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Gas Tankless BTU Calculator
Quick answer: Required gas BTU/h ≈ GPM × temperature rise × 500. Example: 4 GPM at a 70°F rise ≈ 140,000 BTU/h output; pick a unit whose flow-at-rise chart covers that.
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.
Worked example
Figures below are illustrative examples (2026-10-29), not quotes or guarantees.
Assumptions: 5 GPM, 75°F rise → 187,500 BTU/h. Many whole-home gas units are rated around 150,000–200,000 BTU/h input; efficiency means actual delivered heat is somewhat lower. Gas line sizing must be confirmed by a licensed gas fitter.
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