Heat Index Calculator
At 90 degrees Fahrenheit with 70 percent humidity, it feels like about 106 degrees. This heat index calculator combines air temperature and relative humidity to estimate the feels-like temperature, the value your body actually senses on a humid day. It uses the National Weather Service heat index formula and returns the result in Fahrenheit and Celsius along with a heat risk level. Enter the temperature and humidity to see how hot it really feels.
Quick answer
The heat index is how hot it feels when humidity is factored in with the air temperature.
What this tells you
- •The heat index is how hot it feels when humidity is factored in with the air temperature.
- •High humidity slows sweat evaporation, so your body cools less and the air feels hotter.
- •The calculator uses the National Weather Service formula and is most accurate at 80 degrees Fahrenheit and above.
- •The risk level moves from Caution to Extreme Danger as the heat index climbs.
- •The published heat index assumes shade and light wind, not direct sunlight or strenuous activity.
- •Personal risk can be higher than the category suggests because of age, health, medicines, clothing, exertion, and heat acclimatization.
How to Use
- 1Enter the air temperature in degrees Fahrenheit.
- 2Enter the relative humidity as a percentage from 0 to 100.
- 3Click Calculate to get the feels-like temperature in Fahrenheit and Celsius plus the heat risk level.
- 4Compare the estimate with current local alerts and reduce exposure when conditions or symptoms warrant it.
How It Works
Formula
HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T^2 - 0.05481717R^2 + ... (NWS regression)The heat index uses the National Weather Service Rothfusz regression, where T is the temperature in Fahrenheit and R is the relative humidity. A simpler average formula is used first, and when that value reaches 80 degrees the full regression is applied, with small adjustments for very dry or very humid air. At 90 degrees and 70 percent humidity the result is about 106 degrees Fahrenheit.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
How hot does 90 degrees at 70% humidity feel?
At 90 degrees with 70 percent humidity, the heat index is about 106 degrees Fahrenheit. That falls in the Danger range, where heat cramps and heat exhaustion are likely with prolonged exposure.
A hot, drier day at 100 degrees and 40% humidity
Even at 40 percent humidity, 100 degrees feels like about 109 degrees because the air still limits how fast sweat evaporates.
Very humid air at 86 degrees
The regression gives 105.4 degrees Fahrenheit after the high-humidity adjustment. This shows why a temperature in the 80s can still create dangerous conditions when the air is nearly saturated.
Heat Index Risk Levels
How the National Weather Service classifies heat index ranges.
| Heat Index (Fahrenheit) | Risk Level | What It Means |
|---|---|---|
| 80 to 90 | Caution | Fatigue possible with prolonged activity |
| 91 to 103 | Extreme Caution | Heat cramps and heat exhaustion possible |
| 104 to 124 | Danger | Heat exhaustion likely, heat stroke possible |
| 125 and above | Extreme Danger | Heat stroke highly likely |
Values are for shade. Full sun can raise the heat index by up to 15 degrees Fahrenheit.
Reading Heat Index Safely
The calculation starts with a simpler Steadman-style estimate. The implementation averages that result with the entered air temperature. When the average reaches 80 degrees Fahrenheit, it applies the Rothfusz regression used by the National Weather Service. The full equation uses Fahrenheit and relative humidity as a percentage, so entering Celsius or a decimal humidity produces the wrong result.
The regression also includes two limited adjustments. For relative humidity below 13 percent and temperatures from 80 through 112 degrees Fahrenheit, a dry-air adjustment lowers the estimate. For humidity above 85 percent and temperatures from 80 through 87 degrees, a humid-air adjustment raises it. Outside those specific ranges, neither adjustment applies. The displayed Celsius value is converted from the final Fahrenheit heat index.
Heat index categories describe increasing potential for heat-related illness during prolonged exposure or strenuous activity. They do not predict what will happen to one person. Risk may rise sooner for older adults, infants, pregnant people, outdoor workers, athletes, people without air conditioning, and people with heart, lung, kidney, or other health conditions. Some medicines and substances can affect sweating, circulation, hydration, alertness, or temperature regulation.
The standard estimate is intended for shaded, light-wind conditions. Direct sunlight can increase heat stress beyond the displayed number. Heavy work, sports, dark or impermeable clothing, protective equipment, radiant heat from machinery or pavement, limited airflow, and a hot indoor space can also make exposure more severe. A local weather observation may not match conditions on a roof, field, vehicle, warehouse floor, or poorly ventilated room.
Use the number as one input to a heat-safety decision. Check current weather warnings, workplace procedures, school or sports policies, and public-health advice. Plan water access, rest breaks, shade or cooling, lighter workloads, and supervision when conditions are hot. People returning after time away from heat often need gradual acclimatization rather than an immediate full workload.
Possible warning signs of heat illness include headache, dizziness, nausea, weakness, heavy sweating, cramps, faintness, or unusual fatigue. Confusion, collapse, seizures, loss of consciousness, or very hot skin can signal an emergency. Move the person away from heat, begin rapid cooling, and contact emergency services immediately for severe symptoms. Do not wait for this calculator or a category label to confirm an emergency.
Common mistakes
- Entering humidity as a decimal instead of a percentage. Use 70 for 70 percent, not 0.7.
- Expecting accurate results in cool weather. The heat index formula is built for warm conditions at about 80 degrees Fahrenheit and above.
- Assuming the reading applies in direct sun. The heat index is a shade value, and standing in full sunlight can feel up to 15 degrees hotter.
- Using a humidity reading from another location or time instead of conditions where the exposure occurs.
- Treating a category boundary as a guarantee that activity is safe below it.
- Ignoring workload, protective clothing, indoor radiant heat, medications, or lack of heat acclimatization.
Limitations
This calculator estimates heat index from air temperature and relative humidity using the simple screening calculation and Rothfusz regression implemented here. The regression is intended mainly for warm, humid conditions and is most useful when the apparent temperature reaches about 80 degrees Fahrenheit or more. It assumes shade and light wind. It does not measure core body temperature or account for direct sun, radiant heat, workload, clothing, protective equipment, airflow, indoor conditions, elevation, hydration, acclimatization, age, pregnancy, health conditions, or medicines. Weather-station inputs may differ from the conditions at your exact location. The category is not a diagnosis, exposure limit, or guarantee of safety.
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