Heat Index Calculator.

Formula and practical context.

Formula, variables, and method

NOAA Rothfusz regression using temperature °F and relative humidity %. T is air temperature in Fahrenheit and R is relative humidity percentage; low- and high-humidity adjustments apply in NOAA ranges. Heat index estimates how hot shaded conditions feel when air temperature and relative humidity are considered together. Start by identifying what each entered number represents and whether it is a measured value, a specification, or an assumption. Convert units before combining quantities, preserve full precision through intermediate steps, and round only the displayed result. Normalize temperature to Fahrenheit, apply the Rothfusz polynomial and NOAA humidity adjustments, then convert the result back for a Celsius display. This sequence makes the output reproducible and helps expose a misplaced unit or decimal point before it affects a decision.

Worked example and independent check

At 90 °F and 70% relative humidity, the NOAA Rothfusz regression gives a heat index of approximately 105.9 °F. To check the example independently, write the formula first, substitute the values with their units, and complete one operation at a time. Compare the order of magnitude with a familiar reference rather than accepting a formatted result automatically. Normalize temperature to Fahrenheit, apply the Rothfusz polynomial and NOAA humidity adjustments, then convert the result back for a Celsius display. If a second method or inverse calculation is available, use it to recover an original input. Small differences from hand calculation should come only from displayed rounding, not from a different definition.

Units and result interpretation

Temperature may be entered in Celsius or Fahrenheit but the NOAA regression operates in Fahrenheit; relative humidity is a percentage. Heat index estimates how hot shaded conditions feel when air temperature and relative humidity are considered together. A unit is part of the quantity, not a label added after the arithmetic. Keep decimal and binary prefixes, temperature scales, elapsed-time units, and electrical prefixes explicit. When comparing two scenarios, change one assumption at a time and retain the same unit convention. The result describes only the relationship named by this calculator; it does not automatically establish capacity, safety, compatibility, quality, or compliance.

Validation and boundary cases

The regression is intended for temperatures of at least 80 °F and humidity of at least 40%; inputs outside that range receive an explicit applicability error. Blank values, malformed text, non-finite numbers, impossible ranges, zero divisors, and unsupported combinations receive an explicit message rather than a silent fallback. Boundary values are checked before the formula runs. If an input comes from a specification sheet, weather report, network plan, or measurement instrument, confirm its definition and precision. A syntactically valid value can still be unsuitable for the real situation, so inspect the assumptions alongside the result.

Assumptions, privacy, and practical limits

Direct sun, wind, clothing, exertion, age, health, hydration, and local warnings can change risk substantially; use official alerts for safety decisions. All entered values are processed in the browser and are not sent to a calculation service. Use the result as a transparent estimate or transformation, keep the source values with any saved result, and consult the linked authoritative material for definitions and applicability. For safety-critical, health-adjacent, security, network, chemical, or electrical work, verify the result with current standards, official warnings, qualified professionals, and appropriate test equipment.

Check assumptions and sources.

Direct sun, wind, clothing, exertion, age, health, hydration, and local warnings can change risk substantially; use official alerts for safety decisions.

NOAA/NWS Heat Index