Heat Index Calculator

Calculate the 'feels like' temperature using the official NWS heat index formula.

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Enter your values and click Calculate

The heat index — also called the 'apparent temperature' or 'feels like' temperature — combines air temperature and relative humidity to estimate how hot it actually feels to the human body. High humidity slows the evaporation of sweat from the skin, which is the body's primary cooling mechanism. When sweat cannot evaporate efficiently, the body retains more heat, and conditions feel significantly hotter than the thermometer indicates. This calculator uses the Rothfusz regression equation adopted by the National Weather Service (NWS), the same formula used in official US weather forecasts. It is valid when the temperature is at or above 80°F and relative humidity is at or above 40%. The danger level labels follow the NWS heat index chart, which defines four categories from Caution to Extreme Danger based on risk of heat-related illness. Never leave children or pets in a parked car, limit outdoor activity during peak heat hours (10 a.m.–4 p.m.), stay hydrated, and seek air-conditioned spaces when conditions reach the Danger or Extreme Danger threshold.

How It Works

This calculator uses the Rothfusz regression equation adopted by the National Weather Service: HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − 0.00683783T² − 0.05481717R² + 0.00122874T²R + 0.00085282TR² − 0.00000199T²R², where T is temperature in °F and R is relative humidity as a percentage. The formula was derived by fitting a regression model to a table of heat index values based on human physiology and heat transfer research. It is most accurate when T ≥ 80°F and R ≥ 40%. For extreme low humidity cases (below 13% RH and T between 80–112°F) or high humidity cases (above 85% RH and T between 80–87°F), the NWS applies small adjustment factors — this calculator uses the base formula. The NWS danger categories are: Caution (80–90°F HI), Extreme Caution (91–102°F), Danger (103–124°F), and Extreme Danger (125°F+).

Examples

Hot Summer Afternoon
A typical humid summer day: 95°F with 60% relative humidity.
Result: Heat index ≈ 110°F — Danger category. Heat exhaustion and heatstroke are likely risks.
Dry Heat
A hot but dry desert day: 100°F with 20% relative humidity.
Result: Heat index ≈ 87°F — Caution. Dry air allows sweat to evaporate efficiently, reducing perceived heat.
Extreme Humidity
A tropical heat wave: 90°F with 90% relative humidity.
Result: Heat index ≈ 122°F — Danger. Near-saturated air almost completely prevents sweat evaporation.

Frequently Asked Questions

What is heat index and how does it differ from temperature?
Air temperature measures the actual thermal energy in the air. Heat index combines temperature with relative humidity to estimate how hot conditions feel to the human body. The body cools itself by sweating — as sweat evaporates, it carries heat away from the skin. High humidity slows that evaporation, making the body retain more heat. At 95°F with 60% humidity, the body's cooling system is significantly impaired and conditions can feel as hot as 110°F.
When is the heat index most dangerous?
The NWS classifies heat index values above 103°F as 'Danger' and above 125°F as 'Extreme Danger'. At these levels, heat cramps and heat exhaustion are likely, and heatstroke — a life-threatening condition where the body's core temperature rises above 104°F — becomes a serious risk. Children, the elderly, outdoor workers, and athletes are most vulnerable. High overnight temperatures (above 80°F) that prevent the body from recovering are particularly dangerous during multi-day heat waves.
Does heat index apply to shaded areas?
The heat index values published by the NWS assume shaded conditions with light wind. Direct sunlight can add 10–15°F to the perceived temperature beyond what the heat index formula calculates. Working or exercising in direct sun on a humid day can feel dramatically hotter than the official heat index value suggests. Seek shade, wear light-colored clothing, and stay hydrated.
Why does the formula require at least 80°F?
The Rothfusz equation was calibrated for hot weather conditions where heat illness is a risk. Below 80°F, the formula can produce inaccurate results and heat index is not a meaningful safety concern. In cooler conditions, wind chill (for cold weather) or simple temperature are the appropriate metrics. The NWS only issues heat index advisories and warnings when conditions exceed the thresholds where physiological heat stress becomes significant.

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