TryBuildCalcTankless Water Heater Size Calculator (2 Showers + Kitchen Sink, Moderate Climate)
Calculate required GPM and BTU/hr for a tankless water heater.
🕒 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 tankless units show required kW and approximate breaker amperage. Heat Pump isn't a standard tankless product, so it's only offered for Storage Tank.
Required Flow Rate
ℹ️Already know your required flow rate? Enter it directly instead of building a fixture list.
Add the fixtures you expect running at the same time during peak demand.
Fixture 1
Fixture 2
Up to 20 fixture rows total.
Temperature Rise
ℹ️120°F is a widely-recommended default balancing usability against scald risk.
ℹ️Use your coldest expected month's incoming water temperature, not a summer/annual average — the most common tankless sizing mistake.
ℹ️Defaults to 95% for gas (condensing) or 99% for electric (resistance has no combustion/flue loss) when you switch Fuel Type — adjust to your specific model's rated efficiency if known.
🔧 Required input: 221,053 BTU/hr at 7 GPM
Fuel: Natural Gas / Propane
Flow Rate
Shower (x2): 5 GPM
Kitchen Sink (x1): 2 GPM
Required GPM: 7 GPM
Temperature Rise & Required Input
Desired Output Temp: 120°F
Groundwater Temp: 60°F
Temperature Rise: 60°F
Efficiency: 95%
Required Input: 221,053 BTU/hr
Model-Size Class
Closest Standard Class: Exceeds standard classes
⚠️ Single unit not feasible — two units likely needed.
This demand exceeds a single standard residential tankless unit's typical capacity.
Two ~199,000 BTU/hr units (installed in parallel) would cover this with a oversized margin.
Splitting the load across separate zones is another option worth discussing with a licensed professional.
Design Notes
Always size to your coldest-month groundwater temperature, not a summer/annual average — the single most common tankless sizing mistake. A unit rated for a given GPM at a mild rise delivers noticeably less flow at a steeper rise.
This uses the simultaneous-flow-rate method every tankless manufacturer uses, not the WSFU demand-curve method — see this site's Pipe Sizing by Fixture Units calculator for that supply-side sizing approach.
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.
2 showers + kitchen sink tankless sizing example
This page is set up for 2 showers (2.5 GPM each) and a kitchen sink (2.0 GPM) running simultaneously — a common morning-peak scenario — giving a required flow rate of 7.00 GPM.
At a moderate 60°F groundwater temperature and a 120°F desired output, the temperature rise is 60°F. At 95% efficiency, that works out to a required input of 221,053 BTU/hr (7.00 GPM × 60°F × 500 ÷ 0.95) — look for a tankless unit rated at or above this figure at this rise.
- 2 showers + kitchen sink → 7.00 GPM required.
- 60°F rise (120°F desired − 60°F groundwater).
- Required input: 221,053 BTU/hr at 95% efficiency.
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 | Tankless (On-Demand) |
| Fuel Type | Natural Gas / Propane |
| Required Flow Rate | Shower x2, Kitchen Sink x1 |
| Desired Output / Groundwater Temp | 120°F / 60°F |
| Efficiency | 95% |
Step 1 — Flow Rate, Temperature Rise & BTU/hr
| Calculation | Substitution | Result |
|---|---|---|
| Required GPM | Shower x2 + Kitchen Sink x1 | 7 GPM |
| Temperature Rise | 120°F − 60°F | 60°F |
| Required BTU/hr | (7 × 60 × 500) ÷ 0.95 | 221,053 BTU/hr |
| Model-Size Class | Nearest standard class | Exceeds standard classes |
Therefore, this fixture combination needs a tankless unit rated for at least 7 GPM at a 60°F rise — 221,053 BTU/hr input.
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.