Tankless Water Heater Sizing: GPM & BTU

Fuel Type Select

Brand Filter Brand

1. Groundwater & Target Temperatures ΔT = Target − Incoming

Incoming:

Winter Groundwater Temp

°F

Desired Output Temp

°F (120°F Code Standard)

2. Peak Hot Water Fixture Demand Select fixtures operating concurrently during morning rush

Active	Fixture Type	Qty	Flow (GPM)
	Standard Shower (2.0 GPM)
	Bathroom Sink Faucet (1.0 GPM)
	Kitchen Sink Faucet (1.5 GPM)
	Bathtub Filler (4.0 GPM)
	Dishwasher (1.5 GPM)
	Clothes Washer (2.0 GPM)

Required Peak Tankless Flow Rate

6.0 GPM at 70°F Rise

Metric Equivalent: 22.7 L/min at 38.9°C Rise

Required Heating Power 210,000 BTU/hr 61.5 kW Power

Groundwater Temperature Rise 70°F 50°F in → 120°F out

Estimated Electrical Draw 256 Amps 240V Dedicated Electrical Service

Total Connected Fixture Capacity 15.0 GPM If all home fixtures opened simultaneously

Your peak simultaneous hot-water demand is 6.0 GPM (22.7 L/min) with a 70°F (38.9°C) temperature rise (incoming 50°F → desired 120°F). To deliver this flow reliably, your Gas tankless heater must produce approximately 210,000 BTU/hr (61.5 kW). Ensure the chosen model provides at least 6.0 GPM at a 70°F rise.

Dynamic Flow vs. Temperature Rise Matrix Required BTU/hr (kW)

Temperature Rise (ΔT)	2.0 GPM	3.0 GPM	4.0 GPM	5.0 GPM	6.0 GPM
40°F Rise	40k BTU (11.7 kW)	60k BTU (17.6 kW)	80k BTU (23.4 kW)	100k BTU (29.3 kW)	120k BTU (35.2 kW)
50°F Rise	50k BTU (14.7 kW)	75k BTU (22 kW)	100k BTU (29.3 kW)	125k BTU (36.6 kW)	150k BTU (44 kW)
60°F Rise	60k BTU (17.6 kW)	90k BTU (26.4 kW)	120k BTU (35.2 kW)	150k BTU (44 kW)	180k BTU (52.8 kW)
70°F Rise	70k BTU (20.5 kW)	105k BTU (30.8 kW)	140k BTU (41 kW)	175k BTU (51.3 kW)	210k BTU (61.5 kW)
80°F Rise	80k BTU (23.4 kW)	120k BTU (35.2 kW)	160k BTU (46.9 kW)	200k BTU (58.6 kW)	240k BTU (70.3 kW)

Highlighted cell indicates your exact calculated demand requirement.

Recommended Compatible Water Heater Models Matches Your Exact Load

RINNAI

Rinnai RX199iN / RXP199iN

SENSEI® RX Series (Condensing)

5.5 GPM at 70°F Rise (11.1 GPM max)

199,000 BTU/hr 0.98 UEF GAS

Best for large households (3-4 simultaneous showers in cold climates).

RINNAI

Rinnai RX160iN

SENSEI® RX Series (Condensing)

4.4 GPM at 70°F Rise (9 GPM max)

160,000 BTU/hr 0.97 UEF GAS

Ideal for standard 2-3 bathroom homes with moderate to cold groundwater.

RINNAI

Rinnai RX130iN

SENSEI® RX Series (Condensing)

3.6 GPM at 70°F Rise (7 GPM max)

130,000 BTU/hr 0.96 UEF GAS

Ideal for 1-2 bathroom homes or warmer climate zones.

RINNAI

Rinnai V65iN

High Efficiency Plus (Non-Condensing)

3.8 GPM at 70°F Rise (6.5 GPM max)

150,000 BTU/hr 0.82 UEF GAS

Cost-effective non-condensing gas solution for small-to-medium demand.

