Heat Index Calculator

About the Heat Index Calculator

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.

When This Page Helps

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.

How to Use the Inputs

1. Enter the current air temperature in °F (or toggle to °C).
2. Enter the relative humidity as a percentage.
3. View the calculated heat index and danger level.
4. Check the color-coded danger assessment and safety recommendations.
5. Review the heat index chart for surrounding conditions.
6. Use presets for common weather scenarios.
7. The wet bulb globe temperature estimate is also provided for occupational exposure limits.

Example Calculation

Tips & Best Practices

• Add 10-15°F to the heat index when in direct sunlight.
• Schedule strenuous outdoor activities before 10 AM or after 6 PM on high heat index days.
• Hydrate before you feel thirsty — thirst is a late indicator of dehydration.
• Dark-colored clothing absorbs more heat; wear light-colored, loose-fitting clothing.
• Acclimate over 7-14 days when starting outdoor work in heat — gradually increase exposure.
• Recognize heat exhaustion signs: heavy sweating, weakness, nausea, dizziness. Move to shade and hydrate immediately.

The Rothfusz Heat Index Equation

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.

Occupational Heat Safety Standards

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.

Climate Change and Extreme Heat

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.

Frequently Asked Questions

• What is the heat index?

  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.

• At what heat index is it dangerous?

  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.

• When does the heat index apply?

  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.

• What is wet bulb temperature?

  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.

• How does direct sunlight affect the heat index?

  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.

• Who is most vulnerable to high heat index?

  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.