🌡️ Heat Index Calculator: Feels Like Temperature from Heat and Humidity
By Shihab Mia · Reviewed by ToolNimba Review Team, heat safety content review · Updated 2026-06-28
This heat index calculator gives a general safety estimate based on the U.S. National Weather Service formula, not medical advice. In hot, humid conditions seek shade or air conditioning, drink water, and consult a medical professional for any signs of heat illness.
This is a safety estimate using the U.S. National Weather Service heat index, not medical advice. On hot, humid days move to shade or air conditioning, drink water and watch for signs of heat illness.
The heat index, often called the feels like temperature or apparent temperature, combines the air temperature with the relative humidity to estimate how hot it actually feels to the human body. When humidity is high, sweat evaporates more slowly, so your body cannot cool itself as well and the air feels far hotter than the thermometer reads. This heat index calculator uses the official National Weather Service formula to turn a temperature and humidity reading into a single feels like number in both Fahrenheit and Celsius, along with the matching heat risk category.
What is the Heat Index Calculator?
The heat index calculator answers a simple but important question: given the temperature outside and how humid it is, how hot does it really feel? Our bodies cool themselves mainly by sweating, and sweat works by evaporating off the skin. When the air is already full of moisture, evaporation slows down, cooling becomes less effective, and the same air temperature feels much more oppressive. That is why 90 degrees Fahrenheit at 30 percent humidity feels manageable, while 90 degrees at 70 percent humidity feels dangerous.
The number this tool produces is based on the Rothfusz regression, the equation the National Weather Service uses to publish its official heat index charts. For milder conditions the calculator first uses a simpler averaging formula, and only switches to the full regression when the result climbs to about 80 degrees Fahrenheit or higher, exactly as the NWS specifies. Two small correction factors are applied at the extremes: a downward adjustment for very dry air and an upward adjustment for very high humidity in the low 80s. These details matter, because a heat index calculator that skips them can be several degrees off in precisely the conditions where accuracy counts most.
The heat index assumes shady, light wind conditions. Standing in full sunshine can raise the apparent temperature by up to 15 degrees Fahrenheit, so the figure you see here is best treated as a floor rather than a ceiling on a sunny day. Wind, clothing, hydration, age, and individual health all change how a given heat index actually affects a person, which is why the same reading can be merely uncomfortable for one person and genuinely hazardous for another.
Understanding the feels like temperature is the first step in planning a safe day outdoors. Athletes, outdoor workers, parents of young children, and anyone caring for elderly relatives can use the heat index and its risk category to decide when to reschedule activity, add water breaks, or move indoors. The categories used here, from Caution through Extreme Danger, map directly to the warning levels the National Weather Service issues during heat waves.
When to use it
- Deciding whether an outdoor run, ride, or sports practice is safe before you head out in summer heat.
- Planning water breaks and rest periods for a crew of outdoor workers on a hot, humid afternoon.
- Checking the feels like temperature before taking young children or elderly relatives outside.
- Comparing two cities or two times of day to see which conditions feel hotter despite a similar reading.
- Teaching students how temperature and relative humidity combine to drive heat stress and apparent temperature.
- Understanding a heat advisory or excessive heat warning by seeing the underlying heat index number.
How to use the Heat Index Calculator
- Choose Fahrenheit or Celsius with the unit toggle; the temperature field updates to match.
- Enter the current air temperature from a thermometer or weather report.
- Enter the relative humidity as a percentage, from 0 to 100.
- Read the heat index, shown in both Fahrenheit and Celsius, along with its risk category.
- Use the copy button to save the feels like temperature and category for your notes or a message.
Formula & method
Worked examples
A muggy summer afternoon at 90 degrees Fahrenheit with 70 percent relative humidity.
- Simple formula: HI = 0.5 x (90 + 61 + (90 - 68) x 1.2 + 70 x 0.094) = about 91.4
- Average of 91.4 and 90 is 90.7, which is 80 or higher, so use the Rothfusz regression
- Plug T = 90 and R = 70 into the full equation
- Humidity is below 85 and above 13, so no correction factor applies
- Result is about 106 degrees Fahrenheit, or about 41 degrees Celsius
Result: Feels like about 106 F (41 C), Danger category
A hot, dry desert reading of 100 degrees Fahrenheit with only 10 percent relative humidity.
