What Size Tankless Water Heater Do I Need? (Sizing Calculator + Charts)

"I'm replacing my old tank water heater and want to go tankless. But every model lists a different GPM, BTU, or kW rating — how do I know which size is right for my house?"

If you've started shopping for a tankless water heater, you already know the sizing confusion is real. Unlike tank water heaters that are simply rated by gallon capacity (40, 50, 75 gallons), tankless units are sized by flow rate in Gallons Per Minute (GPM) and heating capacity in BTU (gas) or kW (electric). Pick the wrong size and you're either shivering mid-shower or overspending on a unit you'll never max out.

That's why we built the Tankless Water Heater Sizing Calculator below. It will tell you the exact GPM, BTU, and kW rating you need based on your fixtures, climate, and desired water temperature.

Before you use the calculator, you'll need to know three things:

1. How many fixtures you'll run simultaneously (shower + dishwasher + faucet, etc.)
2. Your inlet water temperature (determined by your state/region — we have a table below)
3. Your desired output temperature (typically 120°F for general use, 110°F for showers mixed at the tap)

Let's size your tankless water heater.

Tankless Water Heater Sizing Calculator

How to Size a Tankless Water Heater (3-Step Method)

Sizing a tankless water heater boils down to one core question: how much hot water do you need, and how fast does the unit need to heat it? The answer depends on three variables. Let's walk through them.

Step 1: Count Your Simultaneous Fixtures and Add Up the GPM

First, figure out the maximum number of hot water fixtures you'd realistically run at the same time. Not every fixture in your house — just the ones that could overlap during peak use (think: morning rush with two showers running while the dishwasher finishes a cycle).

Each fixture has a standard flow rate measured in Gallons Per Minute (GPM). Here's what to use for sizing:

Federal maximums set by the Energy Policy Act of 1992 (EPAct): 2.5 GPM for showerheads at 80 psi, 2.2 GPM for faucets at 60 psi. EPA WaterSense-labeled showerheads max out at 2.0 GPM; WaterSense faucets at 1.5 GPM. Sources: DOE FEMP BMP #7, EPA WaterSense Showerhead Spec.

Add up the GPM for all fixtures you'll use simultaneously. That's your required flow rate.

Example: 2 showers (2.0 GPM each) + 1 kitchen faucet (1.5 GPM) = 5.5 GPM total.

Step 2: Find Your Inlet Water Temperature by State

Your inlet water temperature — the temperature of cold water entering the tankless unit — depends on where you live. Groundwater in southern Florida enters at 72–77°F. In Minnesota, it can be as low as 37–42°F.

This matters enormously. A unit in Miami only needs to raise water temperature by 43–48°F to reach 120°F, while that same unit in Minneapolis needs to push through an 78–83°F rise.

More temperature rise = more energy = fewer GPM at the output.

Here's the full breakdown by state and region:

Data compiled from Rinnai North America Ground Water Temperature Map, HydroFLOW Interactive Groundwater Database (NOAA/USGS monitoring stations), and Bradley Corp US Groundwater Temperature Map. Sources: HydroFLOW, Bradley Corp.

We have a detailed 50-state table further below. For now, find your region and note the required temperature rise.

Step 3: Calculate Your Required Temperature Rise and Match to a Unit

Now combine your GPM from Step 1 with your temperature rise from Step 2. The formulas are straightforward.

For gas tankless water heaters:

BTU/hr = GPM × 500 × ΔT

Where 500 is a constant (8.33 lbs per gallon of water × 60 minutes), GPM is your total simultaneous flow rate, and ΔT is the temperature rise in °F.

For electric tankless water heaters:

kW = (GPM × ΔT) ÷ 3.1

Where 3.1 is a conversion constant derived from the relationship between BTU and kilowatts (1 kW = 3,412 BTU/hr).

Quick example: You need 5.0 GPM and live in Ohio (52°F inlet, so ΔT = 68°F).

• Gas: 5.0 × 500 × 68 = 170,000 BTU
• Electric: (5.0 × 68) ÷ 3.1 = 109.7 kW — that's not practical for a single electric unit. This is why gas tankless heaters dominate in cold climates.

Tankless Water Heater Sizing Chart by Family Size

This is the table most people are looking for. We've cross-referenced family size with typical fixture use, climate, and unit recommendations.

What Size Tankless Water Heater for a Family of 2?

