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Thermometer and warm weather

Heat Index Calculator

Feels-like temperature from the official NWS heat index formula

About the Calculator

Temperature alone does not tell you how hot it feels outside. Humidity slows sweat evaporation, so the same thermometer reading can feel much worse when the air is muggy. This calculator uses the National Weather Service method: Steadman's simple formula when conditions are mild, and the full Rothfusz regression with humidity adjustments when heat stress is meaningful. Enter your numbers, choose Fahrenheit or Celsius, and get a clear feels-like result with NWS-style safety guidance.

Enter air temperature and relative humidity to see the NWS heat index — the standard "feels like" temperature used in U.S. weather reports — plus a color-coded safety level for outdoor activity.

Your conditions
Temperature unit

Range: 32130°F

Enter temperature and humidity, then calculate to see your feels-like result.

The Formulas

Steadman (simple): HI = 0.5 × {T + 61 + [(T − 68)(1.2)] + (RH × 0.094)} Average with air temperature: HI_avg = (HI_steadman + T) / 2 (If HI_avg < 80°F, the heat index is HI_steadman.) Rothfusz (when HI_avg ≥ 80°F): HI = −42.379 + 2.04901523(T) + 10.14333127(RH) − 0.22475541(T)(RH) − 0.00683783(T²) − 0.05481717(RH²) + 0.00122874(T²)(RH) + 0.00085282(T)(RH²) − 0.00000199(T²)(RH²) Low humidity adjustment (RH < 13%, 80°F ≤ T ≤ 112°F): subtract [(13 − RH) / 4] × √[(17 − |T − 95|) / 17] High humidity adjustment (RH > 85%, 80°F ≤ T ≤ 87°F): add [(RH − 85) / 10] × [(87 − T) / 5]

Heat Index Chart

The table below matches the familiar NOAA heat index chart: air temperature down the side, relative humidity across the top, and heat index values in each cell. Use it to compare conditions at a glance or to verify a result from the calculator.

Each cell is the calculated heat index (°F) for that air temperature and relative humidity, using the same NWS formulas as the calculator above. Colors follow National Weather Service heat stress categories.

Temp °FRelative humidity (%)
5101520253035404550556065707580859095100
807878787979797980808181828283848485868889
817879797979808081818282838485858687899193
827979808080808181828384848586888990929496
8379808081818182828384858687889091939597100
84808181818282838384858688899092949698101104
858181828282838484858688899193959799102105108
86818283838384858587888991939597100102105109112
878283838484858687888991939598100103106109113116
8883848485858687888991939598100103106110113117121
89848485858687888991939597100103106110113117122126
908485868687888991929597100103106109113117122127132
918586878788899092949799102105109113117122126132137
9286878888899092949699101105108112116121126131137143
93878889899092939598101104107111116120125130136142149
948789909091939597100103106110114119124129135141148155
958889919193949699102105109113118123128134140147154161
9689909293949698101104108112116121126132138145152160168
97909193949597100103106110114119125130136143150158166174
98919294959799102105109113117123128134141148155164172181
999293959698101104107111115120126132138145153161170179188
10093949697100102106109114118124129136143150158167176185195
10193959799101104108112116121127133140147155164173182192203
102949698100103106110114119124130137144152160169179189199210
103959799101104108112116122127134141148157165175185195206218
1049698100103106110114119124131137145153161171181191202214226
1059799102104108112116121127134141149157166176187198209221234
10698100103106109114119124130137145153162172182193204216229243
10799101104107111116121127134141149157167177188199211224237251
108100102105109113118123130137144153162172182193205218231245260
109100103107110115120126133140148157167177188200212225239254269
110101104108112117122129136143152161171182194206219233247262278
111102106109114119125131139147156166176187199212226240255271288
112104107111115121127134142150160170181193205219233248263280297
113104108112117123129137145154164175186198211225240256272289307
114105109113119125132140148158168179191204218232247264280298317
115106110115121127134143152162173184197210224239255272289308327
116107111116122129137146155166177189202216231246263280298317337
117108112118124132140149159170181194208222237253271289307327348
118108113119126134142152162174186199213228244261279297317337358
119109114121128136145155166178191204219235251268287306326347369
120110116122130138148158170182196210225241258276295315336358380
121111117124132141151162174187200215231248265284304324346368392
122111118125134143154165178191205221237254273292312333356379403
123112119127136146157169182196211226243261280300321343366390415
124113120129138148160172186200216232250268288308330353376401427
125114121130140151163176190205221238256275296317339362387412439