RHEEM

Rheem RTGH-95DVLN

Performance Platinum® Condensing Gas

5.3 GPM at 70°F Rise (9.5 GPM max)

199,900 BTU/hr 0.93 UEF GAS

High-output gas unit supporting up to 3-4 simultaneous showers.

RHEEM

Rheem RTGH-84DVLN

Performance Platinum® Condensing Gas

4.7 GPM at 70°F Rise (8.4 GPM max)

180,000 BTU/hr 0.93 UEF GAS

Efficient mid-size whole-house gas tankless for 2-3 bathrooms.

RHEEM

Rheem RTEX-27 (240V)

Performance® Electric Tankless

2.6 GPM at 70°F Rise (6.6 GPM max)

27.0 kW (112.5A) 0.99 Energy Factor ELECTRIC

High-power whole-house electric unit (Requires 3x 40A double-pole breakers).

RHEEM

Rheem RTEX-18 (240V)

Performance® Electric Tankless

1.8 GPM at 70°F Rise (4.4 GPM max)

18.0 kW (75A) 0.99 Energy Factor ELECTRIC

Compact electric unit for 1-2 bathroom apartments or warm southern climates.

Occupants	Low Demand	Average Demand	High Demand
1–2 People	30 Gal (115 L)	40 Gal (150 L)	50 Gal (190 L)
3–4 People	40 Gal (150 L)	50 Gal (190 L)	65–75 Gal (245–285 L)
5+ People	50 Gal (190 L)	65–75 Gal (285 L)	80+ Gal (300+ L)

About the Tankless Water Heater Sizing Calculator

The Tankless Water Heater Sizing Calculator determines the required continuous flow rate (GPM) and burner/element firing rate (BTU/hr or electric kW) for on-demand water heaters. Because tankless units do not store heated water, proper sizing requires calculating the total simultaneous hot-water flow of fixtures running concurrently alongside the temperature lift needed to convert cold winter groundwater into comfortable 120°F water.

Deciding between fuel types? Calculate electrical breaker load with our electric tankless water heater sizing calculator or determine gas line supply requirements with the gas tankless sizing tool. If you are comparing tankless with conventional storage tanks, explore the master water heater sizing calculator, consult the tankless sizing chart, or evaluate specific manufacturer ratings with our Rheem water heater tool and Rinnai sizing calculator.

How to Use the Tankless Sizing Calculator

Select Simultaneous Fixtures

Select how many showers (1.5–2.5 GPM), kitchen faucets (1.5 GPM), and appliances will run at the same time.

Set Incoming Groundwater Temperature

Choose your geographic zone or enter your region's coldest winter municipal water temperature (e.g., 40°F–50°F).

Choose Fuel Source (Gas vs Electric)

Select Natural Gas / Propane (BTU/hr) or Electric (kW / 240V Amperage) to see infrastructure requirements.

Review Required Capacity & Match Units

View exact peak GPM, temperature rise (ΔT), and matching residential tankless units from Rinnai, Rheem, and Navien.

How Tankless Water Heater Sizing Works

Tankless sizing is governed by the thermodynamic heat transfer equation:

ΔT = T_target (120°F) - T_groundwater

BTU/hr = 500.4 × Peak GPM × ΔT / Thermal Efficiency

Where 500.4 is the thermodynamic fluid constant (8.34 lb/gal × 60 min/hr).

Groundwater Temperature & Maximum Tankless GPM by Region

Region / Climate	Winter Groundwater	Temp Rise (ΔT to 120°F)	199k BTU Gas Output	27 kW Electric Output
Northern US / Canada	37°F – 45°F	75°F – 83°F	4.6 – 5.1 GPM	2.2 – 2.5 GPM
Central / Mid-Atlantic	48°F – 55°F	65°F – 72°F	5.3 – 5.9 GPM	2.6 – 2.8 GPM
Southern US / Sunbelt	60°F – 70°F	50°F – 60°F	6.4 – 7.6 GPM	3.1 – 3.7 GPM

Real-World Tankless Flow & Rise Example

Scenario: Sizing a tankless heater for a home in Columbus, Ohio (50°F winter groundwater) to support 2 simultaneous 2.0 GPM showers and a 1.0 GPM kitchen sink.

