Heat Index Calculator
Reviewed by Hamza Gujjar, Algorithm Architect & Software Developer
Last updated: August 19, 2026Why Humidity Makes Heat More Dangerous
The body cools itself mainly through sweat evaporation, and evaporation slows dramatically in humid air because the surrounding air is already close to saturated with moisture. Heat index — the "feels like" temperature the National Weather Service issues advisories around — captures that compounding effect, and at high enough readings it becomes a genuine safety threshold rather than just a discomfort figure.
The Formula
T is air temperature in °F and R is relative humidity in percent. This is the NOAA/NWS Rothfusz regression, the standard formula used for official heat index reporting. It only applies when a simpler Steadman approximation, averaged with air temperature, indicates a result of 80°F or higher — below that the simpler estimate is used directly. Additional adjustment terms apply at low humidity (RH below 13% with T between 80–112°F) and high humidity (RH above 85% with T between 80–87°F).
Worked Examples
| Air temperature | Relative humidity | Heat index |
|---|---|---|
| 85°F | 40% | 84.33°F |
| 90°F | 70% | 105.92°F |
| 95°F | 60% | 113.09°F |
NWS Heat Index Danger Categories
| Heat index range | Category | Risk |
|---|---|---|
| 80–90°F | Caution | Fatigue possible with prolonged exposure or activity |
| 91–103°F | Extreme Caution | Heat cramps and heat exhaustion possible |
| 104–124°F | Danger | Heat cramps and heat exhaustion likely; heatstroke possible |
| 125°F+ | Extreme Danger | Heatstroke highly likely |
Where This Matters
- Outdoor labor and athletics — employers and coaches use heat index thresholds to schedule work breaks, hydration protocols, or cancel outdoor activity entirely.
- Vulnerable populations — young children, older adults, and people with certain health conditions face elevated heat-related illness risk at heat index levels that healthy adults tolerate.
- Event and travel planning — checking heat index rather than raw temperature gives a more accurate picture of what a day will actually feel like outdoors.
How to Use This Calculator
- Enter the Air Temperature in °F.
- Enter the Relative Humidity as a percentage (0–100).
- Select Calculate to see the heat index.
Related Calculations
In cold conditions, use the Wind Chill Calculator instead, or check the underlying humidity reading with the Humidity Calculator.
Principles of Human Biometeorology and Environmental Heat Index Modeling
A heat index calculator computes the apparent "feels-like" equivalent temperature experienced by human tissue resulting from combined high ambient air temperature and high relative humidity. In environmental medicine, occupational OSHA safety, and athletics, high humidity prevents sweat from evaporating off human skin, disabling the body's primary thermoregulatory cooling mechanism and accelerating heat exhaustion, heat cramps, and fatal heat stroke.
The National Weather Service (NWS) Rothfusz Heat Index Formula
The National Oceanic and Atmospheric Administration (NOAA) computes heat index (HI in °F) for air temperatures T ≥ 80.0°F and relative humidity RH ≥ 40.0% using the Rothfusz Multi-Variable Polynomial Regression Equation:
NOAA NWS Heat Danger Classification Categories
| Heat Index Range | Danger Classification | Clinical Physiological Health Disorders |
|---|---|---|
| 80°F to 90°F (27°C to 32°C) | Caution | Fatigue possible with prolonged exposure and physical activity |
| 91°F to 103°F (33°C to 39°C) | Extreme Caution | Heat cramps and heat exhaustion possible; mandatory hydration breaks |
| 104°F to 124°F (40°C to 51°C) | Danger | Heat cramps and heat exhaustion likely; Heat stroke possible |
| 125°F or Higher (≥ 52°C) | Extreme Danger | Heat stroke highly imminent with rapid fatal organ failure |
Step-by-Step Worked Calculation Example
Example: Evaluating Summer Occupational Heat Danger
Problem: An outdoor roofing crew works in ambient air temperature T = 90.0°F with relative humidity RH = 70.0%. Calculate the effective Heat Index (HI) and determine clinical OSHA safety precautions.
