The Heat Index tells you how hot it actually feels when humidity is factored into the air temperature. When humidity is high, sweat evaporates more slowly, reducing the body's ability to cool itself. This calculator uses the exact NOAA National Weather Service (NWS) formula and lets you enter either Relative Humidity or Dew Point for maximum flexibility.

What Is the Heat Index?

The Heat Index (HI), also called the apparent temperature or humiture, is a measure that combines air temperature and relative humidity to estimate how hot the conditions feel to the human body. It is calculated for shaded areas with light wind and assumes a typical adult wearing light clothing.

Because the body's primary cooling mechanism is the evaporation of sweat, high humidity dramatically reduces cooling efficiency. At 90°F (32°C) with 40% relative humidity, the heat index is roughly 92°F (33°C). At the same temperature with 80% humidity, the heat index jumps to 113°F (45°C) — a difference that can turn a warm day into a life-threatening situation.

Why Humidity Matters

This table shows why humidity is often more dangerous than the raw temperature. A "moderate" 90°F day can feel like a deadly 113°F when the air is saturated with moisture.

Heat Index Danger Levels

The National Weather Service divides the Heat Index into four categories. These levels help individuals, employers, and sports coaches decide when to limit outdoor activity.

The Official NOAA/NWS Heat Index Formula

This calculator implements the Rothfusz regression equation, the same formula used by the National Weather Service. It is far more accurate than simple linear approximations found on many websites.

Base Regression Equation

HI = -42.379 + 2.04901523T + 10.14333127RH − 0.22475541TRH − 0.00683783T² − 0.05481717RH² + 0.00122874T²RH + 0.00085282TRH² − 0.00000199T²RH²

Where:

• T = Air temperature in °F
• RH = Relative humidity as a percentage (e.g., 65 for 65%)

Special Adjustments

The NWS applies three corrections to improve accuracy:

1. Steadman Simple Formula Below 80°F: For temperatures between 70°F and 80°F, the NWS averages the air temperature with the Steadman simple-formula value: 0.5 × (T + 61.0 + [(T − 68.0) × 1.2] + (RH × 0.094)). Below 70°F, the heat index is set equal to the air temperature.
2. Low Humidity Correction: If RH < 13% and 80°F ≤ T ≤ 112°F, subtract [(13 − RH) / 4] × √[(17 − |T − 95|) / 17].
3. High Humidity Correction: If RH > 85% and 80°F ≤ T ≤ 87°F, add [(RH − 85) / 10] × [(87 − T) / 5].

These adjustments ensure accuracy across the wide range of conditions where the Heat Index is meaningful.

How to Use the Heat Index Calculator

Step 1: Enter the air temperature. Choose Fahrenheit or Celsius. The calculator will convert internally as needed.

Step 2: Choose your humidity input. You can enter either:

• Relative Humidity (%): The percentage of moisture in the air relative to saturation.
• Dew Point (°F or °C): The temperature at which air becomes saturated.

Step 3: Click Calculate. The calculator returns:

• Heat Index in °F and °C
• Heat risk category
• Safety recommendations
• Optional heat index chart position

Why Offer Dew Point Input?

Dew point is an absolute measure of moisture. Unlike relative humidity, which changes as the air warms or cools, dew point stays constant throughout the day unless the air mass changes. Many meteorologists prefer dew point for this reason.

If you enter dew point, the calculator first converts it to relative humidity using the August-Roche-Magnus approximation:

RH = 100 × [exp((17.625 × Tdp) / (243.04 + Tdp)) / exp((17.625 × T) / (243.04 + T))]

(Temperatures in °C in this formula)

Heat Index Reference Chart

Use this chart to quickly estimate the Heat Index for common temperature and humidity combinations.

Values are approximate and based on the NOAA/NWS formula. Direct sunlight can increase the effective heat index by up to 15°F.

Real-World Worked Examples

Example 1: Hot and Humid Day in Miami

• Air temperature: 92°F
• Relative humidity: 75%
• Heat Index: 119°F (48°C)
• Risk: Extreme Danger
• Advice: Limit outdoor activity, drink water frequently, seek shade or air conditioning.

Example 2: Dry Heat in Phoenix

• Air temperature: 105°F
• Relative humidity: 15%
• Heat Index: 103°F (39°C) after low-humidity adjustment
• Risk: Danger
• Advice: Even with low humidity, extreme heat can cause heat illness. Hydrate and rest.

Example 3: Moderate Summer Day in London

• Air temperature: 82°F
• Relative humidity: 55%
• Heat Index: 84°F (29°C)
• Risk: Caution
• Advice: Comfortable for most people, but prolonged exertion may cause fatigue.

Heat Index Limitations

The Heat Index is a useful public health tool, but it has limitations:

• Shade assumption: It assumes the person is in the shade. Direct sun can raise the effective value by up to 15°F.
• Light wind: It assumes calm to light winds. Stronger winds can increase evaporative cooling.
• Individual variation: Age, fitness, hydration, medications, and acclimatization affect heat tolerance.
• Not for indoor use: It is designed for outdoor, shaded conditions.

For a more comprehensive occupational assessment that includes sun, wind, and workload, use our Heat Stress (WBGT) Calculator.

Heat Index vs. Heat Stress (WBGT)

While the Heat Index is excellent for public weather warnings, Wet-Bulb Globe Temperature (WBGT) is the gold standard for occupational safety. WBGT incorporates:

• Air temperature
• Relative humidity
• Solar radiation (globe temperature)
• Wind speed

Employers, sports teams, and military organizations typically rely on WBGT for work/rest decisions. Our Heat Stress Calculator estimates WBGT from standard weather data and provides OSHA/NIOSH-aligned schedules.