Peak Simultaneous Flow 5.0 GPM

Required Rise (ΔT) 70°F (50°F to 120°F)

Required Gas BTU Input 185,000 BTU/hr

Recommended Model Class 9.0 GPM Rated Gas Condensing

• Simultaneous GPM: 2.0 + 2.0 + 1.0 = 5.0 GPM

• Thermal Heat Demand: 5.0 GPM × 70°F × 500.4 / 0.95 = 184,357 BTU/hr

• Recommendation: Standard 199,000 BTU/hr condensing tankless unit (e.g. Rinnai RU199iN or Rheem Performance Platinum 9.5 GPM).

Expert FAQ Guide

Frequently Asked Questions

6 Questions Answered

Q1 How do I calculate what size tankless water heater I need?

Add together the flow rate (GPM) of all hot-water fixtures you plan to use simultaneously. Then calculate the temperature rise (ΔT) by subtracting your incoming groundwater temperature from 120°F. Choose a tankless heater that delivers your required GPM at that specific temperature rise.

Q2 What GPM tankless water heater do I need for a 2-bathroom house?

For a 2-bathroom home, a tankless water heater should provide between 5.0 to 7.0 GPM in moderate climates and 4.0 to 5.5 GPM in cold northern climates (at a 70°F rise). This allows running two showers (2.0 GPM each) and a sink (1.0 GPM) without temperature drop.

Q3 Can a tankless water heater be too small?

Yes. If your peak demand exceeds the heater's maximum flow rate at your regional temperature rise, modern tankless units will automatically restrict water flow to maintain target temperature, causing low water pressure at your showers.

Q4 Why is winter groundwater temperature so critical for tankless sizing?

Because tankless heaters heat water on demand, cold 40°F winter water requires twice as much heating power (80°F rise) to reach 120°F compared to warm 70°F summer water (50°F rise). Sizing must always be based on winter worst-case conditions.

Q5 How many BTU/hr does a gas tankless water heater need for whole-house use?

Whole-house gas tankless water heaters typically require 160,000 to 199,000 BTU/hr to provide 5.0 to 9.5 GPM depending on regional temperature rise. Units under 150,000 BTU/hr are generally limited to 1–2 simultaneous fixtures in cooler regions.

Q6 Do I need to upgrade my gas line for a gas tankless water heater?

Most whole-house gas tankless units (160k–199k BTU) require a dedicated 3/4-inch gas supply line. Existing 1/2-inch lines from standard 40,000 BTU tank heaters are usually insufficient unless the run from the gas meter is extremely short.

Rinnai RX199iN / RXP199iN

Rinnai RX160iN

Rinnai RX130iN

Rinnai V65iN

Rheem RTGH-95DVLN

Rheem RTGH-84DVLN

Rheem RTEX-27 (240V)

Rheem RTEX-18 (240V)

About the Tankless Water Heater Sizing Calculator

How to Use the Tankless Sizing Calculator

Select Simultaneous Fixtures

Set Incoming Groundwater Temperature

Choose Fuel Source (Gas vs Electric)

Review Required Capacity & Match Units

How Tankless Water Heater Sizing Works

Groundwater Temperature & Maximum Tankless GPM by Region

Real-World Tankless Flow & Rise Example

Frequently Asked Questions

Related Water Heater Sizing Tools

Water Heater Sizing Calculator

Tankless Water Heater Sizing Calculator

Electric Tankless Water Heater Sizing Calculator

Gas Tankless Water Heater Sizing Calculator

Rinnai Tankless Water Heater Sizing Calculator

Rheem Water Heater Sizing Calculator

Water Heater Size Guide

Tankless Sizing for 2 Bathroom House

Tankless Sizing for 3 Bathroom House

Tankless Sizing for 4 Bathroom House

Tankless Water Heater Sizing Chart