Plan Otter

Water-heating planning tool

Water Heater Size Calculator

Planning comparison only — not product selection or installation design. This tool totals uses entered for the busiest single one-hour period. FHR is not nominal tank capacity.

Peak-hour planning

Enter busiest-hour hot-water uses

Planning comparison only — not product selection or installation design. The result is a minimum First Hour Rating (FHR) comparison target for a storage or heat-pump-with-tank heater. FHR is not tank capacity.

Peak-hour events

Add each use during the busiest single hour. Enter your own whole-number quantity and explicit gallons per event; this tool supplies no usage defaults.

Add up to 50 events.

Additional peak-hour gallons

Add only gallons in that same hour that are not already represented by an event row.

Optional; add up to 50 labeled entries.
Optional arithmetic comparison only. Enter a published FHR, not nominal tank capacity; no model is verified or recommended.

Plan responsibly

Before You Build

This project may be subject to local permit, zoning, site, or structural requirements.

  • Permit and inspectionConfirm locally required permits, inspections, approved appliance scope, and manufacturer installation instructions.
  • Seismic restraintVerify locally required seismic restraint and structural support for the selected appliance and location.
  • Local installation requirementsVerify fuel, venting, combustion air, electrical, relief, drainage, scald protection, and locally adopted amendments with qualified professionals.

FHR comparison only: The result is a First Hour Rating comparison target, not tank capacity, product selection, delivered performance, or installation design.

Scald hazard: Hot water can scald. Follow manufacturer instructions and qualified local requirements for temperature control and scald protection.

View code references

Model-code reference

2018 IPC, Section Chapter 5

Water-heater installation: Model-code installation provisions do not convert peak-hour demand arithmetic into tank capacity, product selection, or an approved installation.

Source: ICC — 2018 IPC Chapter 5 Water Heaters · Reviewed 2026-08-24

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.

Model-code references

2018 IPC, Section Chapter 5: Water-heater installation. Source: ICC — 2018 IPC Chapter 5 Water Heaters. Reviewed 2026-08-24.

Local requirements may differ. Verify the adopted code, amendments, zoning rules, and permit requirements with the local authority.

Methodology

How we calculate this

Each positive integer event quantity is multiplied by its explicitly entered gallons per event. Labeled additional peak-hour gallons are added only after acknowledgment that they are not in event rows. Exact bounded scaled-integer arithmetic preserves up to six decimal places.

minimum FHR comparison target = Σ(quantity × entered gallons/event) + Σ(additional peak-hour gallons)

Avoid double counting

Add only peak-hour gallons not already represented in event rows; otherwise demand will be counted twice.

Compare FHR, not tank capacity

Compare the result with manufacturer/EnergyGuide First Hour Rating. FHR is one-hour hot-water delivery beginning with a fully heated tank; it is not nominal tank capacity.

Installation and scald boundary

A qualified plumbing/heating installer must verify the appliance and installation with the manufacturer and authority having jurisdiction, including fuel, venting, combustion air, electrical, seismic restraint, relief, scald protection, drainage, structural support, local code, permits, and inspections. Hot water can scald; hand-test it and follow manufacturer and qualified/local requirements.

Worked examples

Water Heater Size Calculator examples

Common questions

Water Heater Size Calculator FAQ

Does this calculate tank capacity?

No. It returns only a minimum manufacturer/EnergyGuide First Hour Rating comparison target, not tank gallons or a product recommendation.

Is this daily hot-water use?

No. DOE's method concerns the busiest single one-hour period, not total daily use.

Does an entered FHR prove a heater is suitable?

No. The tool performs only an arithmetic comparison of the user-entered FHR and derived target; it does not verify or recommend a model.

Sources

Assumptions and references

Last reviewed: 2026-08-24