Heat Index Calculator
Calculate the apparent "feels like" temperature from air temperature and relative humidity. Includes heat danger level and heat illness risk assessment.
°C
%
Enter your values above to see the results.
Tips & Notes
- ✓The Heat Index formula is valid only when temperature is at or above 80°F (27°C) and relative humidity is at or above 40%. Below these thresholds, the apparent temperature is close to actual temperature.
- ✓Direct sunlight can increase the heat index by up to 15°F (8°C). Shade, breeze, and lower humidity all reduce perceived heat. Evaporative cooling (sweating) is the primary mechanism — high humidity prevents effective evaporation.
- ✓Acclimatization significantly affects heat tolerance. People who regularly work or exercise in heat develop better sweating efficiency and plasma volume expansion over 10-14 days. Unacclimatized individuals are more vulnerable at any given heat index.
- ✓Wet Bulb Globe Temperature (WBGT) is more accurate than Heat Index for occupational heat stress because it also accounts for solar radiation and wind. Military, athletic, and industrial standards typically use WBGT rather than Heat Index.
- ✓Children and elderly people are more vulnerable to heat illness than healthy adults. Children have higher surface area to mass ratio and less efficient sweating; elderly individuals have reduced cardiovascular response and may take medications that impair heat regulation.
Common Mistakes
- ✗Applying heat index below the valid range — the Rothfusz equation is designed for temperatures above 80°F and humidity above 40%. Using it outside this range produces unreliable results; use actual temperature instead.
- ✗Ignoring the compounding effect of multiple risk factors — high heat index + physical exertion + dark clothing + poor fitness + dehydration dramatically increases heat illness risk beyond what any single factor suggests.
- ✗Confusing heat index with dew point — dew point measures absolute moisture content and is a better indicator of discomfort than relative humidity. At 95°F, 60% RH produces Heat Index 114°F; the dew point in that case is about 77°F.
- ✗Assuming indoor temperatures follow outdoor heat index — indoor heat index depends on air conditioning, ventilation, and occupant activity. A 95°F outdoor air temperature with proper AC may be 72°F indoors with much lower heat index.
- ✗Using Celsius temperature directly in the Fahrenheit formula — the NWS Rothfusz equation requires Fahrenheit input. Convert °C to °F (multiply by 9/5, add 32) before calculating, then convert result back if needed.
Heat Index Calculator Overview
Heat index — also called the apparent temperature or "feels like" temperature — quantifies the combined physiological stress of heat and humidity. The human body experiences heat primarily through its ability to cool itself, and high humidity severely impairs that cooling mechanism.
NWS Heat Index equation (simplified):
HI ≈ 0.5 × [T + 61.0 + (T − 68.0) × 1.2 + RH × 0.094] for T near 80°F | Full Rothfusz equation for T ≥ 80°F
EX: Air temperature 95°F, relative humidity 65% → Heat Index ≈ 114°F. A person outdoors feels temperatures equivalent to 114°F despite the actual temperature being 95°F — because evaporative cooling is severely limited at 65% RH.Dew point from temperature and humidity:
T_d ≈ T − (100 − RH) / 5 (approximate, valid for RH > 50%) | Dew point above 65°F feels oppressive; above 75°F is dangerous
EX: T = 90°F, RH = 70% → T_d ≈ 90 − (100−70)/5 = 90 − 6 = 84°F dew point. This is extremely dangerous — rare even in tropical climates. Compare: T = 90°F, RH = 40% → T_d ≈ 90 − 12 = 78°F — still uncomfortable but manageable.Heat Index danger levels (NWS):
| Heat Index | Danger Category | Risk | Recommended Action |
|---|---|---|---|
| 80–90°F (27–32°C) | Caution | Fatigue possible | Limit prolonged exertion |
| 90–103°F (32–39°C) | Extreme Caution | Heat cramps, exhaustion possible | Rest frequently, drink water |
| 103–125°F (39–52°C) | Danger | Heat stroke probable | Avoid outdoor activity |
| Above 125°F (52°C) | Extreme Danger | Heat stroke imminent | Emergency precautions |
| Temp / RH | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|
| 80°F | 79°F | 81°F | 82°F | 85°F | 87°F | 91°F |
| 85°F | 83°F | 86°F | 90°F | 93°F | 97°F | 102°F |
| 90°F | 88°F | 92°F | 98°F | 104°F | 111°F | 122°F |
| 95°F | 93°F | 99°F | 107°F | 115°F | 127°F | — |
| 100°F | 99°F | 108°F | 117°F | 132°F | — | — |
Frequently Asked Questions
The National Weather Service uses the Rothfusz regression equation with nine terms: HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T×RH − 0.00683783T² − 0.05481717RH² + 0.00122874T²×RH + 0.00085282T×RH² − 0.00000199T²×RH², where T is in °F and RH is in %. This formula is valid for T ≥ 80°F and RH ≥ 40%. For temperatures near 80°F, a simpler Steadman formula is used. The NWS adds adjustment factors for RH < 13% or RH > 85% near certain temperature ranges.
