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Tankless Water Heater Size Calculator

Quick answer: Enter your groundwater temperature, your target hot-water temperature and your peak flow. Required size ≈ GPM × temperature rise. Gas input ≈ GPM × rise × 500 BTU/h; electric ≈ GPM × rise × 0.146 kW.

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Estimates for planning only — not professional, engineering, plumbing, gas or electrical advice. Confirm sizing, venting, gas and electrical with a licensed installer and local code.

How the estimate works

Temperature rise = outlet − inlet. A tankless must heat your full peak flow by that rise at the same time. The 500 and 0.146 factors are common rules of thumb (water ≈ 8.33 lb/gal × 60 min, and 1 kW ≈ 3,412 BTU/h) and ignore efficiency losses — real gas units list input BTU/h, so compare against manufacturer flow-at-rise charts.

Worked example

Figures below are illustrative examples (2026-10-29), not quotes or guarantees.

Assumptions: 50°F groundwater, 120°F target, 4 GPM peak → 70°F rise → ≈140,000 BTU/h gas or ≈41 kW electric.

All calculators

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.