- Simple formula average works out to 80 or higher, so use the Rothfusz regression
- Plug T = 100 and R = 10 into the full equation
- Humidity is below 13 and temperature is between 80 and 112, so apply the dry correction
- Subtract ((13 - 10) / 4) x sqrt((17 - |100 - 95|) / 17), about 0.65 degrees
- Result is about 94 degrees Fahrenheit, or about 34 degrees Celsius
Result: Feels like about 94 F (34 C), Extreme Caution category
NWS heat index risk categories and what they mean
| Heat index (F) | Category | Likely effect with exposure or activity |
|---|---|---|
| Below 80 | No significant risk | Comfortable to warm; normal precautions |
| 80 to 90 | Caution | Fatigue possible with prolonged exposure |
| 90 to 103 | Extreme Caution | Heat cramps and exhaustion possible |
| 103 to 124 | Danger | Heat exhaustion likely; heatstroke possible |
| 125 and above | Extreme Danger | Heatstroke highly likely |
Feels like temperature (F) at selected temperature and humidity
| Air temp | 40% RH | 60% RH | 80% RH |
|---|---|---|---|
| 85 F | 84 | 89 | 97 |
| 90 F | 91 | 100 | 113 |
| 95 F | 99 | 113 | 134 |
| 100 F | 109 | 129 | 158 |
Common mistakes to avoid
- Confusing the thermometer reading with the heat index. The air temperature alone does not tell you how hot it feels. At the same 90 degrees, low humidity can feel pleasant while high humidity can be dangerous. Always pair temperature with relative humidity to get a meaningful feels like number.
- Forgetting that the heat index assumes shade. The National Weather Service formula is calibrated for shady, light wind conditions. Direct sunlight can add up to 15 degrees Fahrenheit, so on a sunny day the real feels like temperature is higher than the calculator shows.
- Using the heat index below 80 degrees. The heat index is only meaningful in hot conditions. Below about 80 degrees Fahrenheit the formula is not designed to apply, and a separate wind chill calculation is used for cold weather instead.
- Entering dew point instead of relative humidity. This tool needs relative humidity as a percentage from 0 to 100, not the dew point in degrees. If your weather source only gives a dew point, convert it to relative humidity first or the result will be wrong.
Glossary
- Heat index
- A combined measure of air temperature and relative humidity that estimates how hot it feels to the human body.
- Feels like temperature
- A common name for the heat index, the apparent temperature your body experiences rather than the raw thermometer reading.
- Relative humidity
- The amount of water vapor in the air as a percentage of the maximum the air can hold at that temperature.
- Rothfusz regression
- The multi-term equation the U.S. National Weather Service uses to compute the official heat index.
- Heat advisory
- A warning issued when the heat index is expected to reach a dangerous level, often around 100 to 105 degrees Fahrenheit.
- Heatstroke
- A life threatening condition where the body can no longer regulate its temperature, more likely at high heat index values.
Frequently asked questions
What is a heat index calculator?
A heat index calculator combines air temperature and relative humidity to estimate the feels like temperature using the National Weather Service formula. It tells you how hot the air actually feels to your body, which is often much higher than the thermometer reading on humid days.
What temperature and humidity is dangerous?
A heat index of 103 to 124 degrees Fahrenheit falls in the Danger category, and 125 or above is Extreme Danger. For example, 95 degrees with 55 percent humidity already feels like about 109 degrees, which is in the Danger range.
Why does humidity make it feel hotter?
Your body cools itself by evaporating sweat. When humidity is high the air already holds a lot of moisture, so sweat evaporates slowly and you lose less heat. The same temperature therefore feels much hotter when it is humid.
Is the heat index the same as the real temperature?
No. The heat index is the apparent or feels like temperature, while the real temperature is what a thermometer measures. The heat index is usually higher than the air temperature whenever humidity is significant.
Does the heat index account for direct sunlight?
No. The official formula assumes shady, light wind conditions. Standing in full sun can raise the apparent temperature by up to 15 degrees Fahrenheit, so treat the calculator value as a minimum on a sunny day.
Can I use this heat index calculator in Celsius?
Yes. Toggle to Celsius and enter the temperature in degrees Celsius. The tool converts it to Fahrenheit for the calculation, then shows the heat index in both Celsius and Fahrenheit along with the risk category.
Sources
- The Heat Index Equation , U.S. National Weather Service, Weather Prediction Center
- Heat Index , U.S. National Weather Service