basic

Tankless Water Heater GPM Sizing Calculator

Sizing = simultaneous GPM at your local ΔT.

Estimate the minimum gallons-per-minute flow rating a tankless (on-demand) water heater needs to handle your simultaneous-fixture demand at your inlet-water temperature. Sum the per-fixture GPM (default: kitchen 1.5, shower 2.0, bath 2.5, dishwasher 1.0, washer 2.0). Reports total simultaneous GPM and the temperature rise required from inlet to setpoint.

Marcus Laurent By Marcus Laurent
Published Last reviewed 1 min read

Inputs

GPM
GPM
GPM
GPM
°F
°F

Results

Enter values and click Calculate to see results.
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How to use this calculator

  1. Fill in the inputs above using the units you already have.
  2. Values update automatically as you type — no submit button needed.
  3. Hover any result row for the underlying formula and intermediate values.

Formula

GPM_total = Σ active fixtures · per-fixture GPM; ΔT = setpoint − inlet.

In depth

Estimate the minimum gallons-per-minute flow rating a tankless (on-demand) water heater needs to handle your simultaneous-fixture demand at your inlet-water temperature. Sum the per-fixture GPM (default: kitchen 1.5, shower 2.0, bath 2.5, dishwasher 1.0, washer 2.0). Reports total simultaneous GPM and the temperature rise required from inlet to setpoint.