Heat Index Calculator — What Hot and Humid Weather Really Feels Like

35°C in dry Arizona heat is very different from 35°C in coastal Florida. The humidity in Florida prevents your sweat from evaporating efficiently — your body's primary cooling mechanism stalls, and your core temperature rises faster. The heat index puts a number on that difference.

Check the heat index for any temperature and humidity combination at CalcHub.

Why Humidity Matters for Heat

Human cooling works by sweating. When sweat evaporates, it carries heat away from your body. In dry air, sweat evaporates quickly and cooling is efficient. When relative humidity is high (70%+), the air is already saturated with moisture and sweat evaporates slowly. Your body works harder, produces more sweat, and still struggles to cool down.

The heat index (developed by Robert Steadman, 1979) accounts for this by calculating the "apparent temperature" — what the combination of heat and humidity feels like to the human body.

Heat Index Table (°C)

Air Temp	40% RH	50% RH	60% RH	70% RH	80% RH	90% RH
27°C	27	28	29	30	31	32
30°C	30	32	34	36	38	40
32°C	33	35	38	41	44	47
35°C	37	41	45	49	54	59
38°C	41	47	53	60	—	—

Note: Dashes indicate conditions so extreme that the formula may underestimate risk.

Heat Illness Risk Levels

Heat Index	Risk Level	What to Expect
27–32°C	Caution	Fatigue possible with prolonged activity
32–41°C	Extreme Caution	Heat cramps, heat exhaustion possible
41–54°C	Danger	Heat exhaustion likely; heat stroke possible
54°C+	Extreme Danger	Heat stroke highly likely

How to Use the Calculator

1. Enter air temperature in °C or °F
2. Enter relative humidity percentage
3. Get heat index apparent temperature and risk level

The CalcHub calculator also shows practical advice for each risk level.

High-Risk Groups

Children, the elderly, and people taking certain medications (diuretics, antihistamines, blood pressure drugs) are more vulnerable to heat illness because their thermoregulatory systems are less efficient. Outdoor workers and athletes exercising in humid heat need to pay particular attention — physical exertion generates internal heat on top of ambient thermal load.

The risk compounds quickly: a hard workout at a heat index of 38°C creates effective thermal stress equivalent to sitting in a sauna.

What is the difference between heat exhaustion and heat stroke?

Heat exhaustion is the warning stage: heavy sweating, cool pale clammy skin, weakness, nausea. The body is struggling but still able to cool itself. Move to shade, drink fluids, rest. Heat stroke is a medical emergency: body temperature above 40°C, hot dry skin (sweating stops), confusion, loss of consciousness. Call emergency services immediately — heat stroke can be fatal within hours.

At what heat index should outdoor events be cancelled?

There's no universal standard, but many organized sports leagues and outdoor event guidelines use 40–41°C heat index as the threshold for cancellation or mandatory breaks. Military and occupational health guidelines often set limits at a heat index of 39°C for heavy physical work with mandatory rest periods.

Does wind help in humid heat?

Yes, but less than in dry heat. Wind still helps evaporation somewhat even in humid conditions, which is why a breeze provides relief even on a muggy day. However, in extreme heat (above 35°C air temp), if the wind is also hot, it can add heat to your skin faster than evaporative cooling can remove it — the opposite of relief.

Related Calculators

• Wind Chill Calculator — the cold-weather equivalent
• Dew Point Calculator — understand humidity's moisture content
• UV Index Calculator — sun exposure risk alongside heat