A heat index calculator tells you how hot the air actually feels to your body once humidity is taken into account. A day at 90°F (32°C) with dry air is uncomfortable. The same temperature with 70% relative humidity can feel like 106°F (41°C), because sweat evaporates more slowly and your body sheds heat less efficiently.
Knowing how to calculate the heat index helps you plan outdoor work, workouts, travel and events. It also helps you spot the point where heat stops being a nuisance and becomes a health risk. Below you will find the formula, a manual method, worked examples and a guide to reading the result. You can also check other weather and conversion tools on Calculatorr.
What is the heat index and why does it matter?
The heat index, also called the apparent temperature or feels-like temperature in hot conditions, combines two inputs:
- Air temperature, measured in the shade.
- Relative humidity, expressed as a percentage.
The human body cools itself mainly by sweating. When the air is already loaded with moisture, sweat evaporates slowly and body heat builds up. The heat index translates that effect into a single number on the same scale as temperature, so it is easy to compare with what a thermometer shows.
It matters because heat illness depends on the heat index more than on the thermometer alone. Cramps, exhaustion and heat stroke become more likely as the value climbs.
Heat index formula: the equation behind the calculation
The most widely used method is a regression equation. It works with temperature (T) in degrees Fahrenheit and relative humidity (RH) as a whole number from 0 to 100:
HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T·RH − 0.00683783T² − 0.05481717RH² + 0.00122874T²·RH + 0.00085282T·RH² − 0.00000199T²·RH²
The equation looks heavy, but it is just one long sum. Every term uses only T and RH. Because it was fitted to data in Fahrenheit, you must convert Celsius readings to Fahrenheit before using it.
When the full formula applies
The regression is designed for hot, humid conditions. As a rule of thumb, use it when the temperature is at least 80°F (27°C) and relative humidity is 40% or higher. Below those values, the feels-like temperature is usually close to the actual air temperature and the equation becomes unreliable.
The simple formula for a first estimate
For cooler conditions, or as a first check, use this shorter version:
HI = 0.5 × {T + 61.0 + [(T − 68.0) × 1.2] + (RH × 0.094)}
Average this result with the actual temperature. If the average is below 80°F, that is your heat index. If it is 80°F or higher, switch to the full regression equation.
How to calculate the heat index step by step
Follow these steps to get an accurate result by hand or with a basic scientific calculator.
Step 1: Convert the temperature to Fahrenheit
If your reading is in Celsius, use °F = °C × 9/5 + 32. For example, 32°C becomes 89.6°F.
Step 2: Run the simple formula
Plug T and RH into the simple formula and average the result with T. This tells you whether the full equation is needed.
Step 3: Apply the full regression equation
If the average from Step 2 is 80°F or more, calculate each term of the long equation and add them. Keep at least four decimals in the intermediate steps, because the terms are large and nearly cancel each other out.
Step 4: Apply the adjustments if they fit
Two small corrections are used in specific cases:
- Very dry air: if RH is below 13% and T is between 80°F and 112°F, subtract ((13 − RH) / 4) × √((17 − |T − 95|) / 17).
- Very humid air: if RH is above 85% and T is between 80°F and 87°F, add ((RH − 85) / 10) × ((87 − T) / 5).
Step 5: Convert back if needed
To return to Celsius, use °C = (°F − 32) × 5/9.
Heat index examples with real numbers
Example 1: 90°F and 70% humidity
Start with the simple formula: 0.5 × (90 + 61 + 26.4 + 6.58) = 91.99°F. Averaging with 90°F gives 91°F, which is above 80°F, so the full equation applies.
Calculate the terms:
- −42.379
- +184.4114 (2.04901523 × 90)
- +710.0332 (10.14333127 × 70)
- −1,415.9591 (0.22475541 × 90 × 70)
- −55.3864 (0.00683783 × 90²)
- −268.6041 (0.05481717 × 70²)
- +696.6956 (0.00122874 × 90² × 70)
- +376.0936 (0.00085282 × 90 × 70²)
- −78.9831 (0.00000199 × 90² × 70²)
The sum is about 105.9°F, or roughly 41°C. A 90°F afternoon feels closer to 106°F.
