Calculate the heat index (feels-like temperature) from air temperature and relative humidity. Assess heat danger levels and get safety recommendations.
The Heat Index Calculator computes the "feels-like" temperature from air temperature and relative humidity using the NOAA/NWS regression equation. It helps translate weather numbers into practical heat risk, since humidity limits how well sweat can evaporate and cool the body. That makes it more useful than air temperature alone when planning outdoor activity. The result is a better shorthand for comparing two hot days that do not feel the same.
At 90°F with 40% humidity, the heat index is only a little above the air temperature, but at 90°F with 80% humidity it can jump into a dangerous range. That difference matters for outdoor work, sports, events, and any situation where people may be exposed to heat for long periods.
Enter the air temperature and relative humidity to calculate the heat index, danger level, and recommended precautions. The calculator also shows NOAA-style risk categories so you can quickly decide whether conditions call for extra caution, schedule changes, or moving activity indoors.
Use this calculator to turn weather conditions into a practical heat-risk estimate. It helps you plan outdoor activities safely, protect workers and athletes from heat-related illness, and decide when conditions call for shade, water, or a schedule change. It is also a quick check when you need to compare two hot days that feel very different because of humidity.
Heat Index (Rothfusz regression): HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T² - 0.05481717R² + 0.00122874T²R + 0.00085282TR² - 0.00000199T²R². Where T = air temperature (°F), R = relative humidity (%). Adjustments apply for low and high humidity extremes.
Result: 119°F Heat Index — Danger
At 95°F and 65% relative humidity, the Rothfusz regression gives a heat index of approximately 119°F. This falls in the "Danger" category: heat cramps and heat exhaustion are likely; heatstroke is possible with prolonged exposure or physical activity. Limit outdoor exertion and hydrate frequently.
The NWS heat index uses the Rothfusz (1990) multiple regression equation with nine terms. It is valid for temperatures above 80°F and relative humidity above 40%. Outside this range, adjustments are applied: for low humidity at high temperatures, a correction reduces the heat index; for very high humidity at moderate temperatures, a correction increases it. The equation was derived from Steadman's 1979 table of apparent temperatures.
OSHA recommends using the heat index for workplace heat hazard assessment. At 91°F+ heat index, employers should provide water, rest, and shade. The ACGIH TLV (Threshold Limit Values) for heat stress uses WBGT (Wet Bulb Globe Temperature) to set work/rest ratios: light work up to 86°F WBGT, moderate work up to 82°F WBGT, heavy work up to 79°F WBGT. Workers should be monitored for heat illness symptoms.
Global warming is increasing both the frequency and intensity of extreme heat events. Heat index values that were once rare "record" events are becoming more common. Many climate models project wet bulb temperatures exceeding 95°F in parts of South Asia, the Persian Gulf, and the US Gulf Coast by 2070-2100. This represents a fundamental limit to outdoor human activity and agricultural labor.
The heat index (also called apparent temperature) is what the temperature "feels like" when humidity is factored in. High humidity prevents sweat from evaporating efficiently, reducing your body's primary cooling mechanism. The NWS heat index was developed from extensive research on human physiology and heat transfer.
NOAA danger levels: 80-90°F Caution (fatigue possible). 90-103°F Extreme Caution (heat cramps/exhaustion possible). 103-124°F Danger (heat cramps/exhaustion likely, heatstroke possible). 125°F+ Extreme Danger (heatstroke highly likely). Vulnerable populations face risks at lower thresholds.
The heat index is most useful above 80°F. Below 80°F, humidity has minimal effect on perceived temperature. The formula assumes shade and light wind — direct sun can add up to 15°F to the heat index, and strong winds may reduce it slightly.
Wet bulb temperature (WBT) is the lowest temperature achievable through evaporative cooling alone. It's always equal to or lower than the dry bulb temperature. Above ~95°F WBT, the human body cannot cool itself and heatstroke is inevitable. Climate change is pushing more regions toward this limit.
The standard heat index formula assumes shaded conditions. Direct sunlight can add 10-15°F to the effective heat index. On a 95°F day with 50% humidity in full sun, the effective heat index may be 120-130°F instead of the calculated 107°F in shade.
The elderly, young children, people with chronic illnesses, outdoor workers, athletes, and people on certain medications (diuretics, beta blockers, anticholinergics) face elevated risk. These groups should take precautions at lower heat index thresholds.