🌡️ Heat Index & Real-Feel Calculator
A heat index calculator running the full NWS procedure, including the low-humidity and high-humidity corrections most calculators skip.
Heat cramps and heat exhaustion likely; heat stroke possible with prolonged exposure or activity.
In shade the heat index is 121°F / 49°C, against an air temperature of 96°F / 36°C — humidity is adding 25°F. In full sun, NWS says it can feel like 136°F / 58°C.
NWS algorithm, including both adjustments and the below-80°F path. 96°F at 65% humidity gives 121°F, matching the example NWS publishes. The categories above are the NWS’s long-standing four; its current chart no longer prints them, noting that “the Heat Index level that is considered dangerous can vary by person and region”. The equation · NWS heat tools
What Heat Index & Real-Feel Calculator Does
The heat index answers how hot it feels once humidity is accounted for. Sweating is how the body sheds heat, and sweat only cools you when it evaporates. Humid air is already close to saturated, so evaporation slows, and the same air temperature becomes far more dangerous. At 96°F the heat index is 96°F in dry air at 30% humidity and 121°F at 65%.
The National Weather Service computes it in three steps, and most calculators only implement the middle one. First a simple formula is evaluated and averaged with the air temperature; if that lands below 80°F, that is the answer and the regression is not used. Above 80°F the Rothfusz regression applies, with a downward correction in very dry air and an upward one in very humid air near 80°F.
Skipping those corrections is the standard error. At 95°F and 5% humidity the regression alone reads 90.2°F while the NWS procedure gives 88.2°F — two degrees of overstatement in exactly the desert conditions where people check.
The larger caveat is one the NWS states plainly and almost nobody repeats: heat index values assume shade and light wind. Standing in full sun can add up to 15°F. That is a bigger correction than anything in the equation, and it is why a 103°F heat index reading is not a reason to work outside unshaded.
How to Use Heat Index & Real-Feel Calculator
- Enter ambient air temperature in Fahrenheit (≥80°F)
- Input relative humidity percentage
- View the apparent feels-like temperature and heat danger classification
Formula Used by Heat Index & Real-Feel Calculator
The full NWS procedure, not just the regression
HI_simple = 0.5·(T + 61 + (T−68)·1.2 + RH·0.094); if (HI_simple + T)/2 < 80 use it, else use the Rothfusz regression plus adjustments
- T
- air temperature in °F
- RH
- relative humidity in percent
- dry adjustment
- RH < 13% and 80 ≤ T ≤ 112: subtract ((13−RH)/4)·√((17−|T−95|)/17)
- humid adjustment
- RH > 85% and 80 ≤ T ≤ 87: add ((RH−85)/10)·((87−T)/5)
Worked example
Air temperature 96°F, relative humidity 65% — the worked example NWS publishes.
- Simple formula: 0.5 × (96 + 61 + 33.6 + 6.11) = 98.4; averaged with 96 gives 97.2, which is ≥ 80
- So the Rothfusz regression applies
- RH is neither below 13% nor above 85%, so no adjustment
Result: 121°F, matching the NWS figure exactly.
How much humidity adds at the same air temperature
Heat index in °F, computed with the full NWS procedure. A dash means the combination is outside the range of conditions the regression was fitted to — the NWS chart leaves those cells blank too.
| Air temp | 40% RH | 55% RH | 70% RH | 85% RH |
|---|---|---|---|---|
| 84°F | 83 | 86 | 90 | 96 |
| 88°F | 88 | 93 | 100 | 110 |
| 92°F | 94 | 101 | 112 | 126 |
| 96°F | 101 | 112 | 126 | — |
| 100°F | 109 | 124 | — | — |
What the categories mean
The NWS’s long-standing four bands. Its current chart no longer prints them, noting that the level considered dangerous varies by person and region.
| Heat index | Category | Risk with prolonged exposure or activity |
|---|---|---|
| 80–90°F | Caution | Fatigue possible |
| 90–103°F | Extreme caution | Heat cramps and heat exhaustion possible |
| 103–124°F | Danger | Heat cramps and exhaustion likely; heat stroke possible |
| 125°F and above | Extreme danger | Heat stroke highly likely |
How to Read Your Result
Add 15°F if you are in the sun
This is the single most useful correction and the least published. The index was derived for shade and light wind; NWS says direct sunshine can raise the felt temperature by up to 15°F. A 100°F heat index in shade is an extreme-danger 115°F on an unshaded site.
The chart and the algorithm disagree at high humidity
Above 85% humidity between 80 and 87°F, the NWS documents an upward adjustment, but the cells on its published chart match the unadjusted regression. This calculator follows the documented algorithm, so it can read one to two degrees higher than the chart in that one corner. Neither is a mistake; they are two different published artefacts.
It describes air, not people
The index assumes a notional adult in shade. Age, medication, acclimatization, fitness, clothing and whether you are working all change the actual risk substantially. NWS now also publishes HeatRisk and Wet Bulb Globe Temperature, which account for more of that.
Limitations & Accuracy Notes
- Assumes shade and light wind. Full sun adds up to 15°F, and strong hot dry wind carries its own risk that the index does not capture.
- The Rothfusz regression is not valid outside the range of conditions Steadman studied, so extreme combinations are extrapolation.
- Below about 80°F the simpler formula is used and humidity barely matters — the number is not telling you much there.
- One number cannot represent individual risk. Age, health, medication, acclimatization and exertion all matter more than a few degrees of index.
- Not medical advice. If someone shows signs of heat exhaustion or heat stroke, treat it as an emergency rather than consulting a calculator.
Frequently Asked Questions
What is the Heat Index?
What temperature triggers the Heat Index formula?
Why does humidity make heat so much worse?
When does the formula apply?
Does it assume shade or sun?
At what point does it become dangerous?
Why does wind not appear in the calculation?
Is my data stored?
References & Further Reading
- NOAA Weather Prediction Center — the heat index equation — The Rothfusz regression, both adjustments, and the below-80°F procedure, exactly as implemented here
- NWS — heat forecast tools — Source of the 96°F/65% = 121°F example and the "full sunshine can increase heat index values by up to 15°F" figure