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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)
2
Bathtub (4 GPM)
1
Kitchen Sink (1.5 GPM)
1
Bathroom Sink (1 GPM)
2
Dishwasher (1.5 GPM)
1
Washing Machine (2 GPM)
1

Sizing Result

Recommended Unit
Commercial (250K BTU)
16 GPM @ 50°F rise
Required BTU
401,500
Annual Cost
$293/yr

Cost Estimate

Equipment$2,500
Installation$1,500 – $3,000
Total$4,000 – $5,500

Flow Rate Reference

FixtureGPMTemp
Shower2.5104°F
Bathtub4102°F
Kitchen Sink1.5110°F
Bathroom Sink1100°F
Dishwasher1.5120°F
Washing Machine2105°F

What You'll Need

JACO ThreadPro PTFE Thread Seal Tape 1/2 x 125ft

$6-$94.7
View on Amazon
SharkBite Push-to-Connect Contractor Kit

SharkBite Push-to-Connect Contractor Kit

$18-$284.6
View on Amazon

Dixon Valve PTFE Industrial Sealant Tape

$3-$64.6
View on Amazon

JACO ThreadPro PTFE Thread Seal Tape 1/2 x 125ft

$6-$94.7
View on Amazon
SharkBite Push-to-Connect Contractor Kit

SharkBite Push-to-Connect Contractor Kit

$18-$284.6
View on Amazon

Dixon Valve PTFE Industrial Sealant Tape

$3-$64.6
View on Amazon

As an Amazon Associate, we earn from qualifying purchases.

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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.

Frequently Asked Questions

Q

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
Q

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
Q

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
Q

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: Mar 19, 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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