Tankless Water Heater Size Calculator
This free tankless water heater sizing calculator answers the question a tank calculator can't: a tankless unit has no reserve, so it must heat every gallon the instant you draw it. That makes sizing a two-part question — how much hot water runs at once (GPM), and how far the unit has to raise the temperature (ΔT). Multiply them and you get the required output. Get either wrong and you get lukewarm showers, which is the number-one complaint about undersized tankless installs.
Count the fixtures that could run simultaneously during your peak — two showers plus a kitchen faucet, say — using hot-water flow rates from Rinnai's sizing guideline, with WaterSense and California Title 20 presets for efficient fixtures. Pick your climate so the calculator knows how cold your incoming water starts; northern groundwater near 40°F demands far more energy per gallon than southern water near 68°F. The result is your peak GPM, temperature rise, and the BTU/hr (gas) or kW (electric) a unit must deliver.
The most important thing the results tell you is to shop on the curve, not the box. A unit advertised at "9 GPM" hits that only at a small temperature rise; at a real 68°F winter rise it might deliver half that. Always confirm the model delivers your peak GPM at your ΔT on its published GPM-vs-rise chart. And if you're considering electric tankless, the electrical demand is enormous — size the circuits with our tankless breaker calculator before committing. Free, no signup.
Tankless Water Heater Size Calculator
What GPM tankless do you need? A tankless heats water on demand, so size it by how much runs at once and how cold your water starts — not by tank gallons. Enter your peak fixtures and climate for the flow and BTU/kW to shop for. Free, no signup.
What runs at the same time?
Groundwater is coldest in winter — that's when a tankless is most stressed, so size for your cold-season inlet, not the annual average.
Calculation Formulas
A tankless heats water as you draw it, so it must serve everything running simultaneously — not an hourly total. Fixture GPM defaults to Rinnai's hot-water sizing values.
Example:
2 showers (2 × 1.5) + 1 kitchen faucet (1.4) = 4.4 GPM.
Incoming groundwater is far colder in winter and in the north — the hidden variable that shrinks a unit's deliverable flow.
Example:
120°F target − 52°F inlet = 68°F rise.
The 500 constant is 8.34 lb/gal × 60 min. Electric output is the same energy expressed in kW.
Example:
4.4 × 500 × 68 = 149,600 BTU/hr ≈ 43.9 kW.
A unit's advertised max GPM is measured at a small rise; the real deliverable flow drops as ΔT climbs. Always confirm on the GPM-vs-rise chart.
Example:
A "9 GPM" unit may deliver only 4–5 GPM at a 68°F rise.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| Energy constant | 500 (BTU per GPM per °F) | 8.34 lb/gal × 60 min/hr. |
| Rinnai fixture GPM (hot) | shower/tub 1.5 · kitchen 1.4 · lav 0.5 · DW/washer 1.5 | Manufacturer sizing-guideline hot-water values (mixed at the valve). |
| Groundwater inlet | north 40°F · moderate 55°F · south 68°F | Regional estimates (Rinnai/Bradley maps) — size for cold-season inlet. |
| Residential gas ceiling | ≈199,000 BTU/hr | Beyond a single unit; heavy simultaneous demand needs two units or trimmed fixtures. |
| Worked example | 4.4 GPM, ΔT 68 → 149,600 BTU/hr | The research anchor this calculator reproduces. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
Manufacturer GPM-vs-rise curves(Rinnai / Navien / Rheem sizing charts)
View StandardThe authoritative deliverable-flow data — a unit's output at your exact temperature rise, not its headline max.
Key Requirements:
- •Confirm peak GPM at your ΔT
EPA WaterSense(Showerhead 2.0 GPM / lavatory 1.5 GPM)
View StandardEfficient-fixture flow rates offered as a preset — lower demand means a smaller unit suffices.
Key Requirements:
- •WaterSense fixtures reduce peak GPM
California Title 20(CEC Title 20 (shower 1.8 / lav 1.2 / kitchen 1.8 GPM))
View StandardThe strictest US fixture-flow standard, offered as a preset for California and similar jurisdictions.
Key Requirements:
- •Lowest fixture GPM of the three presets
Electric circuit sizing(NEC Article 424)
View StandardElectric tankless draws enormous current — size the circuit separately.
