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Heat Index Calculator | Compute Feels-Like Temp and Heat Risk

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Calculates the heat index — how hot the air actually feels — from temperature and humidity. Enter the air temperature and relative humidity, and the tool applies the National Weather Service Rothfusz equation to return the feels-like temperature, then classifies it into caution, extreme caution, danger, or extreme danger categories.

Last updated: June 2026 · Free · No sign-up required

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Results

Enter values above and click Calculate to see your result instantly.

Quick reference

Heat indexRisk category
80 – 90 °FCaution — fatigue possible
90 – 103 °FExtreme caution — cramps, exhaustion
103 – 124 °FDanger — heat exhaustion likely
125 °F and upExtreme danger — heat stroke risk

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Interpretation guide

ConditionEffect
Higher humiditySweat evaporates slower → feels hotter
Applies above~80 °F and 40% humidity
Direct sunCan add up to ~15 °F

lightbulb Worked example: 92°F and humid

It is 92°F outside with 70% relative humidity — a typical July afternoon in the US Southeast. You want to know how hot it actually feels before going for a run.

Result: The Rothfusz equation combines temperature and humidity in a nine-term polynomial. Plugging in T = 92 and RH = 70 gives −42.379 + 188.51 + 710.03 − 1447.42 − 57.88 − 268.60 + 728.00 + 384.45 − 82.53 ≈ 112°F. (No humidity adjustment applies here, since RH is under 85%.)

What this means: 112°F falls in the NWS 'danger' band (103–124°F), where heat exhaustion is likely with prolonged activity. The thermometer says 92°F, but your body's cooling is impaired enough that the run should move to early morning — and this is why the heat index, not the temperature, drives official heat advisories.

Formula & methodology

Formula: NWS Rothfusz regression: HI = -42.379 + 2.04901523*T + 10.14333127*RH - ... (T in °F, RH in %)

How the heat index is calculated

The tool applies the NWS Rothfusz regression HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T×RH − … — a multi-term polynomial combining temperature T in °F and relative humidity RH as a percentage. It estimates the perceived temperature from humidity's effect on sweat evaporation.

Example: at 95°F with 60% humidity, the heat index reaches about 112°F — the body's cooling mechanism is far less effective than the thermometer alone suggests.

Reading the result: the heat index is the 'feels like' temperature used in heat advisories; above 105°F, heat stroke becomes a real risk with prolonged exposure. It applies to people and animals — objects do not feel it — and it matters most in humid climates where sweat cannot evaporate.

Authoritative source: timeanddate.com

Frequently asked questions

What is the heat index?
The heat index is the 'feels-like' temperature that combines the actual air temperature with relative humidity. Because high humidity slows the evaporation of sweat — the body's main cooling method — humid air feels hotter than dry air at the same temperature. The National Weather Service uses it to warn about heat-related health risks.
How is the heat index calculated?
It uses the NWS Rothfusz regression, a multi-term equation taking temperature (°F) and relative humidity (%) as inputs, with adjustments at very high or low humidity. The formula is too complex for mental math, but at 90 °F and 70% humidity it returns about 105 °F. It is valid mainly above 80 °F and 40% humidity.
Why does humidity make it feel hotter?
Your body cools itself by evaporating sweat. When the air is already humid, it holds less additional moisture, so sweat evaporates slowly and removes less heat. The result is that your body struggles to cool down, making the same temperature feel significantly hotter and raising the risk of heat illness.
What heat index is dangerous?
Caution begins around 80-90 °F. From 90-103 °F is 'extreme caution' (cramps and exhaustion possible), 103-124 °F is 'danger' (heat exhaustion likely with activity), and 125 °F or above is 'extreme danger' with a high heat-stroke risk. Limit exertion, hydrate, and seek shade or air conditioning as the index climbs.
Does the heat index account for sun exposure?
No. The official heat index assumes shady, light-wind conditions. Standing in full sunshine can raise the effective feels-like temperature by up to about 15 °F. So on a sunny day, the real heat stress on your body can be considerably higher than the reported shaded heat index.
When is the heat index more useful than the thermometer?
On humid days, when the thermometer understates how hot your body actually feels — for example, 92°F air at 70% humidity feels like 112°F, which changes how much water you need and how long you can safely exercise. Use the heat index to plan workouts, yard work, and outdoor events in summer. In dry or mild weather the two numbers converge and the thermometer alone is fine.
Does the heat index work for temperatures below 80°F?
Not reliably. The Rothfusz equation is validated for temperatures at or above 80°F with relative humidity of at least 40% — below that range, sweat evaporation is not impaired enough to matter and the air temperature is the better measure. The calculator will still produce a number outside that range, but treat it as informational only; heat risk is minimal there.
Why does full sun feel so much worse than the heat index?
The official heat index assumes shade and a light breeze. Direct sunlight can add up to about 15°F to what your body experiences, because radiant heat loads you on top of the warm, humid air. On a sunny day with a computed heat index of 100°F, the real heat stress can feel closer to 115°F — another reason to schedule outdoor activity for morning or evening.
How does the heat index trigger official heat warnings?
The National Weather Service issues heat advisories when the heat index is forecast around 100°F or higher, and excessive heat warnings at more extreme levels — exact thresholds vary by region. When the calculator shows 103°F or above, the 'danger' band, limit outdoor exertion, drink water before you feel thirsty, and check on children, older relatives, and anyone without air conditioning.

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