Heat index is what the temperature feels like when you factor in humidity
The heat index is a single number that combines air temperature and relative humidity to show how hot it actually feels on your skin. On a 90-degree day with low humidity, the heat index might be 90 degrees. On the same 90-degree day with 70 percent humidity, the heat index could feel like 106 degrees. Your body loses heat through sweat evaporation, and high humidity blocks that process, so the air feels hotter than the thermometer says.
The National Weather Service uses a specific formula to calculate heat index. You need two pieces of information: the actual air temperature (in Fahrenheit) and the relative humidity percentage. Once you have both numbers, you can plug them into the formula or use a heat index chart or calculator to find the result.
Key Takeaways
- Heat index combines temperature and humidity into a single "feels like" number that tells you how hot conditions actually are for your body.
- The National Weather Service formula requires the air temperature in Fahrenheit and the relative humidity percentage as inputs.
- For temperatures below 80 degrees Fahrenheit or humidity below 40 percent, the heat index is close enough to the actual temperature that the formula is not necessary.
- Heat index charts and online calculators do the math for you if you do not want to work through the formula by hand.
- Heat index matters for health and safety because it shows when conditions become dangerous for outdoor work, exercise, or being outside for long periods.
The National Weather Service heat index formula
The standard formula used by the National Weather Service is:
HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541(T)(RH) − 0.00683783(T)² − 0.05481717(RH)² + 0.00122874(T)²(RH) + 0.00085282(T)(RH)² − 0.00000199(T)²(RH)²
In this formula, HI is the heat index, T is the air temperature in Fahrenheit, and RH is the relative humidity as a whole number (so 65 percent humidity is entered as 65, not 0.65). The formula includes eight separate terms that account for how temperature and humidity interact. You do not need to memorize it—the point is that it is precise enough to match real conditions.
If you are working through the formula by hand, do the math in order: start with the constant −42.379, add each term one at a time, and keep your intermediate results to at least four decimal places until the final answer. A small rounding error early on will throw off the result.
When you can skip the formula
The heat index formula is most accurate when the temperature is 80 degrees Fahrenheit or higher and the humidity is 40 percent or higher. Below those thresholds, the formula can give results that are slightly lower than the actual temperature, which does not make physical sense. In those conditions, the heat index is close enough to the actual air temperature that you can just use the thermometer reading.
If the temperature is below 80 degrees or the humidity is below 40 percent, report the actual temperature instead of calculating a heat index. The National Weather Service does the same thing—you will not see a heat index issued for mild or dry conditions.
Using a heat index chart
The National Weather Service publishes a standard heat index chart that shows the heat index for every combination of temperature (from 80 to 120 degrees Fahrenheit) and humidity (from 40 to 100 percent). To use it, find your air temperature along the left side of the chart, find your humidity percentage along the top, and read where the row and column meet.
The chart is faster than the formula if you are doing this once or twice. It is also easier to spot patterns—you can see at a glance that humidity matters much more at higher temperatures. At 90 degrees, the difference between 40 percent and 100 percent humidity is about 20 degrees of heat index. At 110 degrees, the same humidity range creates a difference of nearly 40 degrees.
You can find the official National Weather Service heat index chart on their website, or you can search for "heat index chart" and find versions published by weather services and educational sites. They all use the same formula, so the numbers will match.
Online heat index calculators
The easiest method is to use an online calculator. Type in the air temperature and relative humidity, and the calculator returns the heat index when ready. The National Weather Service has one on their website, and many weather apps include a heat index calculator as a built-in tool.
Online calculators are accurate as long as you enter the correct temperature and humidity. Make sure you are using Fahrenheit if the calculator asks—some calculators let you choose between Fahrenheit and Celsius, and entering the wrong scale will give you a nonsense result. If you are not sure what your relative humidity is, many weather websites and apps show it alongside the temperature.
How to measure relative humidity for the calculation
Relative humidity is the amount of moisture in the air compared to the maximum amount the air can hold at that temperature. A weather station measures it with an instrument called a psychrometer or hygrometer. You do not need to own one—your local weather service reports the current relative humidity for your area, and most weather websites and smartphone apps display it.
If you are calculating heat index for a specific location, check the weather report for that place, not your own location. A shaded area near trees may have different humidity than an open parking lot a few blocks away. For the most accurate heat index, use the humidity reading from the nearest weather station.
What the heat index number means for safety
The heat index tells you how your body will experience the temperature. A heat index of 91 to 103 degrees means heat exhaustion is possible with prolonged exposure or physical activity. A heat index of 103 to 125 degrees means heat exhaustion and heat cramps are likely, and heat stroke is possible. Above 125 degrees, heat stroke is likely with continued exposure.
These ranges are general guidelines, not hard cutoffs. Individual factors like age, fitness level, medications, and acclimatization matter. Older adults, young children, and people with certain health conditions are at higher risk at lower heat index values. If you are planning outdoor work or exercise, calculate the heat index and compare it to these ranges to decide whether conditions are safe.
Frequently Asked Questions
Do I need to use Fahrenheit or Celsius for the heat index formula?
The National Weather Service formula requires Fahrenheit. If you have a temperature in Celsius, convert it first by multiplying by 9/5 and adding 32. For example, 30 degrees Celsius is (30 × 9/5) + 32 = 86 degrees Fahrenheit. Some online calculators let you choose the scale, so check before you enter your number.
Can heat index be lower than the actual air temperature?
No. Heat index is always equal to or higher than the air temperature. If a calculator shows a heat index lower than the temperature, something went wrong—either the formula was applied outside its valid range (below 80 degrees Fahrenheit or below 40 percent humidity) or there was an input error. Use the actual air temperature in that case.
What is the difference between heat index and wind chill?
Heat index combines temperature and humidity to show how hot it feels. Wind chill combines temperature and wind speed to show how cold it feels. They use different formulas and explore to opposite seasons. Heat index matters in summer; wind chill matters in winter.
If the heat index is 110 degrees, is that the actual temperature?
No. The heat index is what the temperature feels like to your body. The actual air temperature might be 95 degrees, but with high humidity, it feels like 110 degrees. Your thermometer will still read 95 degrees. The heat index is useful for understanding how your body will respond, not for knowing what the thermometer says.
Can I calculate heat index if I do not know the exact relative humidity?
You can estimate it using a weather report, but the result will only be as accurate as your humidity number. If the weather service reports 65 percent humidity and you use that, your heat index will be close. If you guess at the humidity, your heat index could be off by several degrees. Use the reported humidity from your local weather service for the best result.