The air temperature says 90°F, but your body tells a different story. When humidity is high, sweat evaporates slowly, and the air feels much hotter than the thermometer reads. That perceived temperature is the heat index.
Use the calculator above to enter your current air temperature and relative humidity. The tool returns an estimated "feels like" temperature in seconds, so you can decide whether outdoor plans are safe or need adjusting.
The heat index is sometimes called the apparent temperature. It was originally devised for shady, light wind conditions using assumptions about the human body, clothing, and natural cooling. Think of it as a quick gauge of how much heat stress your body may actually face.
Calculate the Heat Index
Enter two values into the calculator:
- Air temperature in degrees Fahrenheit (or Celsius).
- Relative humidity as a percentage.
Click calculate. The result is your estimated heat index temperature.
That number tells you what the combination of heat and moisture feels like to a person standing in the shade with a light wind. If the heat index is noticeably higher than the actual temperature, your body will struggle more to cool itself.
A few quick reference points:
- 90°F air temperature with 70% relative humidity produces a heat index near 106°F.
- A heat index of 105°F or higher is considered dangerous for prolonged exposure.
- A heat index of 110°F or above indicates extreme danger, where heat stroke can develop quickly.
This calculator is an estimate. It is a planning aid, not a medical assessment. If you feel symptoms of heat illness, seek shade, hydration, and medical help.
How the Heat Index Calculator Uses Temperature and Relative Humidity
The heat index calculation is based on a regression equation developed by Robert Steadman and later adapted by the National Weather Service. The formula takes into account how humidity reduces the rate of evaporation of sweat from your skin.
Here is the core idea: your body cools itself through perspiration. When sweat evaporates, it pulls heat away from your skin. High humidity slows that evaporation, so your body retains more heat. The heat index formula translates that effect into a single "feels like" number.
The NWS heat index equation uses a polynomial regression with multiple coefficients applied to air temperature and relative humidity. The math is complex, but the inputs are simple: just temperature and RH. The calculator handles the rest.
What the formula assumes:
- You are in a shaded area.
- There is a light wind (about 6 mph).
- Your body type and clothing are average.
These assumptions matter. Exposure to full sunshine can increase heat index values by up to 15°F. Wind speed, clothing, and physical activity also shift the real risk, but the standard formula does not account for them.
Heat Index Chart and Heat Index Values
A heat index chart is a quick lookup table. You find your air temperature on one axis and your relative humidity on the other. Where they intersect is your heat index value.
The NWS groups heat index values into four risk categories:
| Heat Index Range | Risk Level | Possible Effects |
|---|---|---|
| 80°F to 90°F | Caution | Fatigue possible with prolonged exposure |
| 91°F to 103°F | Extreme Caution | Heat cramps and heat exhaustion possible |
| 104°F to 124°F | Danger | Heat cramps, heat exhaustion, and heat stroke likely |
| 125°F and above | Extreme Danger | Heat stroke highly likely |
Heat index values were devised for shady, light wind conditions. Direct sun, intense physical activity, or poor hydration can push your actual risk higher than the chart suggests.
An 80°F heat index on its own is generally comfortable for most people. Once the index climbs past 90°F, take precautions. Above 104°F, continuing activity could result in heat stroke without rest, shade, and fluids.
Relative Humidity vs. Dew Point Temperature
Both relative humidity and dew point temperature describe moisture in the air, but they do it differently. Understanding the difference helps you choose the right input and interpret results more accurately.
Relative humidity is a percentage. It tells you how close the air is to being fully saturated at its current temperature. An RH of 70% means the air holds 70% of the moisture it could hold at that temperature.
Dew point temperature is a fixed value in degrees. It indicates the temperature at which water vapor in the air would start to condense. A higher dew point means more moisture, regardless of the current air temperature.
What Is Dew Point and How Dew Affects the Index
Dew point is often considered a more reliable measure of how muggy it feels. Why? Because relative humidity changes as temperature changes throughout the day, even if the actual moisture in the air stays the same.
A dew point of 60°F feels comfortable. At 70°F, the air feels sticky. Above 75°F, conditions become oppressive and significantly limit your body's ability to cool through evaporation.
Some extended heat index models accept dew point as an input and convert it internally. If your weather station reports dew point instead of RH, you can convert between the two. The calculator on this page uses relative humidity directly.
Globe Temperature and Other Meteorological Inputs
Globe temperature is part of the Wet Bulb Globe Temperature (WBGT) system. A WBGT reading uses three measurements: a standard air temperature, a wet bulb temperature (which captures humidity and evaporation), and a black globe temperature (which captures solar radiation and wind).
WBGT is used in military, athletic, and occupational settings because it factors in sunlight, wind speed, and radiant heat. The standard heat index does not include those variables. That is an important distinction.
The heat index is simpler and more accessible. WBGT is more comprehensive for outdoor work and sports in direct sun. If you need a planning tool for everyday weather decisions, the heat index calculator is the right starting point.
Heat Index During a Heat Wave
A heat wave amplifies every risk the heat index measures. During a heatwave, high temperatures persist overnight, which means your body never fully recovers from daytime heat stress.
Multiple consecutive days of extreme heat push the danger beyond what a single day's heat index reading suggests. Buildings, pavement, and soil absorb and re-radiate heat, raising the ambient temperature in urban areas even further.
