Detailed method (your actual fixture tally) is more precise for an unusual household or high simultaneous-use pattern.
Approximate results for planning only. Verify with a professional.
Looking for the verification checklist, reference tables, or tips and mistakes?See the complete Water Heater Sizing Calculator.
1 shower + 1 bath detailed demand example
This page is set up for a single shower (20 gallons) plus a single bath/tub fill (20 gallons) used within the same busy hour, giving a total peak-hour demand of exactly 40.00 gallons.
A storage heater with a First Hour Rating of 40 gallons or slightly more would cover this exact demand — remember that First Hour Rating (not raw tank volume) is what should be matched against this figure, since a 40-gallon-labeled tank's actual FHR varies by fuel type and recovery rate.
- 1 shower + 1 bath → 40.00 gal peak-hour demand.
- Target: a heater with First Hour Rating ≥ 40 gal.
- Actual tank volume needed varies by fuel type — check the EnergyGuide label FHR, not just tank size.
How Water Heater Sizing Is Calculated
The calculation differs by water heater type and, for storage tanks, by sizing method.
Step 1 — Storage Tank Quick Sizing (Household Size)
A direct lookup from A.O. Smith's published household-size guidance — no entry beyond 5 occupants, so larger households are directed to the Detailed method instead of an extrapolated guess.
Step 2 — Storage Tank Detailed Sizing (Peak-Hour Tally)
Sum every fixture used during your busiest hot-water hour (not necessarily simultaneous — cumulative within one hour), then shop for a storage heater with a First Hour Rating at or slightly above the total.
Step 3 — Tankless Flow Rate & BTU/hr
Temperature Rise (°F) = Desired Output Temp − Groundwater Temp
Required BTU/hr = (GPM × Rise × 500) ÷ Efficiency
500 comes from water's weight (8.33 lb/gal) × 60 minutes/hour. This sums the actual simultaneous fixtures you expect running at once — a different diversity model than the WSFU demand-curve method used on this site's Pipe Sizing by Fixture Units calculator, since a tankless unit must supply real-time flow rather than a statistically-averaged peak. If you already know your required flow rate, enter it directly instead of building a fixture list.
Step 4 — Tankless Electric Load (kW & Amperage)
Amps = (kW × 1000) ÷ 240V
Recommended Minimum Breaker = next standard size ≥ (Amps × 1.25)
For an electric fuel type, the same BTU/hr requirement converts to kW using the standard 3412.14 BTU-per-kWh constant, then to amperage at the standard 240V US residential circuit voltage. A 125% continuous-load safety margin (consistent with NEC continuous-load practice) is applied before rounding up to a real, purchasable breaker size — a planning estimate only, not a substitute for a licensed electrician's circuit design.
Step 5 — Model-Size Class & Feasibility
Margin % = (Model Class − Requirement) ÷ Requirement × 100
The requirement (target FHR for storage, BTU/hr or kW for tankless) is matched against common off-the-shelf model-size classes to show what to actually shop for, with a margin percentage classified as comfortable (15-50% headroom), borderline (under 15% — consider the next size up), or oversized (over 50% — more capacity than this specific requirement needs). If the requirement exceeds every listed class, this calculator flags a feasibility warning rather than guessing a bigger, unlisted size — a single residential unit may not cover very high demand, and multiple units or a commercial-grade unit should be considered.
Worked Example
This example uses the active inputs above and follows the same steps as the Formula section.
Input Values Used
| Input | Value |
|---|---|
| Water Heater Type | Storage Tank |
| Fuel Type | Natural Gas / Propane |
| Peak-Hour Fixtures | Shower x1, Bath (Tub Fill) x1 |
Step 1 — Peak-Hour Fixture Tally & Target FHR
| Fixture | Substitution | Result |
|---|---|---|
| Shower | 20 gal × 1 | 20 gal |
| Bath (Tub Fill) | 20 gal × 1 | 20 gal |
| Target First Hour Rating | 40 gal | |
| Model-Size Class | ~40 gal (borderline) | |
Therefore, this fixture combination needs a storage heater with a First Hour Rating of at least 40 gallons — closest to a ~40 gallon standard class (borderline).
This does not cover venting, gas/electrical sizing, or code compliance. Cross-check against the Reference Tables in the complete Water Heater Sizing Calculator.
This page keeps things focused on the calculation above. For the full construction guide — verification checklist, reference tables, usage steps, tips, common mistakes, and limitations.See the complete Water Heater Sizing Calculator.