Heat Index Calculator
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Quick Guide to Use the Heat Index Calculator
- Enter the temperature in the provided field.
- Select the temperature unit (Celsius or Fahrenheit).
- Enter the relative humidity in % in the respective field.
- Click the “Calculate” button to obtain the Heat Index in the selected Unit.
- The result will display the Heat Index value along with an alert indicating potential risks.
The alert follows the four National Weather Service heat index categories. Note that the Danger band begins at 103°F and Extreme Danger at 125°F. Many calculators still show the older 105°F and 130°F cut-offs, which understate the risk in between.
| Temperature in °C | Temperature in °F | Alert |
| 26.7-32.2°C | 80-90°F | Caution: Fatigue possible with prolonged exposure and/or physical activity. |
| 32.2-39.4°C | 90-103°F | Extreme Caution: Heat stroke, heat cramps, or heat exhaustion possible with prolonged exposure and/or physical activity. |
| 39.4-51.7°C | 103-124°F | Danger: Heat cramps or heat exhaustion likely, and heat stroke possible with prolonged exposure and/or physical activity. |
| 51.7°C or higher | 125°F or higher | Extreme Danger: Heat stroke highly likely. |
Formula
For Fahrenheit
Where:
- HI is the Heat Index in degrees Fahrenheit
- T is the temperature in degrees Fahrenheit
- H is the relative humidity as a percentage
That regression is only one step of three, and applying it on its own is the most common way a heat index calculator goes wrong. It was fitted to Steadman’s table in the hot part of the range, so below roughly 80°F it drifts badly and even rises as the air gets cooler. The National Weather Service procedure is:
- Try the simple form first. HI = 0.5 × [T + 61.0 + ((T − 68.0) × 1.2) + (RH × 0.094)]. Average that answer with the air temperature. If the average is under 80°F, that simple value is the heat index and the regression is never used.
- Otherwise use the regression above.
- Then apply the two corrections at the humidity extremes. If RH is below 13% and T is between 80°F and 112°F, subtract [(13 − RH) / 4] × √([17 − |T − 95|] / 17). If RH is above 85% and T is between 80°F and 87°F, add [(RH − 85) / 10] × [(87 − T) / 5].
This calculator runs all three steps. Two checks against the NWS’s own published examples: 96°F at 65% relative humidity returns 121.0°F, and 100°F at 15% returns 95.8°F, both matching the values the NWS quotes.
For Celsius
HI=c1+c2T+c3R+c4TR+c5T2+c6R2+c7T2R+c8TR2+c9T2R2
Where:
- HI is the heat index in degrees Celsius
- T is the temperature in degrees Celsius
- R is the relative humidity
c1, c2, c3, …c9 are constants
C1 = -8.78469475556;
C2 = 1.61139411;
C3 = 2.33854883889;
C4 = -0.14611605;
C5 = -0.012308094;
C6 = -0.0164248277778;
C7 = 2.211732e-3;
C8 = 7.2546e-4;
C9 = -3.582e-6;
The two coefficient sets describe the same index, one fitted in Fahrenheit and one in Celsius; evaluated across 20°C to 45°C and 20% to 100% humidity they agree to within 0.001 of a degree. This tool evaluates in Fahrenheit and converts the answer, so the Celsius and Fahrenheit modes return the same reading and fall into the same category. The heat index is a temperature, not a separate unitless score, so the dropdown selects the units of both the input and the result.
What is the Heat Index?
The Heat Index, often referred to as the “apparent temperature,” represents the perceived warmth experienced by individuals when considering both air temperature and humidity levels.
By incorporating humidity into the equation, the Heat Index provides insight into the actual sensation of heat on the body, aiding in the evaluation of potential heat-related health hazards during extended periods of exposure.
What the Heat Index Assumes, and Where It Falls Short
The number is a shade reading. Steadman’s model assumes shade and light wind, so it describes someone standing still, out of the sun, in ordinary clothing. Four limits follow from that, and they matter most to exactly the outdoor users this tool is built for.
- Full sun adds up to 15°F. The National Weather Service publishes that figure directly; OSHA quotes up to 13.5°F. A shade heat index of 95°F can be an apparent 110°F on an unshaded roof.
- It ignores workload. Temperature and humidity are the only inputs. Metabolic heat from heavy labour, protective clothing and radiant heat from plant or asphalt are all outside the model.
- The regression itself carries about ±1.3°F of error, which is the NWS’s own stated fit quality against Steadman’s table. Treat the result as a band, not a precise reading.
- A low reading is not a safe reading. OSHA records outdoor workers dying of heat stroke on days whose maximum heat index was only 86°F, and treats the heat index as a screening tool rather than a hazard assessment. Where the stakes are real, OSHA points to Wet Bulb Globe Temperature, which also accounts for radiant heat and wind.
Who Can Use This Calculator?
Anyone, including individuals, outdoor workers, athletes, and event organizers, can use this calculator to assess the heat-related risks based on temperature and humidity.
Industries That Can Use This Calculator
- Construction and Outdoor Work
- Athletics and Sports Events
- Agriculture
- Emergency Response Services
- Event Planning
Benefits of Using This Calculator
- Risk Assessment: Evaluate the risk of heat-related illnesses.
- Safety Planning: Plan outdoor activities considering potential heat stress.
- Health Protection: Safeguard individuals from heat-related health issues.
- Industry Compliance: Meet safety standards in various industries.
FAQs
How is the heat index calculated?
The National Weather Service uses three steps, not one. A simple form is computed first and averaged with the air temperature; if that average is below 80 F, it is the heat index. Only above 80 F is the Rothfusz regression used, and it is then corrected when relative humidity is below 13 percent or above 85 percent. Applying the regression on its own is the usual reason a calculator reports a high heat index on a mild day.
What is the heat index in Celsius?
It is the same index expressed in Celsius rather than a different scale. The four National Weather Service thresholds convert to 26.7 C for Caution, 32.2 C for Extreme Caution, 39.4 C for Danger and 51.7 C for Extreme Danger. This calculator accepts Celsius or Fahrenheit and returns the result in the unit you selected.
Is the heat index the same as humidex or RealFeel?
No, they are separate apparent temperature scales. The heat index is the United States National Weather Service scale derived from Steadman’s work and uses air temperature and relative humidity. Humidex is the Canadian index and is derived from air temperature and dew point. RealFeel is a proprietary AccuWeather index that also factors in sunshine, wind and other variables. This calculator returns the NWS heat index.
What heat index does OSHA treat as dangerous?
OSHA does not publish a single legal heat index trigger. It treats the heat index as a screening tool, notes that it is measured in shade and accounts for neither sunlight nor workload, and recommends Wet Bulb Globe Temperature for assessing the hazard itself. OSHA also records outdoor workers dying of heat stroke on days whose maximum heat index was only 86 F, so a reading below the Caution band is not on its own evidence of a safe day.
Why does humidity matter so much?
Sweat cools the body by evaporating, and humid air slows evaporation, so the body sheds less heat and the same air temperature becomes more dangerous. At 96 F, raising relative humidity from 40 percent to 65 percent moves the heat index from about 100.9 F to 121.0 F, a shift from Extreme Caution into Extreme Danger.
Conclusion
The heat index converts a temperature and a humidity reading into the shade apparent temperature, and sorts it into the four National Weather Service categories that begin at 80°F, 90°F, 103°F and 125°F.
Two things decide whether that number is useful on the day. Add for sun and workload, because the index accounts for neither. And where a real heat-safety decision rests on it, use it to screen and then measure Wet Bulb Globe Temperature, which is what OSHA recommends for assessing the hazard itself.