What size tankless water heater do I need?
Quick answer: Size a tankless by peak flow (GPM) × temperature rise. Add up the fixtures you run at once (e.g. 2 showers + a sink ≈ 5 GPM), subtract your groundwater temperature from your target (e.g. 120°F − 45°F = 75°F rise), then match a unit rated for that flow at that rise.
Worked example
Figures below are illustrative examples (2026-10-29), not quotes or guarantees.
Assumptions: 2 showers at 2.0 GPM + 1 bathroom sink at 1.0 GPM = 5.0 GPM peak; groundwater 45°F; target 120°F → 75°F rise. Gas rough input: 5.0 × 75 × 500 ≈ 187,500 BTU/h. Electric rough: 5.0 × 75 × 0.146 ≈ 55 kW — usually too much for one electric whole-home unit, which is why cold-climate homes often choose gas.
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Tankless Water Heater Size CalculatorEstimate the tankless size you need from fixtures and groundwater temperature.Tankless GPM CalculatorAdd up fixture flow rates to find peak GPM demand.Temperature Rise CalculatorFind the temperature rise from your groundwater to your target hot water.Gas Tankless BTU CalculatorEstimate BTU input needed for a gas tankless at your flow and rise.Electric Tankless Amps CalculatorEstimate kW and amps an electric tankless needs.Tankless Cost Savings CalculatorEstimate yearly savings of tankless vs a tank heater.
Guides
What Size Tankless Water Heater Do I Need?Size by peak GPM and temperature rise, not house size alone.GPM Explained for TanklessWhat gallons per minute means and why it drives tankless sizing.BTU Explained for Gas TanklessHow BTU input relates to flow and temperature rise.Tankless vs Tank Water HeaterEndless hot water and space vs upfront cost: how they compare.Electric vs Gas TanklessWhich fuel fits your home, climate and panel.Propane Tankless Water HeatersCan a tankless run on propane? Yes — sizing notes.Tankless in Cold ClimatesCold groundwater means a bigger rise and a bigger unit.Does Tankless Run Out of Hot Water?No, but an undersized unit can't keep up with flow.How a Tankless Water Heater WorksHeats water on demand as it flows through.How Much Does a Tankless Water Heater Cost?Unit, install and upgrade cost ranges to plan for.Tankless Install PlanningWhat to discuss with a licensed installer: space, venting, gas, panel.Indoor vs Outdoor Tankless InstallQuestions to ask a licensed pro about location and venting.Tankless MaintenanceDoes tankless need maintenance? Yes — mainly descaling.How to Flush a Tankless Water HeaterOverview of the yearly flush and what you need.Tankless Descaler GuideVinegar vs commercial descaler for tankless flushing.How Long Does a Tankless Water Heater Last?Typical lifespan and what shortens it.Point-of-Use Tankless HeatersWhen a small single-fixture unit makes sense.