NWS Heat Index danger categories: 80-90°F (27-32°C) — Caution: fatigue possible with prolonged exposure and physical activity. 90-103°F (32-39°C) — Extreme Caution: heat cramps and heat exhaustion possible; heat stroke possible with prolonged activity. 103-125°F (39-52°C) — Danger: heat cramps and heat exhaustion likely; heat stroke probable with continued activity. Above 125°F (52°C) — Extreme Danger: heat stroke imminent. Even the Caution category can be dangerous for elderly people, children, and those with medical conditions.
The human body cools itself primarily through evaporative cooling — sweat evaporates from the skin surface, removing heat. This process requires a vapor pressure gradient: the skin (nearly 100% humidity at skin surface temperature) must have higher humidity than the surrounding air. When relative humidity is high, the air is already close to saturation, limiting evaporation rate. At 95°F and 20% RH, sweat evaporates quickly and the body can maintain core temperature. At 95°F and 80% RH, evaporation is severely limited, core temperature rises, and heat exhaustion becomes likely even at rest.
Heat index combines temperature and relative humidity into one number. Dew point is the absolute moisture content of the air — the temperature at which condensation forms. Dew point is actually a more stable indicator of discomfort because relative humidity changes dramatically with temperature (same dew point gives lower relative humidity at higher temperatures). Comfort thresholds by dew point: below 55°F — comfortable; 55-65°F — somewhat uncomfortable; 65-70°F — oppressive; above 70°F — extremely uncomfortable, dangerous with heat. A dew point of 75°F+ is considered suffocating and is relatively rare but occurs regularly in Gulf Coast states in summer.
At Heat Index above 90°F (Extreme Caution): stay in air-conditioned spaces during peak heat (noon-4 PM); drink water before feeling thirsty — 8 oz every 20 minutes during outdoor activity; wear light-colored, loose, breathable clothing; avoid alcohol and excessive caffeine which promote dehydration; check on elderly neighbors and relatives; never leave children or pets in vehicles. At Heat Index above 103°F (Danger): outdoor physical exertion should be stopped or moved to very early morning hours. Workers in these conditions require mandatory rest breaks in shade or AC, and buddy systems to watch for heat illness symptoms.
Heat exhaustion: heavy sweating, cold/pale/clammy skin, fast/weak pulse, nausea or vomiting, muscle cramps, tiredness, weakness, dizziness, headache. Core temperature below 104°F. Treatment: move to cool location, loosen clothing, apply cool wet cloths, sip water. Heat stroke is a medical emergency: high body temperature above 103°F, hot/red/dry or damp skin, rapid/strong pulse, possible unconsciousness, confusion, slurred speech. Call 911 immediately. While waiting: move to cool location, cool with whatever means available (ice packs to neck/armpits/groin, cool water immersion if possible). Heat stroke can cause organ failure and death within minutes without treatment.