Heat Index Calculator
Calculate the heat feels-like temperature using the official NWS Rothfusz formula. Enter air temperature with relative humidity or dew point — get heat index and danger level instantly.
Heat Index Calculator
NWS Rothfusz equation
Actual Temp
95°F
Feels Like
121°F
Heat Index
Danger — Danger
Avoid strenuous outdoor activity — move to air conditioning
SAFETY REFERENCE
Heat Index Risk Levels
Official NWS heat danger categories and recommended precautions.
| Heat Index (°F) | Heat Index (°C) | Category | Health Effects | Action |
|---|---|---|---|---|
| 80-90°F | 27-32°C | Caution | Fatigue possible with prolonged activity | Stay hydrated, take breaks in shade |
| 91-103°F | 33-39°C | Extreme Caution | Heat cramps and heat exhaustion possible | Limit strenuous activity, monitor closely |
| 103-124°F | 40-51°C | Danger | Heat exhaustion likely, heat stroke possible | Avoid strenuous outdoor activity |
| 125°F+ | 52°C+ | Extreme Danger | Heat stroke highly likely | Stay indoors in AC, call 911 if symptoms |
REFERENCE CHART
Heat Index Chart (°F)
Temperature is column · Relative Humidity is row · color = risk category.
| RH \ °F | 80°F | 85°F | 90°F | 95°F | 100°F |
|---|---|---|---|---|---|
| 40% | 80 | 84 | 91 | 99 | 109 |
| 50% | 81 | 86 | 95 | 105 | 118 |
| 60% | 82 | 89 | 100 | 113 | 129 |
| 70% | 83 | 93 | 106 | 123 | 143 |
| 80% | 84 | 97 | 113 | 134 | 158 |
| 90% | 86 | 102 | 122 | 147 | 176 |
| 100% | 89 | 108 | 132 | 161 | 195 |
☀️ Direct Sun Adds Up To 15°F More
The NWS heat index formula assumes shaded conditions with light wind. Standing in direct sunlight can add up to 15°F (about 8°C) to your calculated value — a 100°F heat index in the shade could feel like 115°F in full sun, potentially life-threatening territory.
🥵 Heat Exhaustion vs Heat Stroke
Heat exhaustion: heavy sweating, pale clammy skin, nausea, dizziness, muscle cramps. Move to a cool area, sip water, apply cool wet cloths.
Heat stroke (emergency): body temp above 104°F, hot dry or profuse sweating skin, confusion, throbbing headache. Call 911 immediately — do not give fluids if the person is unconscious.
🧊 Staying Safe in High Heat Index
- Hydration: drink water before you feel thirsty — 1 cup every 20 minutes during activity
- Acclimatization: it takes 10-14 days for the body to adjust to heat
- Timing: schedule outdoor activity for early morning or evening, out of peak sun
- Clothing: light, loose, light-colored fabric helps sweat evaporate
How the Heat Index Is Calculated
The modern heat index traces back to Robert G. Steadman's 1979 biometeorological model of apparent temperature, later fitted to a practical polynomial — the Rothfusz regression — by the National Weather Service so it could be computed quickly in the field.
The NWS Rothfusz Equation (°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
where T = air temperature (°F), RH = relative humidity (%)
Adjustment if RH<13% and 80≤T≤112: subtract ((13−RH)/4)×√((17−|T−95|)/17)
Adjustment if RH>85% and 80≤T≤87: add ((RH−85)/10)×((87−T)/5)The Metric Version (°C)
HI = -8.78469475556 + 1.61139411·T + 2.33854883889·RH − 0.14611605·T·RH
− 0.012308094·T² − 0.0164248277778·RH² + 0.002211732·T²·RH
+ 0.00072546·T·RH² − 0.000003582·T²·RH²
where T = air temperature (°C), RH = relative humidity (%)Our calculator converts °C inputs to °F, computes with the Rothfusz equation above, and converts the result back — this keeps a single source of truth for the math while still supporting metric units.
When the Formula Applies
The Rothfusz regression is only meaningful at 80°F (27°C) and above. Below that, we use the National Weather Service's simpler average formula, and below 80°F entirely the heat index is treated as equal to the actual air temperature.
Converting Dew Point to Relative Humidity
RH = 100 × exp((17.625×Td)/(243.04+Td)) / exp((17.625×T)/(243.04+T))
where T = air temperature (°C), Td = dew point (°C)When you switch to Dew Point mode, we run this Magnus-formula approximation first, then feed the resulting humidity into the heat index equation above.
Why Humidity Makes Heat More Dangerous
Your body regulates core temperature primarily through sweat evaporation. When humidity is high, that cooling mechanism loses efficiency at exactly the moment your body needs it most.
The Dew Point Threshold
Dew point is a more reliable comfort indicator than relative humidity because it measures absolute moisture in the air rather than a ratio. Once dew point climbs above roughly 65°F (18°C), sweat evaporation starts to feel noticeably impaired — above 75°F (24°C) it becomes oppressive.
Heat Acclimatization
The human body can adapt to working in heat through a gradual process called heat acclimatization, which typically takes 10-14 days of repeated exposure. Newcomers to a hot climate — or anyone returning from a cooler stretch of weather — face meaningfully higher risk during that adjustment window.
The Urban Heat Island Effect
Asphalt and concrete absorb solar radiation during the day and re-radiate it after sunset, which can raise nighttime lows in dense urban areas 2-10°F (1-6°C) above surrounding rural temperatures — denying the body the overnight cooldown it needs to recover from a hot day.
— FAQ