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Heat Index Calculator: Feels-Like Temperature & Risk Levels

Calculate heat index (feels-like temperature) from temperature and humidity. NWS algorithm. Risk levels: caution, extreme caution, danger.

By Jeff Beem

Updated

01

Settings

02

Air temperature

03

Moisture input

%

Adds 15°F (8.3°C) to the heat index when checked.

Results

Caution

Fatigue possible with prolonged exposure.

Feels like

86.3 °F

30.2 °C

Note: The Steadman heat index algorithm is designed for ambient air temperatures between 80°F and 120°F. Results for extreme temperatures are theoretical approximations.

Information hub

Heat index is the perceived temperature or apparent temperature: how hot it actually feels when you step outside. Also called humiture, it combines air temperature and relative humidity into a single “feels like” value, similar to how wind chill estimates perceived cold from wind and temperature.

On a humid day, the heat index is usually higher than the thermometer reading. For example, 83°F (28°C) with 70% humidity can feel like 88°F (31°C) or more. That gap reflects how moisture in the air changes the way your body cools itself.

Your body mainly cools by sweating; when sweat evaporates, it pulls heat away from your skin. High humidity slows that evaporation, so the same air temperature feels hotter. The heat index quantifies that effect so you can plan outdoor activities, assess heat stress risk, and understand heat advisories.

Heat index is most commonly used for outdoor conditions; the NWS formula assumes light wind and shade. Some factors, such as fitness, hydration, and sun exposure, also affect how hot it feels, but the heat index focuses on the core relationship between temperature and humidity.

Heat Index & Why Humidity Matters

Ever notice 90°F in Arizona feels different from 90°F in Florida? Humidity is the reason. Heat index combines temperature and moisture into a single "feels like" value so you can plan outdoor activities, assess heat stress, and understand NWS heat advisories.

What You Need to Know

Humidity Makes It Feel Hotter

Sweat cools you by evaporating. Humid air is already saturated, so sweat stays on your skin. The same temperature feels much hotter when humidity is high.

Two Ways to Enter Moisture

Use Relative Humidity (%) or Dew Point. Both sync °C and °F. If you have a hygrometer that reads dew point, switch modes, the calculator derives RH for you.

When Heat Index Matches Air Temp

Below 80°F or 40% RH, the model assumes normal evaporative cooling. The result equals the air temperature, no adjustment applied.

Direct Sun Adds 15°F

The NWS recommends adding 15°F (8.3°C) when in full sun. Check the box if you're outdoors in direct sunlight, it shifts the feels-like value accordingly.

Heat Index Calculator: Feels-Like Temperature & Risk Levels

Find how hot it really feels from temperature and humidity or dew point. NWS Rothfusz formula. Risk levels from Caution to Extreme Danger. Direct sunlight option. Free, runs locally.

What Is Heat Index?

Heat index (also called "apparent temperature" or "feels-like temperature") tells you how hot it feels when humidity is factored in. On a muggy day, your body can't cool as efficiently because sweat doesn't evaporate well. The National Weather Service uses heat index to issue heat advisories and warn when conditions become dangerous for outdoor work, exercise, or prolonged exposure. This calculator follows the NWS Rothfusz regression, the same equation used by NOAA and weather.gov.

The NWS Rothfusz Formula

The base equation is HI=−42.379+2.04901523T+10.14333127RH−0.22475541 TRH−0.00683783 T2−0.05481717 RH2+…HI = -42.379 + 2.04901523T + 10.14333127RH - 0.22475541\,TRH - 0.00683783\,T^2 - 0.05481717\,RH^2 + \ldots where TT is air temperature (°F) and RHRH is relative humidity (%). Additional terms and two adjustments, one for low humidity (RH<13%RH < 13\%, temps 80–112°F) and one for high humidity (RH>85%RH > 85\%, temps 80–87°F), complete the Rothfusz regression. Valid for T≥80∘FT \geq 80^\circ\text{F} and RH≥40%RH \geq 40\%; otherwise heat index equals air temperature.

How to Use This Calculator

Start by entering the current air temperature in Fahrenheit or Celsius—the calculator converts between units automatically. Next, choose your moisture input method: Relative Humidity (as a percentage) or Dew Point (if your sensor reports dew-point temperature instead). Enter the value for your chosen method. If you are outdoors in direct sunlight, toggle the sun checkbox; the NWS recommends adding roughly 15°F (8.3°C) to the computed heat index under full-sun conditions. The feels-like temperature and color-coded NWS risk level update instantly as you type.
  • Air Temperature:
    Type the reading from your thermometer or weather app. Both °F and °C are accepted; the tool syncs both.
  • Moisture Input:
    Select Relative Humidity and enter a percentage, or switch to Dew Point and enter the dew-point temperature.
  • Direct Sun Toggle:
    Check this when standing in direct sunlight. It adds approximately 15°F to the calculated heat index per NWS guidance.
  • Result Panel:
    Displays the feels-like temperature in both °F and °C alongside the NWS risk band (Caution, Extreme Caution, Danger, or Extreme Danger).

Risk Levels

  • Caution (80–90°F / 27–32°C)
    Yellow. Fatigue possible with prolonged exposure. Sensitive groups should take breaks.
  • Extreme Caution (91–103°F / 33–39°C)
    Orange. Heat cramps and exhaustion likely. Increase rest and hydration.
  • Danger (104–124°F / 40–51°C)
    Red. Heatstroke possible. Outdoor work and strenuous activity require frequent breaks.
  • Extreme Danger (≥125°F / 52°C)
    Dark red. Heatstroke highly likely. Reschedule outdoor activities when possible.

Heat Index Calculator FAQ

What is heat index?

Heat index is the "feels-like" temperature, how hot it actually feels when humidity is factored in. Your body cools by sweating; high humidity slows evaporation, so 90°F at 80% humidity feels much worse than 90°F at 30%. The NWS Rothfusz equation applies when air temp T≥80∘FT \geq 80^\circ\text{F} and relative humidity RH≥40%\text{RH} \geq 40\%. Below that, heat index equals air temperature. In direct sun, add about 15∘F15^\circ\text{F} (8.3°C).

Can I enter dew point instead of humidity?

Yes. Switch to "Dew Point" under Moisture Input, enter air temperature and dew point in °C or °F, and the tool derives relative humidity before computing heat index. Handy if you have a hygrometer that reports dew point.

What do the risk levels mean?

Caution (80–90°F): fatigue with long exposure. Extreme Caution (91–103°F): cramps and exhaustion possible. Danger (104–124°F): heatstroke risk. Extreme Danger (≥125°F): heatstroke very likely. These align with NWS heat advisory guidelines.

Why does heat index equal air temperature sometimes?

The Rothfusz formula only applies when temp is at least 80°F and humidity at least 40%. Outside that range, cooler or drier air, the model assumes evaporative cooling works normally, so the "feels like" value matches the thermometer.

When is it dangerous to work outside?

When heat index reaches Danger (104°F+) or Extreme Danger (125°F+), outdoor work becomes risky. Take extra breaks, drink water, and watch for heat exhaustion. Employers and OSHA use heat index to guide work-rest schedules in hot weather.

Mathematical Reference Note

Calculation Logic: This tool uses standard mathematical algorithms. While we strive for accuracy, errors in logic or user input can result in incorrect data.

Verification: Results should be cross-checked if used for important academic, professional, or personal calculations.

Standard Terms: This tool is provided free of charge and as-is. CalcRegistry provides no warranty regarding the accuracy or fitness of these results for your specific needs.

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