A household of 1–2 people typically runs 1 shower + 1 faucet simultaneously — about 3.0–3.5 GPM. In warm climates, a mid-range gas unit (120,000 BTU) or an 18–24 kW electric unit handles this easily.

What Size Tankless Water Heater for a Family of 4?

The most common search. A family of 3–4 usually peaks at 2 showers + 1 faucet — roughly 5.0–5.5 GPM. You'll want at least 150,000–199,000 BTU (gas) or 27–36 kW (electric, warm climates only).

What Size Tankless Water Heater for a Family of 5–6?

Large families with 3+ bathrooms can hit 3 showers + 1 faucet + dishwasher — that's 8.0–9.5 GPM. In cold climates, you'll likely need a top-tier 199,000 BTU gas condensing unit or even dual units. Electric tankless heaters generally can't keep up with this demand in cold regions.

How Many GPM Do I Need for a Tankless Water Heater?

Your required GPM is simply the sum of flow rates for every fixture you'd run at once. Not every fixture in the house — just the worst-case simultaneous scenario.

Here's the deal: most people overestimate their simultaneous use. Unless you have a very large family, you're probably looking at 3–6 GPM for typical residential use.

How Many Gallons Per Minute Does a Shower Use?

A standard shower head uses 2.5 GPM (the federal maximum set by EPAct 1992). If you've replaced your shower heads in the last decade, you likely have WaterSense-labeled heads rated at 2.0 GPM — and many low-flow models go down to 1.5 GPM.

For tankless sizing, use 2.0 GPM per shower unless you know your exact fixture specs.

GPM Flow Rate by Fixture Table

Sources: EPA WaterSense specifications, DOE 10 CFR 430, manufacturer specifications. Low-flow values reflect WaterSense-labeled fixtures. Standard values reflect federal maximums. High-flow values reflect older or specialty fixtures.

Simultaneous Use Scenarios: How Much GPM for Common Combinations?

Inlet Water Temperature by State (Detailed Table)

This is the data you need for accurate sizing. Groundwater temperature determines your required temperature rise — and temperature rise is the single biggest factor in choosing a tankless water heater size. The numbers below are annual averages; winter temps may be 5–10°F colder than listed.

Data compiled from Rinnai Ground Water Temperature Map, HydroFLOW USA groundwater database, and Bradley Corp US Groundwater Temperatures. Values represent annual averages; seasonal variation of ±5–10°F is typical. Sources: HydroFLOW, Rinnai, Bradley Corp.

Temperature Rise Chart: How Many BTU or kW Do You Need?

This is where it all comes together. Find your inlet temperature on the left, read across to your GPM demand, and you have the BTU or kW you need.

Temperature Rise × GPM → Required Gas BTU

Assumes desired output of 120°F.

Formula: BTU = GPM × 500 × ΔT. These are gross input values. At 8+ GPM in cold climates (shaded cells over 199,000 BTU), you'll need multiple gas units or a tank water heater instead.

Temperature Rise × GPM → Required Electric kW

Formula: kW = (GPM × ΔT) ÷ 3.1. Notice how quickly the kW requirement becomes impractical — the largest residential electric tankless unit is 36 kW (Stiebel Eltron Tempra 36 Plus / EcoSmart ECO 36). In cold climates, electric whole-house tankless is only feasible for low-demand households.

Gas Tankless Water Heater Sizing (BTU Guide)

Gas tankless water heaters are sized by BTU per hour (BTU/hr) input rating. Residential models range from 120,000 to 199,900 BTU. Higher BTU = more GPM capacity at any given temperature rise.

There are two types:

• Condensing units (0.90–0.97 UEF): Extract additional heat from exhaust gases. More efficient, can vent with PVC pipe. Brands like Navien, Rinnai Sensei, and Rheem RTGH series.
• Non-condensing units (0.80–0.85 UEF): Simpler design, lower cost, require metal venting. Rinnai RE series, Rheem RTG series.

Whole House Gas Tankless Water Heater Sizing

For whole-house applications, you want a condensing unit with at least 150,000 BTU (moderate climates) to 199,000 BTU (cold climates). Here's what the major manufacturers offer:

Sources: Navien NPE-240A2, Navien Spec Sheet, Rinnai FAQ, Rheem RTGH Spec Sheet.

