What the heat index measures
Air temperature alone does not tell you how hot the air feels on skin. Humidity does most of the rest of the work. The heat index, sometimes called the "apparent temperature", combines the two into one figure that estimates what a shaded, lightly breezy day feels like to an average adult body.
It was built by the US National Weather Service from Robert Rothfusz’s 1990 regression, which itself was fitted to an earlier biometeorological model of how a body exchanges heat with humid air. It is the same figure printed on US weather forecasts and broadcast heat warnings, and it is what this calculator reproduces.
How the maths works
Below about 80°F the regression is not reliable, so the service uses a simpler formula first and only switches to the full equation once conditions call for it:
The full regression is the one behind the official chart:
T is air temperature in °F and R is relative humidity as a whole number (70 for 70%, not 0.7). Two small corrections then apply on top of that result: if humidity is under 13% and the temperature sits between 80°F and 112°F, an amount is subtracted, because very dry air lets sweat evaporate faster than the base regression assumes. If humidity is over 85% and the temperature sits between 80°F and 87°F, an amount is added instead, because that combination is muggier than the regression alone captures. Both corrections are small, generally under two or three degrees, but they are why the official chart is not a perfectly smooth curve.
This calculator works entirely in °F internally, because the regression itself is fitted in Fahrenheit and every published coefficient assumes it. Switch the toggle to °C and your entry is converted to °F before anything is calculated, then the result is converted back for display alongside the °F figure.
Why humidity makes heat dangerous
A body sheds extra heat mainly by sweating: moisture sits on the skin, draws heat from it, and carries that heat away as it evaporates. That evaporation needs somewhere for the moisture to go. In dry air it happens quickly. In humid air the surrounding air is already close to carrying as much water vapour as it can, so sweat evaporates slowly, or barely at all in very humid, still conditions.
The practical effect is that the same air temperature can range from manageable to dangerous depending on what the humidity is doing. A dry 100°F day can feel close to its thermometer reading because sweat still evaporates freely. A muggy 90°F day with 70% humidity can push the feels-like figure past 105°F, because the body's main cooling method is barely working. The heat index exists to put a single, comparable number on that difference.
Reading your result
The band above marks your figure against the four official categories. Caution, from 80°F to 90°F, means fatigue is possible with prolonged exposure or activity. Extreme caution, 90°F to 103°F, means heat cramps, heat exhaustion or heat stroke become possible with continued exposure. Danger, 103°F to 124°F, means heat cramps and heat exhaustion are likely and heat stroke is possible. Extreme danger, 125°F and above, means heat stroke is highly likely with continued exposure. Below 80°F the service does not assign a category at all, because the simple formula it uses there tends to land close to the thermometer reading, so there is little practical difference between the heat index and the air temperature to act on.
Remember what the figure assumes: a typical adult, standing or lightly active, in the shade, with a light breeze. Direct sun can add up to a further 15°F to how hot it feels, strong wind can take the edge off a still-air reading, and anyone working hard, dressed heavily, or more heat-sensitive than average should treat their own category as a floor, not a ceiling.
Heat index, wind chill and "feels like"
These three terms get used loosely, but they answer different questions. The heat index is a warm-weather figure built from temperature and humidity, aimed at how the body struggles to cool itself down. Wind chill is a cold-weather figure built from temperature and wind speed, aimed at how quickly exposed skin loses heat. Neither applies outside its own season: wind chill is not defined for hot days, and the heat index is not defined for cold ones. A forecaster's generic "feels like" figure usually just picks whichever of the two applies to that day's conditions, sometimes folding in extra factors such as sunshine. None of the three measures core body temperature directly; they are all practical estimates for comparing days, not medical readings.
Who is most at risk in high heat index conditions
Public health guidance consistently flags a similar group as more vulnerable in hot, humid weather, though individual circumstances vary a great deal: older adults, whose bodies regulate temperature less efficiently; infants and young children; people with heart, lung or kidney conditions; people on medications that affect fluid balance or sweating; outdoor workers and athletes training or competing in the heat; and anyone without reliable access to a cool space. If you or someone you are checking on falls into one of these groups, treat the category above as a stronger prompt to act, such as limiting outdoor exertion, increasing fluids and finding shade or air conditioning, and seek medical help for symptoms such as confusion, nausea, a rapid pulse or a lack of sweating in the heat.
Questions people ask
Why does my figure differ slightly from a chart I've seen elsewhere?
Most published charts round both the inputs and the result to whole numbers before printing them, and some crop the low-humidity and high-humidity corrections out of a simplified version. This calculator applies the full regression and both corrections at whatever precision you enter, then rounds only the final figure, so a one or two degree difference against a rounded chart is expected and not an error.
Why is there no heat index category below 80°F?
The National Weather Service only defines its four categories from 80°F upward, because below that the simple formula it uses tends to land within a degree or two of the actual air temperature. This calculator still shows a figure below 80°F, worked out the same way the service does it, but there is no official category to place it in.
Does the heat index change if it's windy?
The published equation assumes light wind and does not take wind speed as an input, so this calculator does not either. A stronger breeze can make conditions feel somewhat better than the figure suggests by helping sweat evaporate faster, but there is no official adjustment published for it the way there is for full sun.
How much hotter does full sun make it feel?
The National Weather Service states that direct sunshine can add up to around 15°F to the shaded heat index figure. It is a stated maximum rather than a formula, so this calculator does not build it into the number, but the note beside your result flags it every time.
Is the heat index the same as "real feel" or "feels like" on a weather app?
Often, but not always. Many apps display the heat index directly on hot days and switch to a wind-chill-style figure on cold ones under a single "feels like" label, while a few use their own proprietary blend that can include extra factors. If precision matters, this calculator gives you the specific, published heat index rather than an app's blended figure.
Can I use this for someone with a medical condition or for a child?
The equation itself models a typical, healthy adult and does not adjust for age, medication or health conditions. The category and general notes still apply as a starting point, but treat them cautiously and take extra precautions, or get medical advice, for anyone more heat-sensitive than average.
Why enter humidity as a percentage rather than a dew point?
Relative humidity is what the published Rothfusz regression and NWS chart both take as an input, and it is what most forecasts quote directly, so that is what this calculator asks for. Dew point measures a related but different thing, the actual temperature air would need to cool to for moisture to condense out of it.
The heat index is a weather-safety estimate, not a medical measurement. It assumes a typical adult in the shade with light wind. Use it alongside local heat warnings and common sense, and seek medical attention for symptoms of heat illness.