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Heat Index Calculator,
how hot it really feels.
Calculate the apparent temperature — how hot the air actually feels on your skin — from air temperature and relative humidity using the official National Weather Service (NWS) Rothfusz formula. Includes NWS danger-level categorization and heat illness guidance.
Inputs
Temperature & humidity
Temperature unit
NWS formula applies at ≥ 80°F (27°C) and ≥ 40% humidity. Outside that range the thermometer reading is accurate.
Scenarios
Heat index
95°F · 65% RH
117.6°Ffeels like
Actual 95°F · feels +22.6°F warmer
Heat index danger scale
Danger
Heat cramps and heat exhaustion are likely. Heat stroke is possible.
Recommended action
Avoid outdoor exertion. Move to air conditioning. Watch for symptoms.
Breakdown
Actual vs. feels like
Air temp
95°F
35°C
Humidity
65%
relative humidity
Feels like
117.6°F
47.6°C
Added heat
+22.6°F
+12.6°C warmer
NWS danger scale
Heat index categories
Fatigue is possible with prolonged exposure or physical activity.
Heat cramps and heat exhaustion are possible.
Heat cramps and heat exhaustion are likely. Heat stroke is possible.
Heat stroke is highly likely with continued exposure.
Heat index guide
What is heat index and why does humidity make it feel hotter?
Your body cools itself primarily through sweat. When sweat evaporates from your skin, it carries heat away, the same physics behind an air conditioner. But evaporation requires the surrounding air to be able to absorb more moisture. When relative humidity is high, the air is already close to saturation and sweat evaporates far more slowly. The result: your body cannot dissipate heat efficiently, and the environment feels much hotter than the dry-bulb thermometer reads.
The heat index (also called apparent temperature or feels-like temperature) is a single number that captures this combined effect. At 95°F and 65% humidity, for example, the heat index is approximately 113°F — nearly 18 degrees hotter than the actual temperature.
The Rothfusz regression: how the NWS calculates heat index
The National Weather Service uses a multivariate regression equation developed by Rothfusz (1990), derived from a set of human biophysical models originally created by Steadman (1979). It is a polynomial that takes air temperature in °F and relative humidity as a percentage:
Two correction terms are then applied:
- Low-humidity adjustment (RH < 13% and 80°F ≤ T ≤ 112°F): the heat index is reduced, because very dry air evaporates sweat faster than average.
- High-humidity adjustment (RH > 85% and 80°F ≤ T ≤ 87°F): the heat index is increased slightly in this narrow range where the basic equation slightly underestimates.
The equation is valid when temperature is 80°F (27°C) or higher and humidity is 40% or higher. Below those thresholds, evaporative cooling is effective enough that the thermometer reading is a reliable measure of thermal stress.
Heat index table — °F by temperature and humidity
The table below shows computed heat index values (°F) for common combinations of air temperature and relative humidity. Values in orange fall in the Extreme Caution zone; values in red indicate Danger or Extreme Danger.
| RH \ Temp | 80°F | 85°F | 90°F | 95°F | 100°F | 105°F | 110°F |
|---|---|---|---|---|---|---|---|
| 40% | 79.9 | 84.3 | 90.7 | 99 | 109.3 | 121.5 | 135.7 |
| 50% | 80.8 | 86.5 | 94.6 | 105.2 | 118.3 | 133.9 | 152 |
| 60% | 81.8 | 89.3 | 99.7 | 113.1 | 129.5 | 148.9 | 171.2 |
| 70% | 83 | 92.7 | 105.9 | 122.6 | 142.8 | 166.4 | 193.5 |
| 80% | 84.2 | 96.8 | 113.3 | 133.8 | 158.2 | 186.5 | 218.8 |
| 90% | 86.3 | 101.8 | 121.9 | 146.6 | 175.7 | 209.2 | 247 |
| 100% | 89.3 | 107.6 | 131.6 | 161.1 | 195.3 | 234.4 | 278.2 |
Values above 80°F with RH ≥ 40% only. All values in °F. Yellow = Caution · Orange = Extreme Caution · Red = Danger/Extreme Danger.
Heat-related illness: recognizing and responding
High heat index conditions cause a spectrum of heat-related illnesses, from mild cramps to life-threatening heat stroke. Understanding the progression can save lives.
Symptoms: Painful muscle spasms, usually in legs or abdomen. Often the first sign of overheating.
Response: Stop activity. Move to cool location. Drink water or sports drink. Stretch affected muscles gently.
Symptoms: Heavy sweating, cool pale clammy skin, fast weak pulse, nausea, dizziness, headache, fainting.
Response: Move to cool place. Loosen clothing. Apply cool wet cloths. Sip water. Seek medical attention if symptoms worsen.
Symptoms: Body temperature above 103°F, hot/red/dry skin (no sweating), rapid strong pulse, possible unconsciousness.
Response: Call 911 immediately. Move to cool area. Cool rapidly with ice packs (armpits, neck, groin). Do NOT give water.
Who is most at risk from heat?
While anyone can develop heat illness in extreme conditions, certain groups face significantly higher risk:
- Elderly adults (65+): impaired thermoregulation and often on medications that reduce sweat response.
- Infants and young children: smaller bodies heat up faster and depend on caregivers to keep them cool.
- Outdoor workers: construction, agriculture, and landscaping workers face prolonged exposure during peak heat.
- Athletes: physical exertion generates internal body heat on top of environmental heat; heat exhaustion can occur even in moderate conditions.
- People with chronic conditions: heart disease, diabetes, obesity, and kidney disease impair the body's cooling efficiency.
Heat index vs. humidex — what's the difference?
The humidex is a similar apparent temperature index used primarily in Canada, developed by Masterton and Richardson in 1979. While the heat index uses relative humidity, the humidex uses dew point temperature as its humidity input, which can produce slightly different values. Both indices capture the same physical reality. That humidity inhibits evaporative cooling, but use different formulas and reference points. The NWS heat index is the standard in the United States; the humidex is the Canadian Meteorological Service standard.