CalcaTools

Heat Index Calculator

A Heat Index Calculator is a free online tool that computes feels like temperature using feels like temperature, heat index formula, Rothfusz equation. Tradespeople, planners, and curious owners use it to sanity-check estimates, plan projects, and answer everyday questions.

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

apps Small jobs, done in one click

Everyday calculators are the workshop drawer of the site — quick utilities for the moments when you need a number now and a phone calculator won't cut it. Age, date difference, GPA, fuel cost, time-to-decimal, password strength — pick a tool and you're one click from your answer.

speed Loads in under a second

Every tool is plain HTML + JavaScript with the calculation logic compiled in. No spinner, no "verifying browser" splash, no five-second wait while ads load. Hit the page, fill the form, get the answer. Slow tools waste your time and ours.

devices Works on every device

Responsive design tested down to a 320px-wide phone screen. The input fields use native number/date keyboards on iOS and Android, the calculate button is thumb-sized, and the result still reads big and bold on the smallest screen. No app to download, ever.

sentiment_satisfied No funny business

No newsletter pop-ups, no "create an account to see the answer", no fake countdown, no fake stock counter, no upsell. Two ad slots per page (clearly labelled as Advertisement) keep the site free — that's the whole business model.

Interpretation guide

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

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 heat index, or "feels-like" temperature, combines air temperature and relative humidity to estimate how hot it feels to the body. High humidity slows sweat evaporation, so the air feels hotter than the thermometer reads.

Example

At 90 °F with 70% relative humidity, the NWS heat index is about 105 °F — squarely in the "Danger" range, where heat exhaustion becomes likely with exertion.

The official formula is the NWS Rothfusz regression, valid roughly above 80 °F and 40% humidity. Direct sun can push the real feels-like temperature up to about 15 °F higher than the shaded value.

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.