Water-heating planning tool
Planning comparison only — not product selection or installation design. Enter actual project-specific flow rates and temperatures; this tool supplies no fixture flow, groundwater temperature, outlet setpoint, diversity factor, performance curve, or model.
Flow and temperature-rise planning
Planning comparison only. This arithmetic makes no product, fuel, electrical, venting, installation, or code-compliance claim. Verify an exact manufacturer performance curve and all installation requirements with qualified professionals.
Plan responsibly
This project may be subject to local permit, zoning, site, or structural requirements.
Flow/rise comparison only: The result is a GPM-at-entered-temperature-rise comparison target, not a product recommendation, actual heater output, or installation design.
Scald hazard: Entered outlet temperature is not a safety recommendation. Hot water can cause severe burns; follow manufacturer instructions and qualified local requirements for temperature control and scald protection.
Water-heater installation: Model-code installation provisions do not turn flow-and-temperature-rise arithmetic into product selection, delivered performance, or an approved installation.
Source: ICC — 2018 IPC Chapter 5 Water Heaters · Reviewed 2026-08-26
Local requirements may differ. A model-code reference does not confirm the code edition, amendments, zoning rules, or permit requirements adopted where the project is located.
2018 IPC, Section Chapter 5: Water-heater installation. Source: ICC — 2018 IPC Chapter 5 Water Heaters. Reviewed 2026-08-26.
Local requirements may differ. Verify the adopted code, amendments, zoning rules, and permit requirements with the local authority.
Methodology
For the one acknowledged simultaneous-demand scenario, multiply each positive whole-number quantity by its entered per-item GPM and sum every unrounded contribution. Temperature rise is entered outlet temperature minus entered incoming temperature in the one selected unit. An optional user-supplied derating divides demand by the remaining-capacity fraction exactly once; the adjusted comparison target rounds upward to 0.001 GPM and is never represented as actual heater output.
Include each physical fixture/appliance once and only if it runs in this same simultaneous scenario. Never combine grouped and individual duplicate contributions, sequential loads, or an aggregate already represented by fixture rows.
Compare the exact manufacturer performance curve or table at this same temperature rise and applicable rated conditions. Leave derating blank if that curve is already derated so the reduction is not applied twice.
Entered outlet temperature is not a safety recommendation. Hot water can cause severe burns; CPSC warns that even 120°F exposure can burn. A qualified plumbing/heating installer and the authority having jurisdiction must verify actual flows, seasonal inlet temperature, scald protection, activation flow, manufacturer performance, altitude or other derating, fuel and gas supply, combustion air, venting, condensate, electrical service/circuit/voltage, water pressure/quality, recirculation/distribution, freeze/seismic/structural requirements, local code, permits, inspections, and warranty before purchase or installation.
Worked examples
Common questions
No. It returns only required GPM at the entered temperature rise for comparison with the exact manufacturer performance curve and conditions.
No. Every flow and both temperatures are project-specific user entries; no household mapping, lookup, setpoint, or diversity factor is inferred.
For entered derating d, capacity retains (100 − d)% and the adjusted comparison target divides by 1 − d/100 exactly once. For example, 20% derating means capacity retains 80% and the target divides base flow by 0.80. Enter derating only from the manufacturer or a qualified assessment, and leave it blank for an already-derated curve.
Sources
Last reviewed: 2026-08-26
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