What is the heat index and how is it calculated?

The heat index (also called "feels like" or apparent temperature) measures how hot it feels when humidity is factored in. High humidity reduces the body's ability to cool itself through sweat evaporation, making it feel hotter than the actual temperature. The NWS Rothfusz regression equation is used: HI = -42.379 + 2.04901523*T + 10.14333127*RH - 0.22475541*T*RH - ... (8 terms). Valid for temperature at or above 80°F (27°C) and humidity at or above 40%.

At what heat index is it dangerous to be outside?

Heat index danger levels by category: Caution (80–90°F / 27–32°C feels like): fatigue possible with prolonged activity. Extreme Caution (90–103°F / 32–39°C): heat cramps and heat exhaustion possible. Danger (103–124°F / 39–51°C): heat exhaustion likely, heatstroke possible — limit outdoor time. Extreme Danger (above 124°F / 51°C): heatstroke highly likely — avoid outdoors entirely. Older adults, children, and people with chronic illness face higher risk at lower thresholds.

What is the difference between heat index and feels like temperature?

"Feels like" (apparent temperature) is the general term; heat index is the hot-weather version of it. In cold weather, "feels like" is wind chill — it accounts for wind removing body heat. In hot weather, "feels like" is the heat index — it accounts for humidity preventing sweat evaporation. Weather apps often show a single "feels like" number that uses wind chill below 50°F (10°C) and heat index above 80°F (27°C), blending both formulas.

Why does humidity make hot weather feel hotter?

Your body cools itself by sweating. When sweat evaporates off skin, it carries heat away. High humidity means the air is already saturated with water vapor, so sweat evaporates more slowly — sometimes barely at all. With less evaporative cooling, your body retains more heat, your core temperature rises faster, and exertion becomes more dangerous. At 40°C (104°F) with 60% humidity, the heat index can exceed 50°C (122°F) because virtually no evaporative cooling is occurring.

What is the Rothfusz regression equation used for heat index?

The Rothfusz regression equation is the formula used by the US National Weather Service (NWS) to calculate heat index. It is a multi-variable polynomial that best fits human physiological heat stress data. It requires temperature at or above 80°F and relative humidity at or above 40% to give accurate results. The formula has 8 terms involving temperature (T) and relative humidity (RH) squared and cross-multiplied. It replaced simpler formulas in official NWS heat advisory calculations.

How should I protect myself in high heat index conditions?

In Extreme Caution or Danger heat index conditions: stay in air-conditioned spaces during peak heat (noon–4pm); drink 1 cup of water every 15–20 minutes during outdoor activity; wear light-colored, loose-fitting, breathable clothing; never leave children or pets in parked cars; check on elderly neighbors; reschedule outdoor workouts to early morning or evening. Signs of heat exhaustion include heavy sweating, weakness, cold/pale skin, and nausea. Heatstroke (hot/dry skin, confusion, rapid pulse) is a medical emergency — call 911.

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Heat Index Calculator

The heat index (feels-like temperature) is calculated using the NWS Rothfusz regression equation, valid for temperatures at or above 80°F (27°C) and humidity at or above 40%. At 35°C with 60% humidity, the heat index is approximately 41°C. Danger levels: Caution (27–32°C), Extreme Caution (32–39°C), Danger (39–51°C), Extreme Danger (above 51°C). High humidity prevents sweat evaporation, causing the body to overheat faster than the thermometer reading suggests.

Calculate the heat index (feels-like temperature) based on air temperature and relative humidity. Uses the official NWS Rothfusz regression equation. Supports Celsius and Fahrenheit. Shows heat category (Caution to Extreme Danger) and health risk guidance.

