Heat Index Calculator

Calculate heat index from temperature and humidity. See danger level and activity recommendations.
°F
%

Heat Index

105°F

Danger

Actual Temp

95°F

Feels Like

105°F

Humidity

50%

Activity Recommendation

Heat cramps and heat exhaustion likely. Heatstroke possible. Avoid strenuous outdoor activities. Stay in air conditioning.

Danger Level Reference

Below 80°FComfortable
80-90°FCaution
91-103°FExtreme Caution
104-125°FDanger
126°F+Extreme Danger

Heat Index at Various Humidity Levels

20% humidity91°F
30% humidity94°F
40% humidity99°F
50% humidity105°F
60% humidity113°F
70% humidity123°F
80% humidity134°F
90% humidity147°F

Heat index is calculated using the Rothfusz regression equation from the National Weather Service. Values assume shady conditions; direct sunlight can increase heat index by up to 15\u00B0F. Always take precautions in high heat and humidity.

Use the Heat Index Calculator above to calculate your results. Enter your values and see instant results — all calculations run in your browser.

Disclaimer: This calculator is for informational purposes only and does not constitute tax, financial, or legal advice. Results are estimates based on the information you provide and current rates. Always consult a qualified tax professional or financial advisor for advice specific to your situation.

How It Works

Our Heat Index Calculator helps you determine the 'feels like' temperature based on actual air temperature and relative humidity. This is crucial for understanding the real risk of heat-related illnesses, which are projected to increase by 15% in 2026 due to climate change, impacting outdoor workers and vulnerable populations disproportionately. Knowing the heat index empowers you to make informed decisions about outdoor activities and hydration, potentially preventing heat stroke or exhaustion.

The Heat Index (HI) is calculated using a multiple regression equation developed by Steadman (1984) and refined by the National Weather Service (NWS). This complex formula considers the non-linear interaction between temperature and humidity to estimate the apparent temperature, which is how hot it feels to the average person. While the exact equation involves several terms and coefficients, our calculator simplifies this by taking your input and computing the HI, then mapping it to established danger categories.

When using the calculator, remember that the heat index assumes a shaded environment with light wind. Direct sunlight can add up to 15°F (8°C) to the calculated index, and strenuous activity further increases your body's heat load. Also, individual factors like age, health conditions, and acclimatization significantly influence heat tolerance, so always listen to your body even if the heat index seems moderate.

Example: Planning a Summer Hike in July 2026

  1. 1 Input: Air Temperature = 90°F (32.2°C), Relative Humidity = 70%.
  2. 2 Calculation: Using the NWS heat index formula, our calculator computes the apparent temperature. The formula involves terms like T (temperature), R (relative humidity), and coefficients derived from empirical data. For these inputs, the Heat Index is approximately 105°F (40.6°C).
  3. 3 Result: The calculated Heat Index is 105°F (40.6°C).
  4. 4 Context: A Heat Index of 105°F falls into the 'Danger' category as defined by the NWS. This means heat cramps or heat exhaustion are likely, and heat stroke is possible with prolonged exposure and/or physical activity. Recommendations include avoiding strenuous outdoor activity, staying in air conditioning, and drinking plenty of fluids. This specific heat index level is 20% more likely to occur in July 2026 compared to 2000 averages in many southern US states.

Source: NWS · Last updated: April 2026

Frequently Asked Questions

What is the heat index?
The heat index combines air temperature and relative humidity to show how hot it actually feels. At 90°F with 70% humidity, the heat index is about 106°F. High humidity prevents sweat from evaporating, reducing your body's ability to cool down.
At what heat index should you stop outdoor activity?
Caution begins at a heat index of 80°F. Above 103°F, heat cramps and exhaustion are likely. Above 125°F is extremely dangerous with high risk of heat stroke. Take frequent breaks, hydrate, and reduce outdoor exertion in the danger zone.
What is the difference between heat index and wet bulb temperature?
The heat index estimates perceived temperature. Wet bulb temperature measures the lowest temperature achievable through evaporative cooling. A wet bulb temperature above 95°F is considered potentially lethal because the human body can no longer cool itself through sweating.

You might also need

Snow Day Calculator

Predict the chance of a snow day based on weather forecast and school type. A fun prediction tool.

Wind Chill Calculator

Calculate wind chill factor using NWS formula. See frostbite risk time and danger level.

Moon Phase Calculator

Find the current moon phase, illumination percentage, and next full moon date.