Price as of Jul 24, 2026. Product prices and availability are accurate as of the date indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.

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Disclaimer: This calculator provides preliminary estimates for educational purposes only. It is not a substitute for a licensed professional engineer, architect, or structural engineer. Results depend on material grade and species, site conditions, local climate and snow loads, and amendments to the applicable building code, all of which vary by location. Always verify results against your local building codes and confirm requirements with your Authority Having Jurisdiction before proceeding. Do not use these estimates for permitting, fabrication, or construction without stamped, engineer-sealed drawings from a licensed professional. Improperly sized structural members, foundations, or gas lines can cause structural collapse, fire, gas leaks, serious injury, or death.

Example Calculations

12-Bathroom Home (Gas, Moderate Climate)

Inputs

Fixtures2 showers + kitchen

ClimateModerate (55°F)

FuelNatural Gas

Result

RecommendedMedium (160K BTU)

6.5 GPM total at 55°F rise = 179,000 BTU required. 160K BTU unit at 85% efficiency.

21-Bathroom Apartment (Electric)

Inputs

Fixtures1 shower + sink

ClimateWarm (65°F)

FuelElectric

Result

RecommendedMedium (24 kW)

3.5 GPM at 39°F rise = 68,250 BTU. 24 kW electric unit at 98% efficiency.

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Frequently Asked Questions

How do I size a tankless water heater?

Add up the GPM (gallons per minute) of all fixtures you want to run simultaneously, then calculate the temperature rise from your inlet water temperature to your desired outlet temperature. A family of 4 typically needs 7–10 GPM with a 50–70°F rise.

Shower: 2.0–2.5 GPM at 104°F; kitchen sink: 1.5 GPM at 110°F
Dishwasher: 1.5 GPM at 120°F; washing machine: 2.0 GPM at 105°F
Cold climate (45°F inlet) needs much larger unit than warm climate (65°F inlet)
Gas units handle higher flow rates than electric at the same price point

Gas vs electric tankless water heater?

Gas tankless heaters deliver 8–11 GPM and cost $800–$1,500 for the unit. Electric models deliver 3–8 GPM and cost $500–$1,200. Gas has lower operating costs but higher installation costs due to venting requirements.

Gas: 120K–199K BTU, handles whole-house demand, needs venting and gas line
Electric: 18–36 kW, best for 1–2 bathrooms, needs 200A panel and heavy wiring
Gas operating cost: ~$200–$400/year; electric: ~$300–$500/year (varies by rates)
Electric is simpler to install; gas requires combustion air and stainless steel venting

How much does tankless water heater installation cost?

Professional installation costs $1,500–$3,000 for gas units (includes venting, gas line) and $800–$1,500 for electric units. Total project cost including the unit: $2,300–$4,500 for gas, $1,300–$2,700 for electric.

Gas install includes: stainless vent ($200–$400), gas line sizing, condensate drain
Electric install includes: dedicated circuit, wire run, possible panel upgrade ($1,000+)
Replacing a tank with tankless adds $500–$1,000 for gas line/venting modifications
Permits required in most jurisdictions: $100–$300 depending on location

What temperature rise do I need?

Temperature rise = desired outlet temp minus your incoming water temperature. In cold climates (45°F inlet), you need 55–75°F rise. In warm climates (65°F inlet), only 35–55°F rise is needed, allowing a smaller unit.

Groundwater temp by region: South 65°F, Mid-Atlantic 55°F, Midwest 45°F, North 35°F
Every 10°F of additional rise reduces flow rate capacity by 15–20%
Most homes need 104–110°F outlet temperature for comfortable use
Higher rise = lower GPM capacity, so cold climates need larger or multiple units

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Last Updated: Jul 24, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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Tankless Water Heater Calculator — GPM Sizing Tool

Calculate the right tankless water heater size by fixtures, flow rate, and temperature rise

Required Flow Rate

16 GPM

Temp Rise

50°F

Size

Commercial (250K BTU)

Shower (2.5 GPM)

Bathtub (4 GPM)

Kitchen Sink (1.5 GPM)

Bathroom Sink (1 GPM)

Dishwasher (1.5 GPM)

Washing Machine (2 GPM)