Notice the GPM drop at higher temperature rises. A Navien NPE-240A2 advertises 11.2 GPM — but that's at a 35°F rise (warm climate). At a 67°F rise (cold climate like New York), it delivers only 5.6 GPM.

Always check the GPM at YOUR temperature rise, not the headline max GPM.

Gas Tankless BTU Range: 120,000 to 199,000 BTU

Here's a quick reference for matching your calculated BTU need to an available unit:

Electric Tankless Water Heater Sizing (kW Guide)

Electric tankless heaters are sized in kilowatts (kW). Residential models range from 8 kW (point-of-use) to 36 kW (whole-house, warm climates). They're ~99% efficient (virtually all electricity converts to heat), but they have a hard ceiling: even the largest 36 kW unit can only deliver about 3.0 GPM at a 68°F rise (New York climate).

That's why Rinnai doesn't even make an electric tankless model — they've stated the power requirements for whole-house electric tankless can be "prohibitive," requiring 150+ amps for a single unit.

What Size Electric Tankless Water Heater for a Whole House?

Whole-house electric tankless is realistic in warm climates (South Florida, Hawaii, Arizona, South Texas) where inlet water is 67°F+ and the required temperature rise is under 53°F. In cold climates, electric tankless works best as a point-of-use supplement rather than a whole-house solution.

Electric Tankless Water Heater Sizing Chart

Sources: EcoSmart ECO 27, EcoSmart ECO 36, Stiebel Eltron Tempra Spec Sheet.

How to Size an Electric Tankless Water Heater

Use the formula: kW = (GPM × ΔT) ÷ 3.1.

Then compare to available models. If your calculated kW exceeds 36 kW, either reduce your simultaneous fixture count, switch to gas tankless, or install multiple electric units at different points of use.

Important: check your electrical panel capacity. A 36 kW unit draws 150 amps and requires 3 separate 50-amp double-pole breakers with 300-amp minimum service to the home. Many older homes have only 100–200 amp service. Upgrading your electrical panel can cost $1,500–$3,000+, which should factor into your gas vs. electric decision.

How Many Fixtures Can My Tankless Water Heater Handle? (Reverse Lookup)

Already bought a unit or eyeing a specific model? Here's the reverse question: given your unit's rated GPM at your local temperature rise, how many fixtures can you run simultaneously?

Gas Units — Fixture Capacity by Climate

Electric Units — Fixture Capacity by Climate

The contrast is stark. A 199,000 BTU gas unit in a warm climate handles 5+ showers, while the largest 36 kW electric unit in a cold climate handles barely 1 shower and a faucet.

This is the fundamental reason gas tankless dominates the whole-house market in most of the country.

Tankless Water Heater Size by Number of Bathrooms

Many people size by bathroom count rather than family size. Here's the mapping, assuming standard WaterSense fixtures.

Tankless Water Heater for One Bathroom

One bathroom means at most 1 shower + 1 faucet running simultaneously (3.0 GPM). In any climate, a 120,000 BTU gas unit or 18–24 kW electric unit is sufficient.

Tankless Water Heater for a 2 Bathroom House

Two bathrooms typically means 2 showers running + 1 faucet as worst-case (5.0 GPM). A 150,000–180,000 BTU gas unit covers this in all climates. Electric: 27–36 kW in warm climates only.

Tankless Water Heater for a 3 Bathroom House

Three bathrooms could mean 3 simultaneous showers (6.0+ GPM). You'll want a top-tier 199,000 BTU gas condensing unit. For electric, this is only feasible in warm climates with a 36 kW unit, and even then it's tight.

Worked Examples

Example 1: Small Family in a Warm Climate (2 People, Tampa, Florida)

Inputs:

1. Household: 2 people, 1 bathroom
2. Simultaneous fixtures: 1 shower (2.0 GPM) + 1 bathroom faucet (1.0 GPM) = 3.0 GPM
3. Location: Tampa, FL — inlet water temp: 72°F
4. Desired output: 120°F
5. Temperature rise: 120 − 72 = 48°F

Gas calculation: 3.0 × 500 × 48 = 72,000 BTU. A 120,000 BTU gas unit handles this with significant headroom.

Electric calculation: (3.0 × 48) ÷ 3.1 = 46.5 kW... wait, that's over 36 kW. But that assumes 100% of water at the tap is hot water. In reality, showers mix hot and cold at the tap. At a 105°F mixed shower temp, only about 65–70% of the flow is hot water from the heater. Effective hot water GPM is closer to 2.0 GPM, so: (2.0 × 48) ÷ 3.1 = 31.0 kW.

