Heat Index Calculator
How hot it really feels when humidity joins the heat, with the National Weather Service risk level and what to do about it.
By Manzoor Ahmad, pharmacist turned web designer · Updated How is the heat index calculated?
The National Weather Service combines air temperature and relative humidity in the Rothfusz regression equation. At 90 °F (32 °C) and 60% humidity the heat index is about 100 °F (38 °C), in the “extreme caution” band. At 100 °F and 50% humidity it reaches 118 °F (48 °C), which is “danger”. Full sun can add up to 15 °F more.
Heat index chart (°F)
Air temperature down the side, relative humidity across the top. Values are for shade.
| Air temp | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|
| 80 °F (27 °C) | 80 | 81 | 82 | 83 | 84 | 86 |
| 84 °F (29 °C) | 83 | 85 | 88 | 90 | 94 | 98 |
| 88 °F (31 °C) | 88 | 91 | 95 | 100 | 106 | 113 |
| 92 °F (33 °C) | 94 | 99 | 105 | 112 | 121 | 131 |
| 96 °F (36 °C) | 101 | 108 | 116 | 126 | 138 | 152 |
| 100 °F (38 °C) | 109 | 118 | 129 | 143 | 158 | 176 |
| 104 °F (40 °C) | 119 | 131 | 145 | 161 | 181 | 202 |
| 108 °F (42 °C) | 130 | 144 | 162 | 182 | 205 | 231 |
NWS heat index risk levels
| Heat index | Category | Possible effects with prolonged exposure or activity |
|---|---|---|
| 80–90 °F (27–32 °C) | Caution | Fatigue possible |
| 90–103 °F (32–39 °C) | Extreme caution | Heat cramps and heat exhaustion possible |
| 103–124 °F (39–51 °C) | Danger | Heat cramps or exhaustion likely; heat stroke possible |
| 125 °F+ (52 °C+) | Extreme danger | Heat stroke highly likely |
A pharmacist’s note: medicines and heat
In nine years of counselling patients I have seen how often heat illness is helped along by everyday prescriptions. Diuretics (“water pills”) deplete fluid and salt; anticholinergics and many antipsychotics reduce sweating; beta-blockers blunt the heart’s ability to pump more blood to the skin; ACE inhibitors and ARBs can drop blood pressure further when you are dehydrated; and stimulants raise heat production. If you take any of these, plan for heat days: drink regularly, stay in cooled spaces at the hottest hours and ask your pharmacist whether storage or dose timing should change. Never stop a prescribed medicine without advice. To gauge how sticky the air is, also check the dew point.
Heat index FAQs
The heat index is how hot it feels to the human body when relative humidity is combined with air temperature, measured in the shade. High humidity slows the evaporation of sweat, so the body cools less effectively.
The National Weather Service uses the Rothfusz regression, a nine-term equation in °F: HI = −42.379 + 2.049T + 10.143RH − 0.2248T·RH − 0.00684T² − 0.0548RH² + 0.00123T²RH + 0.00085T·RH² − 0.0000020T²RH², with small adjustments for very dry or very humid air, and a simpler formula below about 80 °F.
About 100 °F (38 °C), which falls in the NWS 'extreme caution' band.
NWS categories: 80–90 °F caution, 90–103 °F extreme caution, 103–124 °F danger, and 125 °F or higher extreme danger. Heat stroke becomes likely with prolonged exposure or exertion in the danger range.
Yes. The heat index assumes shade and light wind. Full sunshine can raise it by up to 15 °F (about 8 °C).
Calculate in °F, then convert: °C = (°F − 32) × 5/9. 35 °C air at 50% humidity has a heat index of about 105 °F, or 41 °C.
Yes. This calculator converts dew point to relative humidity with the Magnus formula, then applies the NWS equation. Dew points above about 21 °C (70 °F) make the heat feel much worse.
Heavy sweating, cold clammy skin, fast weak pulse, nausea, muscle cramps, tiredness, dizziness and headache. Move to a cool place, loosen clothing and sip water; get medical help if symptoms worsen or last more than an hour.
Body temperature of 103 °F (39.4 °C) or higher, hot red dry or damp skin, a fast strong pulse, confusion or loss of consciousness. Heat stroke is a medical emergency: call emergency services and cool the person immediately.
Several common drugs impair sweating, thirst or blood pressure control in heat, including diuretics, anticholinergics, many antipsychotics and antidepressants, beta-blockers, ACE inhibitors and stimulants. Do not stop a prescribed medicine on your own; ask your pharmacist or doctor how to stay safe in hot weather.
Older adults, infants and young children, outdoor workers, athletes, people with heart, lung or kidney disease, and anyone taking medicines that affect hydration or temperature control.
In hot weather, yes: most forecasts' 'feels like' figure is the heat index. In cold, windy weather 'feels like' refers to wind chill instead.
Sources
For information only; not medical advice. If someone shows signs of heat stroke (confusion, hot skin, body temperature 103 °F / 39.4 °C or higher), call emergency services immediately.