Is this calculation free and private?

Yes, 100% of calculations execute locally in your web browser. No inputs or personal data are stored or transmitted.

What does the Tankless Water Heater Sizing & GPM Calculator give me?

It works out Required Temp Rise, Peak Flow Demand, Estimated Gas Input Required and Estimated Electric Power Required, updating live as you change the inputs.

What do I need to enter?

Just Groundwater Incoming Temp, Target Output Temp, Simultaneous Showers, Simultaneous Faucets and Appliances. Every field has a sensible default you can start from.

Is my data uploaded anywhere?

No. The calculation runs in your browser with JavaScript — nothing you type is sent to a server or stored.

Does it work on a phone?

Yes. The layout adapts to small screens and needs no app or signup.

Tankless Water Heater Sizing & GPM Calculator

Calculate simultaneous hot water demand (GPM), required temperature rise (°F), and minimum BTU/kW sizing for tankless heaters.

100% free
Private in browser
No signup
No watermarks

Groundwater Incoming Temp (°F)

Target Output Temp (°F)

Simultaneous Showers (2.0 GPM each)

Simultaneous Faucets (1.0 GPM each)

Appliances (1.5 GPM each)

Required Temp Rise (°F)

Peak Flow Demand (GPM)

6.5

Estimated Gas Input Required (BTU/hr)

277,439

Estimated Electric Power Required (kW)

66.7

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About Tankless Water Heater Sizing & GPM Calculator

Calculate simultaneous hot water demand (GPM), required temperature rise (°F), and minimum BTU/kW sizing for tankless heaters. Enter Groundwater Incoming Temp, Target Output Temp and Simultaneous Showers and the tool works out Required Temp Rise, Peak Flow Demand and Estimated Gas Input Required straight away. Everything runs in your browser, so your data never leaves your device.

How to use Tankless Water Heater Sizing & GPM Calculator

Enter Groundwater Incoming Temp, Target Output Temp and Simultaneous Showers.
Adjust the remaining options (Simultaneous Faucets and Appliances) to match your case.
Read Required Temp Rise, Peak Flow Demand and Estimated Gas Input Required in the results panel — copy or download what you need.

Key features

Shows Required Temp Rise (°F)
Shows Peak Flow Demand (GPM)
Shows Estimated Gas Input Required (BTU/hr)
5 adjustable inputs, results update as you type
No signup, no limits

Frequently asked questions

Is this calculation free and private?: Yes, 100% of calculations execute locally in your web browser. No inputs or personal data are stored or transmitted.
What does the Tankless Water Heater Sizing & GPM Calculator give me?: It works out Required Temp Rise, Peak Flow Demand, Estimated Gas Input Required and Estimated Electric Power Required, updating live as you change the inputs.
What do I need to enter?: Just Groundwater Incoming Temp, Target Output Temp, Simultaneous Showers, Simultaneous Faucets and Appliances. Every field has a sensible default you can start from.
Is my data uploaded anywhere?: No. The calculation runs in your browser with JavaScript — nothing you type is sent to a server or stored.
Does it work on a phone?: Yes. The layout adapts to small screens and needs no app or signup.

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