Calculate the heat index (feels-like temperature) from air temperature and humidity. Get NWS health risk classification and outdoor activity safety guidelines.
The Heat Index Calculator computes the "feels-like" temperature by combining air temperature and relative humidity using the Rothfusz regression equation (the formula used by the National Weather Service). When humidity is high, sweat evaporates more slowly, reducing the body's ability to cool itself, making the effective temperature significantly higher than the measured air temperature.
At 90°F with 70% humidity, the heat index is 106°F — a Danger level where heat stroke is a real risk. The NWS classifies heat index into four risk levels: Caution (80–90°F), Extreme Caution (90–103°F), Danger (103–124°F), and Extreme Danger (125°F+).
This calculator is essential for planning outdoor exercise, work activities, and events during warm weather. It provides activity modification guidelines for each risk level to help prevent heat-related illness. Whether you are a beginner or experienced professional, this free online tool provides instant, reliable results without manual computation. By automating the calculation, you save time and reduce the risk of costly errors in your planning and decision-making process.
Heat-related illness kills more people annually in the U.S. than any other weather event. The heat index can exceed the actual temperature by 15–30°F in high humidity. Athletes exercising at a heat index above 105°F face serious risk of heat stroke, which can be fatal within minutes. This calculator makes the invisible risk of humidity visible.
Rothfusz Regression (NWS): HI = −4.2785eⁱ + 2.04901523T + 10.14333127R − 0.22475541TR − 6.8378e⁻³T² − 5.48172e⁻²R² + 1.2287e⁻³T²R + 8.5282e⁻⁴TR² − 1.99e⁻⁶T²R² where T = temperature (°F), R = relative humidity (%) Adjustments applied when: • RH < 13% and 80 < T < 112°F: subtract correction • RH > 85% and 80 < T < 87°F: add correction NWS Risk Levels: • Caution: 80–90°F HI • Extreme Caution: 90–103°F HI • Danger: 103–124°F HI • Extreme Danger: ≥125°F HI
Result: Heat Index: 117°F — Danger
At 92°F with 65% humidity, the Rothfusz equation yields a heat index of approximately 117°F. This falls in the "Danger" category where heat cramps, heat exhaustion, and heat stroke are likely with prolonged exposure or physical activity. Outdoor exercise should be avoided or significantly modified.
Heat illness progresses through stages: heat cramps (muscle spasms from salt loss), heat exhaustion (volume depletion with heavy sweating), and heat stroke (thermoregulatory failure). The transition from exhaustion to stroke can happen rapidly. Heat stroke is a medical emergency with mortality rates of 10–50% even with treatment. Prevention through monitoring heat index and modifying activity is far more effective than treatment.
Certain groups face elevated heat illness risk: children under 4 (inefficient sweating), adults over 65 (reduced thermoregulatory response), people with chronic conditions (diabetes, cardiovascular disease), those taking certain medications (diuretics, beta-blockers, anticholinergics), and individuals who are not heat-acclimatized. These groups should reduce their heat index thresholds by one category.
Cities can be 5–15°F warmer than surrounding rural areas due to heat absorption by concrete, asphalt, and buildings, plus waste heat from vehicles and air conditioning. This means the official temperature reported by weather stations may underestimate actual street-level conditions in cities, making real heat index even higher than calculated.
The heat index IS the "feels like" temperature in warm conditions. It combines temperature and humidity to estimate what the temperature feels like to the human body. In cold conditions, the equivalent metric is wind chill. Some weather services combine both into a single "feels like" value that uses heat index in summer and wind chill in winter.
The ACSM recommends extreme caution for exercise above 90°F heat index. At Danger level (103°F+), cancel or significantly modify outdoor workouts. At Extreme Danger (125°F+), no outdoor activity should be attempted. Even at Caution levels (80–90°F), extended exertion can cause heat illness in susceptible individuals.
The body cools itself primarily through sweat evaporation. When humidity is high, the air is already saturated with moisture, so sweat evaporates much more slowly. This reduces the body's cooling efficiency dramatically. At 100% humidity, sweat barely evaporates at all, leaving the body unable to regulate its temperature during exertion.
Heat exhaustion symptoms include heavy sweating, weakness, nausea, dizziness, and headache. Core temperature is typically below 104°F and the person is still sweating. Heat stroke is a medical emergency: core temperature exceeds 104°F, sweating often STOPS, and confusion or loss of consciousness occurs. Untreated heat stroke can be fatal. Call 911 immediately.
Yes, the heat index applies anywhere with warm, humid conditions. Poorly ventilated buildings, kitchens, laundry rooms, and parked cars can have extremely high heat indices. Air conditioning is the most reliable way to maintain safe indoor conditions. Fans help with evaporative cooling but are less effective when humidity exceeds 75%.
When relative humidity is below 13% and temperature is between 80–112°F (dry desert conditions), the NWS applies a correction that reduces the heat index. Low humidity enhances evaporative cooling, making high temperatures more bearable. However, dehydration risk increases dramatically in dry heat because sweat evaporates so quickly you may not notice fluid loss.