Recommendation: A Navien NPE-150A2 (120,000 BTU, 6.8 GPM) is overkill. Or for electric: an EcoSmart ECO 27 (27 kW) or Stiebel Eltron Tempra 29 Plus (28.8 kW) both work well in this warm climate.

Example 2: Large Family in a Cold Climate (5 People, Minneapolis, Minnesota)

Inputs:

1. Household: 5 people, 3 bathrooms
2. Simultaneous fixtures: 2 showers (4.0 GPM) + 1 kitchen faucet (1.5 GPM) + 1 dishwasher (1.5 GPM) = 7.0 GPM
3. Location: Minneapolis, MN — inlet water temp: 40°F
4. Desired output: 120°F
5. Temperature rise: 120 − 40 = 80°F

Gas calculation: 7.0 × 500 × 80 = 280,000 BTU. That exceeds the largest residential single unit (199,000 BTU).

Electric calculation: (7.0 × 80) ÷ 3.1 = 180.6 kW. Completely impractical.

Recommendation: A single gas unit won't cut it at peak demand. Options: install a Navien NPE-240A2 (199,900 BTU, delivers 5.6 GPM at 67°F rise) and accept that not all fixtures run full-hot simultaneously. Or install dual gas units in a cascading rack system. Or reduce simultaneous demand with a hot water recirculation pump to minimize cold-water waste. The heating cost implications favor gas over electric here by a wide margin.

Example 3: Bathroom-Only Tankless Unit (Point-of-Use)

Inputs:

1. Single bathroom addition, remote from main water heater
2. Fixtures: 1 shower (2.0 GPM) + 1 faucet (1.0 GPM) = 3.0 GPM
3. Location: Atlanta, GA — inlet water temp: 65°F
4. Desired output: 110°F (shower only, no dishwasher)
5. Temperature rise: 110 − 65 = 45°F

Electric calculation: (3.0 × 45) ÷ 3.1 = 43.5 kW. Still above 36 kW at full hot. But with shower mixing: effective hot GPM ≈ 2.0 GPM, so (2.0 × 45) ÷ 3.1 = 29.0 kW.

Recommendation: A Stiebel Eltron Tempra 29 Plus (28.8 kW, 5.66 GPM max) is ideal for this single-bathroom point-of-use application. No gas venting needed — perfect for a bathroom addition.

Example 4: Whole-House Gas Tankless (3 Bathroom Home, Charlotte, NC)

Inputs:

1. Household: 4 people, 3 bathrooms
2. Simultaneous fixtures: 2 showers (4.0 GPM) + 1 kitchen faucet (1.5 GPM) = 5.5 GPM
3. Location: Charlotte, NC — inlet water temp: 61°F
4. Desired output: 120°F
5. Temperature rise: 120 − 61 = 59°F

Gas calculation: 5.5 × 500 × 59 = 162,250 BTU.

Recommendation: A Navien NPE-210A2 (180,000 BTU, 7.8 GPM @ 45°F rise) handles this with room to spare. A Rinnai RSC199 or Rheem RTGH-95 would also work. All three are condensing units with UEF ratings of 0.93–0.96, meaning you'll spend significantly less on gas compared to a standard tank heater.

Example 5: Whole-House Electric Tankless (2 Bathroom Condo, Phoenix, AZ)

Inputs:

1. Household: 3 people, 2 bathrooms
2. Simultaneous fixtures: 1 shower (2.0 GPM) + 1 kitchen faucet (1.5 GPM) = 3.5 GPM
3. Location: Phoenix, AZ — inlet water temp: 67°F
4. Desired output: 120°F
5. Temperature rise: 120 − 67 = 53°F

Electric calculation: (3.5 × 53) ÷ 3.1 = 59.8 kW. Over the 36 kW limit. However, accounting for shower mixing (effective hot water ≈ 2.5 GPM): (2.5 × 53) ÷ 3.1 = 42.7 kW. Still tight.

More realistic: this condo rarely runs kitchen and shower at the same time. For 2.0 GPM effective hot water: (2.0 × 53) ÷ 3.1 = 34.2 kW.

