What heat index means
Heat index estimates how hot the air may feel when temperature and relative humidity are combined.
Heat Index Calculator
Calculate the feels-like temperature from heat and humidity.
Weather inputs
Enter air temperature and relative humidity. The estimate assumes shaded conditions.
percent
Heat index temperature
99.7 °F
99.7 °F, 37.6 °C
Heat risk category
Increased caution
Feels-like difference
9.7 °F
Relative humidity used
60%
Temperature used
90.0 °F
Comfort interpretation
Extended outdoor activity may require more careful planning, rest, and hydration.
Formula used
Rothfusz regression approximation
Copy result
Copy heat index, air temperature, humidity, risk category, and formula note.
Formula approximation
The result uses a common heat index approximation for hot, humid conditions.
Shade assumption
Heat index is usually based on shaded conditions. Direct sun can feel hotter.
Humidity impact
High humidity can slow sweat evaporation and increase feels-like temperature.
Weather safety
Use official local forecasts and alerts for severe heat decisions.
Local calculation
Inputs are processed in your browser. No external weather API is required.
Dynamic heat index insights
Risk reading
Extended outdoor activity may require more careful planning, rest, and hydration.
Humidity effect
Humidity affects heat comfort by changing how efficiently sweat can evaporate.
Planning note
Consider shorter outdoor sessions, more breaks, shade, and checking official local forecasts.
Heat risk breakdown
Heat index value
99.7 °F
Air temperature
90.0 °F
Relative humidity
60%
Feels-like difference
9.7 °F
Risk category
Increased caution
Planning note
Consider shorter outdoor sessions, more breaks, shade, and checking official local forecasts.
Formula range note
The Rothfusz regression is most useful for hot, humid conditions.
Heat index risk guide
Below 80 °F or 27 °C
Lower concern
Generally lower heat stress for many people
80 to 90 °F or 27 to 32 °C
Warm
Extended activity may feel uncomfortable
90 to 103 °F or 32 to 39 °C
Increased caution
Plan breaks and monitor conditions
103 to 124 °F or 39 to 51 °C
Higher heat stress risk
Careful planning and official guidance matter
125 °F or 52 °C and above
Extreme concern
Check official alerts and avoid unnecessary heat exposure
Humidity impact guide
Low humidity
Under 40%
Sweat may evaporate more easily, but heat can still be stressful
Moderate humidity
40% to 60%
Feels-like temperature may begin rising in hot weather
High humidity
60% to 80%
Sweat evaporation slows and conditions can feel hotter
Very high humidity
Above 80%
Heat can feel oppressive, especially with little airflow
Outdoor planning guide
Shade
Shade can reduce direct sun exposure and make outdoor time more manageable
Shade can reduce direct sun exposure and make outdoor time more manageable
Hydration
Plan fluids before long outdoor activity in hot weather
Plan fluids before long outdoor activity in hot weather
Rest breaks
Frequent breaks can help reduce heat strain during extended activity
Frequent breaks can help reduce heat strain during extended activity
Time of day
Morning or evening may be more comfortable than peak afternoon heat
Morning or evening may be more comfortable than peak afternoon heat
Official alerts
Use local forecasts and public health guidance for severe heat decisions
Use local forecasts and public health guidance for severe heat decisions
Heat index guide
These notes explain heat index concepts without repeating the calculator result.
Heat index vs air temperature
Air temperature is the actual measured temperature. Heat index adds humidity to estimate a feels-like value.
Why humidity makes heat feel worse
When humidity is high, sweat evaporates less efficiently, which can make cooling harder during hot weather.
When heat index is most useful
Heat index is most helpful in hot, humid conditions and is less meaningful in cooler or very dry weather.
Sun exposure and outdoor planning
Direct sun, low wind, dark clothing, high activity, and limited shade can make conditions feel hotter.
Limitations of heat index calculations
The estimate does not replace medical advice, workplace heat rules, emergency guidance, or official local alerts.
Formula
Heat Index = Rothfusz regression approximation using T and RH
Variables
- T = air temperature.
- RH = relative humidity.
- Result = estimated feels-like temperature.
- The full regression is applied in Fahrenheit, then converted when needed.
Worked example
At 90 °F and 60% relative humidity, the heat index is about 100 °F, or about 38 °C.
Assumptions
The estimate assumes shaded conditions and typical heat index use in hot, humid weather.
Limitations
Direct sun, wind, clothing, activity, hydration, age, health, and acclimatization can affect real comfort and safety.
Frequently asked questions
What does heat index mean?
Heat index estimates how hot conditions may feel when air temperature and relative humidity are combined.
Why does humidity make it feel hotter?
High humidity slows sweat evaporation, which can make it harder for the body to cool itself during hot weather.
Is heat index the same as feels-like temperature?
Heat index is one type of feels-like temperature. It focuses on hot weather using air temperature and humidity.
Does heat index work in direct sunlight?
Heat index is usually based on shaded conditions. Direct sun can make conditions feel hotter than the calculated value.
What heat index is dangerous?
Higher heat index values may indicate greater heat stress risk, but personal risk varies. Check official local alerts and public health guidance during severe heat.
How accurate is a heat index calculator?
It gives a practical estimate using a common approximation. Real comfort and safety can vary with sun, wind, clothing, activity, hydration, and health.
Can heat index be lower than the air temperature?
In some lower humidity or cooler conditions, the calculated adjustment may be small. Heat index is most useful in hot, humid weather.
What is the difference between heat index and dew point?
Dew point describes moisture in the air. Heat index estimates how hot the air may feel by combining temperature and humidity.
Should I use heat index for outdoor workouts?
It can help with planning, but it should not replace official alerts, coach guidance, workplace rules, or medical advice.
Related tools