Units

Quick Examples

Conditions

Temperature (deg C)

Heat index applies at 27 deg C (80 deg F) or above

Relative Humidity (%)

Heat index applies at 40% or above

Heat Index Reference

Feels Like	Category	Health Risk
Below 27 deg C / 80 deg F	Comfortable	Low
27-32 deg C / 80-90 deg F	Caution	Fatigue possible with prolonged activity
32-39 deg C / 90-103 deg F	Extreme Caution	Heat cramps, heat exhaustion possible
39-51 deg C / 103-124 deg F	Danger	Heat exhaustion likely, heatstroke possible
Above 51 deg C / 124 deg F	Extreme Danger	Heatstroke highly likely

Formula: NWS Rothfusz regression equation. Valid for temperature at or above 80 deg F (27 deg C) and relative humidity at or above 40%. Pair with the Wind Chill Calculator for cold-weather conditions.

How to Use

1
Select Celsius or Fahrenheit
Choose your preferred temperature unit. The calculator automatically converts the heat index output to match your selection.
2
Enter the air temperature
Type the actual air temperature. Heat index only applies at or above 27°C (80°F). Below this threshold, the calculator will note there is no significant heat index effect.
3
Enter the relative humidity
Type the relative humidity percentage (0–100%). Heat index applies at 40% or higher. Use a quick example preset to see realistic summer conditions.
4
Read the feels-like temperature and risk level
The heat index (feels-like temperature) appears with a color-coded category: Comfortable, Caution, Extreme Caution, Danger, or Extreme Danger — along with health guidance for that level.

Frequently Asked Questions

What is the heat index and how is it calculated?: The heat index (also called "feels like" or apparent temperature) measures how hot it feels when humidity is factored in. High humidity reduces the body's ability to cool itself through sweat evaporation, making it feel hotter than the actual temperature. The NWS Rothfusz regression equation is used: HI = -42.379 + 2.04901523*T + 10.14333127*RH - 0.22475541*T*RH - ... (8 terms). Valid for temperature at or above 80°F (27°C) and humidity at or above 40%.
At what heat index is it dangerous to be outside?: Heat index danger levels by category: Caution (80–90°F / 27–32°C feels like): fatigue possible with prolonged activity. Extreme Caution (90–103°F / 32–39°C): heat cramps and heat exhaustion possible. Danger (103–124°F / 39–51°C): heat exhaustion likely, heatstroke possible — limit outdoor time. Extreme Danger (above 124°F / 51°C): heatstroke highly likely — avoid outdoors entirely. Older adults, children, and people with chronic illness face higher risk at lower thresholds.
What is the difference between heat index and feels like temperature?: "Feels like" (apparent temperature) is the general term; heat index is the hot-weather version of it. In cold weather, "feels like" is wind chill — it accounts for wind removing body heat. In hot weather, "feels like" is the heat index — it accounts for humidity preventing sweat evaporation. Weather apps often show a single "feels like" number that uses wind chill below 50°F (10°C) and heat index above 80°F (27°C), blending both formulas.
Why does humidity make hot weather feel hotter?: Your body cools itself by sweating. When sweat evaporates off skin, it carries heat away. High humidity means the air is already saturated with water vapor, so sweat evaporates more slowly — sometimes barely at all. With less evaporative cooling, your body retains more heat, your core temperature rises faster, and exertion becomes more dangerous. At 40°C (104°F) with 60% humidity, the heat index can exceed 50°C (122°F) because virtually no evaporative cooling is occurring.
What is the Rothfusz regression equation used for heat index?: The Rothfusz regression equation is the formula used by the US National Weather Service (NWS) to calculate heat index. It is a multi-variable polynomial that best fits human physiological heat stress data. It requires temperature at or above 80°F and relative humidity at or above 40% to give accurate results. The formula has 8 terms involving temperature (T) and relative humidity (RH) squared and cross-multiplied. It replaced simpler formulas in official NWS heat advisory calculations.
How should I protect myself in high heat index conditions?: In Extreme Caution or Danger heat index conditions: stay in air-conditioned spaces during peak heat (noon–4pm); drink 1 cup of water every 15–20 minutes during outdoor activity; wear light-colored, loose-fitting, breathable clothing; never leave children or pets in parked cars; check on elderly neighbors; reschedule outdoor workouts to early morning or evening. Signs of heat exhaustion include heavy sweating, weakness, cold/pale skin, and nausea. Heatstroke (hot/dry skin, confusion, rapid pulse) is a medical emergency — call 911.