Recommendation: An EcoSmart ECO 36 (36 kW) or Stiebel Eltron Tempra 36 Plus (36 kW) works — but with limited simultaneous use. If you want two showers at once, you'll push the limits. Alternatively, an ECO 27 (27 kW) handles 1 shower comfortably with headroom for a faucet. Check your average electricity usage to estimate running costs — electric tankless in AZ can be very cost-effective given the low temperature rise.

Example 6: Simultaneous Heavy Use (2 Showers + Dishwasher, Denver, CO)

Inputs:

1. Household: 4 people, 2 bathrooms
2. Simultaneous fixtures: 2 showers (4.0 GPM) + 1 dishwasher (1.5 GPM) = 5.5 GPM
3. Location: Denver, CO — inlet water temp: 50°F
4. Desired output: 120°F
5. Temperature rise: 120 − 50 = 70°F

Gas calculation: 5.5 × 500 × 70 = 192,500 BTU.

Electric calculation: (5.5 × 70) ÷ 3.1 = 124.2 kW. Not happening.

Recommendation: A 199,000 BTU gas condensing unit — Navien NPE-240A2, Rinnai RSC199, or Rheem RTGH-RH11. At a 70°F rise, the Navien delivers approximately 5.6 GPM, which is tight but workable. Insulating your hot water pipes reduces heat loss and helps maintain temperature at the fixtures. A recirculation pump can also reduce the GPM "wasted" while waiting for hot water to arrive.

Understanding UEF Ratings for Tankless Water Heaters

The Uniform Energy Factor (UEF) is the DOE's current standard for measuring water heater efficiency, replacing the older Energy Factor (EF) metric in 2017. A higher UEF means lower operating costs.

Important: UEF ratings are only comparable within the same "bin" (usage category). A tankless 0.93 UEF and a tank 0.93 UEF don't mean the same real-world performance. Sources: ENERGY STAR, Rinnai, DOE 10 CFR 430.

For condensing gas tankless units, look for a UEF of 0.90 or higher. For electric tankless, most units already hit 0.94–0.99 — efficiency isn't the differentiator; capacity (kW) is.

Note on condensing units and dew point: condensing tankless heaters produce acidic condensate (pH 3–5) because they cool exhaust gases below the dew point to extract additional heat. This condensate needs a drain line — something to factor into installation planning.

FAQ — Tankless Water Heater Sizing

What size tankless water heater do I need for a family of 4?

A family of 4 typically needs 5.0–5.5 GPM to cover 2 simultaneous showers plus a faucet. In cold climates, that requires a 180,000–199,000 BTU gas condensing unit. In warm climates, a 150,000 BTU gas unit or 27–36 kW electric unit works.

How many GPM do I need for a tankless water heater?

Most homes need 3–6 GPM. Add up the flow rates of fixtures you'd realistically use at the same time: 2.0 GPM per shower, 1.0–1.5 GPM per faucet, 1.5 GPM for a dishwasher, and 2.0 GPM for a washing machine.

Can a tankless water heater run 2 showers at once?

Yes — if sized correctly. Two showers at 2.0 GPM each = 4.0 GPM total, requiring 96,000 BTU (warm climate) or about 31 kW electric.

A mid-range gas unit handles this easily. For electric, you'll need at least 27–36 kW in a warm climate. In cold climates, two simultaneous showers require a 180,000+ BTU gas unit.

What is the difference between gas and electric tankless sizing?

Gas units are sized by BTU and deliver much higher GPM at any temperature rise — the largest gas unit (199,000 BTU) can push 11+ GPM. Electric units are sized by kW and max out at 36 kW, delivering about 7 GPM in ideal conditions.

Gas is better for whole-house use in cold/moderate climates. Electric is simpler to install (no venting) and works well in warm climates or as point-of-use.

Is a tankless water heater worth it for a small house?

For a small home with 1–2 bathrooms and 1–3 occupants, a tankless water heater is often a smart choice. You get endless hot water, save space (wall-mounted vs. a 40–50 gallon tank), and reduce energy costs by 20–34% since there are no standby losses. Just size it properly — even a small home needs the right GPM and temperature rise match.

Do I need 2 tankless water heaters for a large home?

Possibly. If your calculated BTU exceeds 199,000 (the max for residential single units) — common for 4+ bathrooms in cold climates — you'll need either dual units configured in a cascade/rack system, or a primary tankless plus a point-of-use electric unit at remote bathrooms. Navien, Rinnai, and Rheem all offer cascading systems that link multiple units together.