Heat index / heat disorder legend

CategoryHeat index (°F)Heat disorder risk
Caution80–90Fatigue possible with prolonged exposure. Take water breaks every 20–30 minutes.
Extreme Caution91–103Heat cramps and heat exhaustion are possible. Limit strenuous outdoor activity.
Danger104–124Heat exhaustion likely. Heat stroke possible. Avoid outdoor activity — stay in cool spaces.
Extreme Danger125+Heat stroke highly likely. This is a medical emergency risk. Seek air conditioning immediately.

Chart values are computed with this site's NWS implementation and rounded to whole degrees, similar to the NOAA heat index chart. Direct sun, wind, and individual health factors are not included.

Official guidance and sources

Heat index values on this page follow the National Weather Service Steadman and Rothfusz methods, including published low- and high-humidity adjustments. For official forecasts, watches, and warnings in your area, rely on your local NWS office or weather.gov.

FAQ

What is a dangerous heat index?

The NWS treats 103°F and above as Danger, and 125°F or higher as Extreme Danger. At those levels, heat exhaustion or heat stroke become much more likely during outdoor exertion. Use the safety badge on this calculator as a quick reference, and follow local heat advisories.

How is heat index calculated?

The NWS starts with a simple Steadman equation, averages it with the actual temperature, and if that average is at least 80°F, applies the Rothfusz formula with up to nine polynomial terms. Low- and high-humidity adjustments apply in specific temperature and RH ranges. This page can show each step when you turn on Show the Math.

What is the difference between heat index and feels like?

“Feels like” is a general label apps use for perceived temperature. Heat index is the specific NWS model based on temperature and relative humidity in the shade. Other “feels like” values may also fold in wind, sun, or different formulas.

Does heat index apply in the shade?

Yes. The heat index is defined for shaded, ventilated conditions. Direct sun can make it feel 5–15°F hotter than the listed index, while wind can make it feel somewhat cooler.

What was the highest heat index ever recorded?

The highest widely cited heat index is about 178°F, recorded in Dhahran, Saudi Arabia on July 8, 2003. That afternoon the air temperature was 108°F with a world-record dew point near 95°F—not 95% relative humidity. At 108°F, a 95°F dew point works out to roughly 68% RH; plug those numbers into the NWS formula and you get about 177°F, matching the reported value. If you mistakenly enter 95% humidity instead, the calculator shows about 245°F, but that does not reflect what was measured that day. Dew point (°F) and relative humidity (%) are different inputs. Other Persian Gulf heat waves have also produced heat index values well above 150°F when extreme moisture is present.

Could it ever be 125°F with 100% humidity?

It depends what you mean. A natural air temperature of 125°F at 100% relative humidity is effectively impossible on Earth: saturated surface air cannot sustain that pairing because moisture and temperature are physically linked (wet-bulb temperatures in the worst real events stay near the mid-90s °F). A heat index of 125°F at 100% humidity is plausible at lower air temperatures—for example, about 89°F with 100% RH yields a heat index near 126°F on the NWS formula. Typing 125°F and 100% RH into any heat index calculator extrapolates beyond the range the equation was built for, so treat that output as mathematical, not as realistic weather.

Tips & Strategies

Quick tip. Heat index is most meaningful at 80°F (27°C) and above with meaningful humidity.

Quick tip. Shade lowers radiant heat but does not change the heat index value — it still uses air temperature and humidity.

Quick tip. Re-check after weather changes; a small humidity shift can move the index several degrees.

Quick tip. Pair this tool with hydration and rest breaks when the danger level is Caution or higher.

Cross-check when the decision matters. Run a second scenario with rounded inputs or a different path to the same quantity so you do not rely on a single fragile chain of arithmetic.