Example 2: 85°F and 50% humidity
The simple formula gives about 85.6°F, and the average with 85°F is 85.3°F, so the full equation applies. Adding the nine terms gives about 86.5°F (30.3°C). Here the feels-like value is only slightly above the air temperature, because moderate humidity adds little extra load.
Example 3: working in Celsius
Suppose the weather app shows 35°C and 55% humidity. Convert first: 35 × 9/5 + 32 = 95°F. Use 95 and 55 in the equation, then convert the result back to Celsius. The key habit is to never mix units inside the formula.
How to use an online heat index calculator
An online calculator removes the arithmetic and the risk of typing errors. The process is quick:
- Enter the air temperature and choose Celsius or Fahrenheit.
- Enter the relative humidity as a percentage.
- Read the feels-like temperature and the risk level.
Use a shaded temperature reading and a humidity value from the same place and time. A forecast from a different location can give a result that is off by several degrees. For related calculations such as dew point, wind chill or temperature conversions, browse the tools at Calculatorr.com.
How to interpret heat index results
The table below shows the commonly used risk ranges.
| Heat index (°F) | Heat index (°C) | Risk level | What it means |
|---|---|---|---|
| 80 to 90 | 27 to 32 | Caution | Fatigue is possible with prolonged exposure or activity. |
| 91 to 103 | 33 to 39 | Extreme caution | Heat cramps and heat exhaustion are possible. |
| 103 to 125 | 39 to 52 | Danger | Heat exhaustion is likely, and heat stroke is possible. |
| Above 125 | Above 52 | Extreme danger | Heat stroke is highly likely. |
In Example 1, a heat index near 106°F falls in the danger range, even though the thermometer reads 90°F. That is the main value of this calculation: it shows risk that the air temperature hides.
Practical actions by risk level
- Caution: drink water regularly and take breaks in the shade.
- Extreme caution: shorten outdoor tasks, schedule heavy work for early morning and watch for dizziness or heavy sweating.
- Danger: limit exposure, rest often in cool places and never leave children or pets in vehicles.
- Extreme danger: avoid outdoor exertion and seek air conditioning.
Common mistakes when calculating the heat index
- Using Celsius in the formula. The equation only works with Fahrenheit inputs. Convert first, then convert back.
- Entering humidity as a decimal. Use 70, not 0.70.
- Applying the formula to mild weather. Below about 80°F or 40% humidity, the result can be misleading. Use the simple formula or just the air temperature.
- Ignoring direct sun. The values assume shade and light wind. Full sunshine can raise the effective heat index by up to about 15°F (8°C).
- Rounding too early. The large positive and negative terms nearly cancel, so rounding them can shift the answer by several degrees.
- Confusing it with dew point or wind chill. Dew point measures moisture in the air, wind chill applies to cold weather, and the heat index applies to hot weather.
Heat index vs WBGT: which one should you use?
The heat index is simple and works well for everyday decisions, such as whether to go for a midday run or how to plan a family outing. It only needs temperature and humidity.
Wet bulb globe temperature (WBGT) also accounts for solar radiation, wind and ground heat. That makes it better suited to workplaces, sports events and military or industrial settings. For daily personal planning, the heat index is usually enough. For regulated heat stress management, WBGT is the stronger choice.
Practical ways to use the heat index in daily life
- Exercise: reduce intensity or move indoors when the heat index enters the extreme caution range.
- Outdoor work: build rest-and-water schedules around the value, not just the temperature.
- Travel: compare destinations by feels-like temperature, since humid coastal areas can feel hotter than dry inland ones with higher readings.
- Home cooling: decide when fans are enough and when air conditioning is worth the cost.
- Vulnerable people: older adults, young children and people with chronic conditions should act earlier, because they are affected at lower values.
Check the heat index with a reliable heat index calculator, compare it against the risk ranges, and adjust your plans before the heat forces you to.
