Understanding your result
The headline is the heat index, with the category and a plain sentence in the sub-line. The stats give the feels-like temperature, the National Weather Service category and the dew point. The table repeats the inputs, the difference between the heat index and the air temperature, and the method the calculation used, so you can see whether the low- or high-humidity adjustment applied.
The heat index is for shade and light wind; in full sun add up to about 15 °F. Indoors, where air conditioning holds both temperature and humidity down, the heat index usually equals the air temperature, which is the reason air conditioning matters so much in a heat wave.
The dew point calculator describes humid air in the way that stays constant as it warms and cools; the central AC size calculator sizes the equipment that keeps indoor heat index down, and the dehumidifier calculator handles damp spaces the air conditioner does not reach. A ceiling fan makes a room feel cooler without changing the heat index.
How we calculate this
This is the procedure the National Weather Service publishes for the heat index: Steadman’s simple formula for mild conditions, the Rothfusz regression otherwise, and the two adjustments at the extremes of humidity. Temperatures are in °F; metric inputs are converted first and the result converted back. The dew point uses the Magnus approximation. The temperature conversions are on the unit conversions page.
The assumptions behind the numbers
| Assumption | Value | Where it comes from |
|---|---|---|
| Heat index formula | Rothfusz regression with NWS adjustments | NOAA / National Weather Service, heat index equation |
| Simple formula | Steadman | NOAA / NWS |
| Categories | Caution 80–90, extreme caution 91–103, danger 103–124, extreme danger ≥ 125 °F | National Weather Service |
| Conditions | Shade, light wind | Basis of the heat index; full sun adds up to about 15 °F |
| Dew point | Magnus approximation | Alduchov and Eskridge (1996) |
Assumptions last reviewed October 8, 2026.
The heat index does not account for sun, wind, clothing, activity or the person; a worker in full sun on a still day faces a much higher effective heat than the number shows, and the wet-bulb globe temperature is the measure used for work and sport. The guide to humidity and comfort explains how humidity and temperature combine, and the guide on sizing an air conditioner covers keeping the indoor side comfortable.
Two worked examples
A humid summer afternoon
92 °F at 60% relative humidity.
- Simple estimate: 93.7 °F, so the full regression is used
- Heat index: 105 °F (104.7 before rounding)
- Category: Danger; dew point 76 °F
A 92 °F day sounds ordinary, but at 60% humidity it is in the danger band, the conditions in which the National Weather Service issues heat advisories in much of the country.
A very hot, moderately humid day
100 °F at 50% relative humidity.
- Heat index: 118 °F (118.3)
- Category: Danger; dew point 78 °F
- 18 °F above the air temperature
At a 78 °F dew point, sweat barely evaporates. Outdoor work should be limited to early morning, and people without air conditioning are at real risk; cooling centres exist for these days.
Where to find your inputs
Temperature. The shade air temperature from a weather service or a thermometer out of the sun.
Relative humidity. From the same weather report or a hygrometer; indoors, many thermostats show it.
Common mistakes
- Measuring in the sun. The heat index is for shade; a sunlit thermometer reads high.
- Using it as a medical threshold. Individual risk varies widely.
- Relying on a fan above 95 °F. It can add heat; air conditioning or a cool space is safer.
- Ignoring the dew point. Above 70 °F the air is oppressive at any temperature.
- Assuming dry heat is safe. At 110 °F the heat index is still high even at low humidity.
- Forgetting people indoors without AC. Indoor heat during heat waves is a major hazard for the elderly.
Questions people ask
- What is the heat index?
- An estimate of how hot it feels to a person in the shade with a light breeze when humidity is taken into account. Humid air slows the evaporation of sweat, which is the body's main way of cooling itself, so 92 °F at 60% relative humidity feels like about 105 °F. The National Weather Service uses the heat index for its heat advisories and excessive heat warnings.
- What heat index is dangerous?
- The National Weather Service bands are 80 to 90 °F caution (fatigue possible with prolonged exposure or activity), 91 to 103 °F extreme caution (heat cramps and heat exhaustion possible), 103 to 124 °F danger (heat cramps and heat exhaustion likely, heat stroke possible), and 125 °F or more extreme danger (heat stroke highly likely). Full sun can add up to about 15 °F to the heat index.
- How is the heat index calculated?
- With the Rothfusz regression, a nine-term equation in temperature and relative humidity fitted to Steadman's model of human heat balance. The National Weather Service first applies a simpler formula; if that gives under 80 °F it is used, otherwise the full regression, with an adjustment subtracted for very dry air (below 13% humidity, 80 to 112 °F) or added for very humid air (above 85%, 80 to 87 °F).
- Why does humidity make it feel hotter?
- The body cools mainly by sweat evaporating from the skin, and evaporation slows as the air approaches saturation. At high humidity sweat drips instead of evaporating, so the body cannot shed heat as fast and core temperature rises. The dew point is a good companion figure; above about 70 °F the air feels oppressive whatever the temperature.
- Does air conditioning reduce the heat index indoors?
- Yes, on both counts. An air conditioner lowers the temperature and, by condensing moisture on its coil, the humidity, so indoor air at 75 °F and 50% has a heat index of about 75 °F. A fan moves air and helps sweat evaporate, but it does not change the heat index and, above about 95 °F, can add heat. During heat advisories, air conditioned spaces are the main protection.
- Is the heat index a medical measure?
- No. It describes how hot it feels to a typical adult and helps decide when heat is dangerous, but individual risk depends on age, health, medication, activity, clothing and acclimatisation. This page is for understanding the weather and the indoor climate, not medical advice. Follow local National Weather Service advisories and public health guidance during heat waves.