CalcaTools

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

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

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

Let's say you are using the Heat Index Calculator. 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 Enter the values that match your situation into the input fields and press calculate — using realistic numbers makes the result directly useful for you.

Result: The calculator instantly applies the formula NWS Rothfusz regression: HI = -42.379 + 2.04901523*T + 10.14333127*RH - ... (T in °F, RH in %) and returns the result with appropriate precision.

What this means: Read the result in the context of what you are measuring. The step-by-step breakdown lets you confirm the math and understand which input most affects the outcome.

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.
What does the Heat Index Calculator do?
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. The calculator takes your inputs, applies the standard calculation, and returns a clear result so you can make an informed decision without doing the math by hand.
What formula does the Heat Index Calculator use?
The Heat Index Calculator applies the standard formula: NWS Rothfusz regression: HI = -42.379 + 2.04901523*T + 10.14333127*RH - ... (T in °F, RH in %). The tool walks through each step of the calculation so you can verify the numbers yourself.
Is the Heat Index Calculator free and private?
Yes — the Heat Index Calculator. There is no sign-up, no paywall, no trial, and no limit on how many calculations you can run. CalcaTools covers its costs with non-intrusive display advertising, so the calculator itself never asks for payment or restricts any feature.
How accurate is the Heat Index Calculator?
The Heat Index Calculator applies the standard formula: NWS Rothfusz regression: HI = -42.379 + 2.04901523*T + 10.14333127*RH - ... (T in °F, RH in %). It is tested against published worked examples before launch and uses native double-precision arithmetic, so results are accurate to typical precision for the values you enter — with no intermediate rounding that could distort the answer.

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