Key Requirements:
- •Use the tankless breaker calculator for the electrical side
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Climate is the biggest variable
Same unit, different homes
A tankless that endlessly serves a Georgia home delivers one or two fixtures in Minnesota, because colder inlet water means a bigger rise for every gallon. Size for your winter inlet temperature.
Regional Examples:
Gas vs electric
A circuit problem, not just a plumbing one
Electric tankless at whole-home sizes needs 54–150 amps — often most or all of a service. Gas units clear the same BTU without touching the panel but need adequate gas supply and venting.
Regional Examples:
Fixture reality
Defaults are conservative
The Rinnai defaults assume standard heads and no luxury fixtures. Rain heads, body-spray systems, and large soaker tubs run well above them and can double a bathroom's draw.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
Plan disposal before you start
Smaller jobs still produce more debris than a few trash bags can hold. Check what's allowed in a dumpster and which disposal option fits the scope.
See disposal options →
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How to Use This Calculator
- Count the fixtures that could draw hot water at the same time during your busiest moment — back-to-back showers, a shower plus the kitchen sink, etc.
- Choose a fixture-efficiency preset: Rinnai typical, WaterSense (efficient), or California Title 20 (strictest) — lower-flow fixtures shrink the unit you need.
- Pick your climate so the calculator uses the right incoming water temperature, and set your target temperature (120°F is standard).
- Choose gas (BTU/hr) or electric (kW) for the output unit.
- Click Calculate for your peak GPM, temperature rise, and required output — then match it against a model's GPM-vs-rise curve.
Tankless sizing vs the circuit
This calculator sizes the appliance — the GPM and BTU/kW the unit must produce. If you're going electric, that unit then needs a circuit, and electric tankless circuits are huge (a whole-home unit can draw 100+ amps across multiple breakers). Size the electrical side separately with the tankless breaker calculator, and check your panel can take it before you buy.
Frequently Asked Questions
What size tankless water heater do I need?
Size it by two numbers, not by household headcount: peak simultaneous flow (GPM) and temperature rise (ΔT). Add up the hot-water GPM of every fixture that could run at once — two showers plus a kitchen faucet is about 4.4 GPM on standard fixtures — then figure your rise from incoming water temperature to your 120°F setpoint. A cold-climate home at a 68°F rise needs roughly 150,000 BTU/hr for that demand; a warm-climate home at a 45°F rise needs far less. The calculator does both and gives you the BTU or kW to shop for.
Why does my tankless water heater's GPM rating drop in winter?
Because deliverable flow is inversely tied to temperature rise. A unit rated '9 GPM' hits that only at a small rise — often 35°F. In winter, northern groundwater arrives near 40°F, so reaching a 120°F shower means an 80°F rise, and the same unit might deliver only 4–5 GPM. This is why you size on the manufacturer's GPM-vs-temperature-rise curve at your actual winter inlet temperature, never on the headline max GPM. Undersizing this way is the top cause of 'my tankless can't keep up' complaints.
How many BTU does a tankless water heater need?
BTU/hr = GPM × 500 × temperature rise. For 4.4 GPM at a 68°F rise: 4.4 × 500 × 68 = about 150,000 BTU/hr, near the top of the residential gas range (units cap around 199,000 BTU/hr). Heavier simultaneous demand or colder climates can exceed a single unit, requiring two in parallel or trimmed peak usage. For electric, divide BTU by 3,412 to get kW — that 150,000 BTU example is about 44 kW, which is a very large electrical load.
Can my electrical panel handle an electric tankless water heater?
Often not without an upgrade — this is the hidden dealbreaker. A whole-home electric tankless can draw 100+ amps across multiple double-pole breakers, which is most or all of a 150-amp service and two-thirds of a 200-amp. Gas tankless sidesteps this entirely. If you're set on electric, size the circuits and check your service capacity with our tankless breaker calculator before buying — the electrical feasibility, not the plumbing, is usually what decides the project.
Is a tankless water heater worth it vs a tank?
Tankless wins on endless hot water (it never runs out mid-shower), a 20+ year lifespan, and near-zero standby loss. Tank wins on install cost (tankless often needs gas-line, venting, or major electrical upgrades), simplicity, and better performance for short simultaneous bursts. If your complaint is running out of hot water, tankless solves it; if it's just an aging tank, a right-sized tank is cheaper. Size a conventional tank instead with our water heater size calculator.