What tips should I keep in mind?

Cold climates need higher BTU — groundwater can be 40F or below Electric tankless may require 200-amp service panel upgrade Gas tankless needs proper venting — budget $500-1,500 for install

Tankless Water Heater Calculator

Calculate BTU requirements and flow rate for a tankless (on-demand) water heater based on simultaneous fixture usage.

Results

Visualization

How It Works

Tankless water heaters heat on demand. Sizing depends on how many fixtures you run simultaneously and the temperature rise needed. Gas units handle higher BTU loads than electric.

The Formula

BTU = GPM x 500 x Temperature Rise
GPM = Number of Fixtures x 2.0 GPM/fixture

Variables

500 — BTU constant for water (weight x specific heat x 60 min)
Temp Rise — Desired outlet temp minus inlet groundwater temp

Worked Example

3 fixtures at 2 GPM each, 50F inlet, 120F outlet: BTU = 6 x 500 x 70 = 210,000 BTU/hr. Gas tankless recommended.

Practical Tips

Cold climates need higher BTU — groundwater can be 40F or below
Electric tankless may require 200-amp service panel upgrade
Gas tankless needs proper venting — budget $500-1,500 for install
Low-flow fixtures reduce GPM demand and allow smaller units

Last updated: March 11, 2026 · Reviewed by the PlumbCalcs Editorial Team