Step 1: Substitute T = 90 and RH = 70 into the Rothfusz equation:
Linear Terms = -42.379 + (2.049015 × 90) + (10.14333 × 70) = -42.379 + 184.411 + 710.033 = 852.065
Cross Term = -0.224755 × 90 × 70 = -1,415.957
Squared Terms = -(0.0068378 × 8,100) - (0.054817 × 4,900) = -55.386 - 268.603 = -323.989
Cubic Terms = (0.0012287 × 8,100 × 70) + (0.0008528 × 90 × 4,900) - (0.00000199 × 8,100 × 4,900)
Cubic Terms = 696.673 + 376.085 - 78.983 = 993.775
Heat Index HI = 852.065 - 1,415.957 - 323.989 + 993.775 = 105.9°F (approx. 106°F / 41.1°C)
Conclusion: Although the thermometer reads 90°F, humidity makes it feel like an extreme 106°F (Danger category), requiring mandatory 15-minute shaded rest breaks every hour.
Wet-Bulb Globe Temperature (WBGT) in Athletics and Military Training
While the standard NWS Heat Index is calibrated for shaded indoor/outdoor conditions with light wind, the military (US Armed Forces), NCAA athletics, and FIFA soccer mandate monitoring the Wet-Bulb Globe Temperature (WBGT) for outdoor activities in direct sunlight:
- Twet-bulb (70% Weight): Measures natural evaporative cooling capacity.
- Tblack-globe (20% Weight): Measures radiant solar thermal energy absorbed inside a 6-inch copper sphere.
- Tdry-bulb (10% Weight): Standard ambient air temperature.
Clinical Pathophysiology of Exertional Heat Stroke
When core body temperature exceeds 104.0°F (40.0°C), systemic inflammatory response cascades trigger cellular protein denaturation, rhabdomyolysis muscle tissue breakdown, disseminated intravascular coagulation (DIC), and acute cerebral edema, requiring immediate whole-body cold-water immersion therapy.
Electrolyte Depletion and Heat Cramps
During prolonged physical exertion in high heat index environments, human sweat rates can exceed 1.5 to 2.5 Liters per hour.
Sweat contains essential dissolved mineral electrolytes (approx. 900 to 1,200 mg sodium and 200 mg potassium per Liter). Drinking massive volumes of plain water without electrolyte replacement causes Exercise-Associated Hyponatremia (water intoxication), triggering severe skeletal muscle spasms (Heat Cramps) and neurological confusion.
Urban Heat Island (UHI) Microclimate Amplification
In dense metropolitan city centers, dark asphalt roadways and concrete structural facades absorb and re-radiate intense solar thermal energy, creating an Urban Heat Island Effect.
Combined with localized vehicle exhaust humidity, urban downtown temperatures average 5°F to 10°F hotter with Heat Index values 12°F to 15°F higher than surrounding rural vegetated outskirts.
ACSM Fluid Replacement Guidelines for Athletes
The American College of Sports Medicine (ACSM) recommends drinking 500 mL (17 fl oz) of fluid 2 hours before exercise and consuming 200 to 300 mL every 15 to 20 minutes during physical exertion in hot, humid environments to maintain cardiac stroke volume.
Heat Acclimatization Physiological Adaptations
Undergoing 10 to 14 days of gradual heat acclimatization stimulates earlier onset of sweating at lower body core temperatures, increases plasma blood volume by 10% to 15%, and lowers sweat sodium salt concentrations, boosting human heat tolerance.
Industrial Fan Air Velocity Cooling Limits
When ambient air temperature exceeds human skin temperature (95.0°F / 35.0°C), electric fans blowing hot dry air increase convective heat transfer into the human body, worsening thermal stress and increasing dehydration risk without room air conditioning.