Check the heat index every morning and afternoon during a heat wave. Conditions can shift quickly, and a 10°F jump in humidity can push the perceived temperature from "caution" to "danger" in an hour.
Extreme Heat, Heat Stroke, and Heat Illness Risk
Heat illness exists on a spectrum. It starts with heat cramps (muscle cramps from dehydration and salt loss), moves to heat exhaustion (heavy sweating, weakness, nausea), and can escalate to heat stroke.
Heat stroke is a medical emergency. Your body's core temperature rises above 104°F, and sweating may stop entirely. Confusion, loss of consciousness, and organ damage can follow within minutes.
Risk factors that increase heat illness beyond what the heat index predicts:
- Prolonged physical activity outdoors
- Dehydration or poor hydration habits
- Age (very young children and adults over 65)
- Certain medications that affect sweating
- Lack of acclimation to high heat
- Direct sun exposure
A heat index of 121°F or above indicates extreme danger for nearly everyone, even at rest in the shade. Take the number seriously.
How Perspiration and Evaporation Cool Your Body
Your body's primary natural cooling mechanism is perspiration. Sweat glands release moisture onto the skin. As that moisture evaporates, it absorbs heat energy and lowers your skin temperature.
This process depends on a gap between the moisture on your skin and the moisture in the surrounding air. In dry air, evaporation is fast and cooling is efficient. When relative humidity is high, the air is already holding a lot of moisture, and evaporation slows dramatically.
That is why 95°F with 40% humidity can feel manageable, while 88°F with 85% humidity feels suffocating. The heat index captures this relationship. When the calculator returns a number well above the actual temperature, it means your body's cooling system is being compromised by the moisture in the air.
National Weather Service and NOAA Heat Index Guidelines
The heat index used in the United States comes from work published in the Journal of Applied Meteorology by Robert Steadman in 1979, later refined by the NWS into the regression formula most calculators use today.
NOAA and the National Weather Service issue heat advisories and excessive heat warnings based on heat index thresholds. These thresholds vary by region because populations in different climates are acclimated to different baselines.
General NWS guidelines:
- Heat Advisory: Heat index of 105°F to 109°F for at least 2 hours, or nighttime lows above 75°F.
- Excessive Heat Warning: Heat index of 110°F or higher for at least 2 hours.
These thresholds are region-specific. A heat index of 100°F may trigger warnings in the Pacific Northwest while being a normal summer afternoon in the Gulf Coast states.
The NWS recommends checking the heat index before outdoor work, exercise, or events. Their guidelines emphasize shade, hydration, frequent breaks, and awareness of heat illness symptoms. This calculator provides an estimate based on the NWS formula, but it does not replace official forecasts or medical guidance.
Air Temperature, Wind Speed, and Sunlight: What the Calculator Does Not Cover
The standard heat index formula assumes shade and a light wind. Real conditions rarely match those assumptions exactly. Here is what the calculator does not factor in:
- Sunshine: Exposure to full sunshine can increase the felt air temperature by up to 15°F beyond the calculated heat index. If you are in direct sun, add roughly 10°F to 15°F to your result for a more realistic estimate.
- Wind speed: A strong breeze can improve evaporation and lower perceived temperature. Calm air does the opposite. The formula assumes roughly 6 mph.
- Surface type: Asphalt, concrete, and sand radiate stored heat. Standing on a dark parking lot feels hotter than standing on grass, even at the same air temperature.
- Physical activity: Exercise generates internal heat. The calculator estimates passive exposure, not active exertion.
For situations involving direct sun, heavy labor, or athletics, the WBGT system is more appropriate. For general planning (should I reschedule the cookout, walk the dog later, or check on an elderly neighbor), the heat index is a practical starting point.
Dew Point Temperature vs. Relative Humidity: Which to Use in the Calculator
This calculator accepts relative humidity. That is the standard input for the NWS heat index equation and the value most weather apps and home weather stations display.
If you only have the dew point temperature, you can convert it to relative humidity with a separate formula or lookup table. The relationship depends on the current air temperature, so you need both the dew point and the air temperature to find RH.
Quick comparison:
- Relative humidity is intuitive (0% to 100%) but changes as air temperature rises and falls throughout the day.
- Dew point stays constant as long as the actual moisture content does not change, making it a better measure of absolute mugginess.
For this calculator, use relative humidity. If your weather source reports dew point, convert first. Many weather apps display both values, so check your settings.
Heat Index vs. Wind Chill Index
The heat index and the wind chill index are opposite sides of the same idea. Both describe a perceived temperature that differs from the actual air temperature. One applies to summer, the other to winter.
| Factor | Heat Index | Wind Chill Index |
|---|---|---|
| Season | Warm months | Cold months |
| Key variable | Relative humidity | Wind speed |
| Effect | Makes it feel hotter | Makes it feel colder |
| Risk | Heat cramps, heat exhaustion, heat stroke | Frostbite, hypothermia |
| Assumption | Shade, light wind | No sun, open exposure |
The heat index tells you how much harder your body must work to cool itself when humidity is high. The wind chill index tells you how fast your body loses heat when wind strips warmth from exposed skin.
Both are estimates. Neither replaces paying attention to how you actually feel. Use the heat index calculator during warm weather to plan outdoor time, and check the wind chill before heading out in winter. Each tool helps you prevent a different category of weather-related illness.