Tankless Water Heater Sizing Chart

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)

Master Tankless Sizing Matrix: Fixture GPM vs Winter Temp Rise

The Tankless Water Heater Sizing Chart serves as an engineering reference matrix illustrating the direct mathematical correlation between fixture demand (GPM), incoming groundwater temperature, and required thermal energy output (in BTU/hr or electric kW). It illustrates why nominal flow ratings must be adjusted for seasonal temperature swings.

Need automated calculations for your exact household? Use our Tankless Water Heater Sizing Calculator or compare storage tank sizes in the Water Heater Size Guide.

Groundwater Temperature Zones & Regional ΔT Requirements

Identify Household Flow Rate (GPM)

Add up fixture flow rates for items running at once (e.g., 2 showers = 4.0 GPM).

Find Regional Groundwater Temperature

Locate your zone on the climate column (e.g. 70°F Rise for Northern US, 50°F Rise for Southern US).

Cross-Reference Required BTU / kW

Locate the matrix intersection point to find minimum certified burner or electrical heating output.

Verify Unit Specifications

Check model specification sheets to ensure the manufacturer's flow curve at your ΔT satisfies demand.

Simultaneous Fixture Load Flow Rates (IPC Code Standards)

Flow Demand	Fixture Scenario	35°F Rise (South)	50°F Rise (Central)	70°F Rise (North)	80°F Rise (Extreme)
2.0 GPM	1 Standard Shower	35k BTU (10 kW)	50k BTU (15 kW)	70k BTU (20 kW)	80k BTU (23 kW)
3.0 GPM	1 Shower + 1 Sink	52k BTU (15 kW)	75k BTU (22 kW)	105k BTU (31 kW)	120k BTU (35 kW)
4.0 GPM	2 Showers Running	70k BTU (20 kW)	100k BTU (29 kW)	140k BTU (41 kW)	160k BTU (47 kW)
5.0 GPM	2 Showers + 1 Sink	87k BTU (25 kW)	125k BTU (36 kW)	175k BTU (51 kW)	199k BTU (58 kW)
6.0 GPM	3 Showers Running	105k BTU (31 kW)	150k BTU (44 kW)	199k BTU Max	Dual Units Req

Sizing Calculators by Home Bedroom & Bathroom Layout

Explore dedicated sizing calculators tailored to specific home configurations, simultaneous fixture counts, and regional climate zones:

1 Bed, 1 Bath 1B / 1B Sizing 3.5 GPM Peak (1 Showers) 2 Bed, 1 Bath 2B / 1B Sizing 4.5 GPM Peak (1 Showers) 2 Bed, 2 Bath 2B / 2B Sizing 5.5 GPM Peak (2 Showers) 3 Bed, 2 Bath 3B / 2B Sizing 6.5 GPM Peak (2 Showers) 3 Bed, 2.5 Bath 3B / 2.5B Sizing 7 GPM Peak (2 Showers) 4 Bed, 2.5 Bath 4B / 2.5B Sizing 7.5 GPM Peak (2 Showers) 4 Bed, 3 Bath 4B / 3B Sizing 8.5 GPM Peak (3 Showers) 5 Bed, 3 Bath 5B / 3B Sizing 9 GPM Peak (3 Showers) 5 Bed, 4 Bath 5B / 4B Sizing 12 GPM Peak (4 Showers)

Field Contractor Sizing Scenario: Northern Climate Home

Scenario: Sizing for 2 simultaneous showers (4.0 GPM) in Philadelphia (winter water temperature 48°F = 72°F rise).

Flow Demand 4.0 GPM

Temperature Lift 72°F (48°F to 120°F)

Matrix Result 150,000+ BTU/hr

Recommended Unit 160k – 199k BTU Gas Condensing

• Result: Cross-referencing 4.0 GPM with a 72°F rise dictates a minimum 160,000 BTU/hr gas unit (such as Rinnai RX160iN or Rheem 8.4 GPM).

Expert FAQ Guide

Frequently Asked Questions

6 Questions Answered

Q1 How do I read a tankless water heater sizing chart?

Find your incoming groundwater temperature on the chart (or required temperature rise) on one axis, and your peak simultaneous hot-water flow rate (GPM) on the other. The intersection shows the minimum heating capacity (BTU/hr for gas or kW for electric) required to maintain continuous 120°F hot water.

Q2 Why does a 9 GPM tankless heater only give 4.5 GPM in the winter?

Manufacturers advertise a unit's maximum flow rate under ideal conditions with a small 30°F–35°F temperature rise (e.g. 85°F incoming water in the southern US). When incoming water drops to 40°F in winter (requiring an 80°F rise), the heater must slow the water flow by half so the burner has sufficient time to heat it.

Q3 What GPM is needed for two simultaneous showers?

Two modern standard 2.0 GPM shower heads running at the same time require a total simultaneous delivery of 4.0 GPM of mixed hot water.

Q4 What temperature rise is typical in the southern United States?

In southern states (like Florida, Texas, and Southern California), groundwater temperatures average 65°F to 72°F year-round, requiring only a modest 48°F to 55°F temperature rise to reach 120°F.

Q5 What temperature rise is typical in the northern United States?

In northern states and Canada (like Minnesota, Illinois, New York), winter groundwater temperatures drop to 37°F to 45°F, requiring an intense 75°F to 83°F temperature rise to achieve comfortable 120°F tap water.

Q6 How does altitude affect gas tankless water heater ratings?

At high altitudes (above 2,000 to 4,000 feet), thinner air reduces combustion oxygen. Atmospheric and standard gas tankless units may experience derating of ~4% per 1,000 feet above sea level unless equipped with electronic altitude-compensating blowers.

Rinnai RX199iN / RXP199iN

Rinnai RX160iN

Rinnai RX130iN

Rinnai V65iN

Rheem RTGH-95DVLN

Rheem RTGH-84DVLN

Rheem RTEX-27 (240V)

Rheem RTEX-18 (240V)

Master Tankless Sizing Matrix: Fixture GPM vs Winter Temp Rise

Groundwater Temperature Zones & Regional ΔT Requirements

Identify Household Flow Rate (GPM)

Find Regional Groundwater Temperature

Cross-Reference Required BTU / kW

Verify Unit Specifications

Simultaneous Fixture Load Flow Rates (IPC Code Standards)

Sizing Calculators by Home Bedroom & Bathroom Layout

Field Contractor Sizing Scenario: Northern Climate Home

Frequently Asked Questions

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