Heat Index Calculator
Enter air temperature (°F or °C) and relative humidity to find the heat index — how hot it actually feels — with the matching NWS heat risk level.
How Hot Does It Feel? Enter Temperature and Humidity
Heat Index Formula
Where:
- \(T\) — air temperature in degrees Fahrenheit (°F)
- \(\mathrm{RH}\) — relative humidity in percent (%)
- \(\mathrm{HI}\) — heat index in degrees Fahrenheit (°F)
This is the Rothfusz regression used by the US National Weather Service (NWS). It is applied only when conditions are hot enough to matter; for cooler conditions the NWS first evaluates a simpler approximation,
\[ \mathrm{HI} = 0.5\left(T + 61.0 + (T - 68.0) \cdot 1.2 + \mathrm{RH} \cdot 0.094\right) \]
and switches to the full regression only if the average of this value and \(T\) is 80°F or higher. Two small corrections are added at very low humidity (below 13%) or very high humidity (above 85%) in the 80–112°F range.
Heat Index – Calculation Example
Example: the air temperature is 90°F and the relative humidity is 60% — a typical humid summer afternoon (these are the calculator's default values).
The simple formula first gives \(\mathrm{HI} \approx 91.5^\circ\mathrm{F}\). Since \((91.5 + 90)/2 \approx 90.8 \ge 80\), the full Rothfusz regression is used:
\[ \mathrm{HI} = -42.379 + 2.04901523(90) + 10.14333127(60) - 0.22475541(90)(60) \\ - 0.00683783(90)^2 - 0.05481717(60)^2 + 0.00122874(90)^2(60) \\ + 0.00085282(90)(60)^2 - 0.00000199(90)^2(60)^2 \approx 99.7^\circ\mathrm{F} \]
Result: the heat index is about 99.7°F (37.6°C) — an "Extreme caution" level, where heat cramps and heat exhaustion are possible with prolonged exposure and physical activity.
How to Use the Heat Index Calculator
- Enter the Air temperature (T) in the field. This should be the shaded air temperature measured by a standard thermometer, not in direct sun.
- Choose the unit — °F or °C — from the dropdown next to the temperature field. Celsius values are converted internally, since the NWS equations are defined in Fahrenheit.
- Enter the Relative humidity (RH) as a percentage between 0 and 100. If you only know the dew point, many weather apps and reports list relative humidity directly.
- Read the results: the heat index in both Fahrenheit and Celsius, the heat risk level, and details about which formula and validity range were used.
What Is the Heat Index?
The heat index is the "feels-like" temperature: what the combination of heat and humidity feels like to the human body, compared with an equivalent dry-air temperature. On a 90°F day with 60% humidity, the air does not feel like 90°F — it feels closer to 100°F, because your body cannot cool itself as effectively as it could in drier air.
The reason is evaporative cooling. Your body's main defense against heat is sweating: as sweat evaporates from your skin, it carries heat away. When the air is already nearly saturated with moisture, sweat evaporates slowly or not at all, so this cooling mechanism fails and body temperature rises. That is why a dry 100°F day in the desert can be more tolerable than a humid 90°F day in the Southeast.
Because it reflects real physiological stress rather than just thermometer readings, the heat index is the number public health agencies use for heat warnings. The NWS classifies heat index values into risk bands, each with recommended precautions:
| Heat index | Category | Main effect |
|---|---|---|
| 80–90°F (27–32°C) | Caution | Fatigue possible with prolonged exposure and activity |
| 90–103°F (32–39°C) | Extreme caution | Heat cramps and heat exhaustion possible |
| 103–125°F (39–52°C) | Danger | Heat exhaustion likely; heat stroke possible |
| ≥ 125°F (≥ 52°C) | Extreme danger | Heat stroke imminent with continued exposure |
A heat index below 80°F is generally comfortable for most people, which is why the full regression is only applied above that threshold.
Assumptions, Limitations and Practical Tips
- Shade and light wind. The heat index assumes you are in the shade with a light breeze. Full sunshine can make it feel up to 15°F (about 8°C) hotter than the calculated value.
- Valid range. The Rothfusz regression is empirically valid for temperatures of 80–112°F and humidities of 20–99%. Outside these ranges the result is an estimate, and below 80°F the simplified formula simply returns a value close to the air temperature.
- It describes typical adults in light clothing. Sensitivity to heat varies with age, fitness, hydration, medication, and acclimatization — children, older adults, and people with chronic conditions can experience heat stress at lower heat index values.
- Not a wind-chill equivalent. The heat index applies to hot weather. In cold and windy conditions, a different model (wind chill) is used.
- Watch the trend, not just the number. A jump in humidity overnight can push an unchanged air temperature into a higher risk category the next day, so re-check the heat index when planning outdoor work or exercise.
For other unit conversions you may need alongside weather and science work, try our PPM to percent converter (useful for moisture and concentration figures) or the time units converter when tracking exposure durations. If you work with heat transfer calculations, the thermal conductivity converter may also come in handy.
Frequently Asked Questions
How can I estimate the heat index without a calculator?
For a quick mental estimate, remember that humidity starts to matter noticeably above about 85–90°F (29–32°C). At 90°F the heat index is close to the air temperature below 30% humidity, near 100°F at 55–60%, and can reach 110°F (43°C) or more above 75% humidity. For anything hotter or more humid, or when safety decisions depend on it, use the calculator or an official NWS chart rather than a rough guess.
At what heat index should I cancel or reschedule outdoor activity?
Most guidance says to use extra caution from a heat index of 90°F (32°C) and to limit or postpone strenuous outdoor activity at 103°F (39°C) and above. If you must be active, do it early in the morning, take frequent shade breaks, and drink water before you feel thirsty. Athletes, outdoor workers, children, and older adults should use stricter limits — heat illness can occur below these thresholds too.
Why does my weather app show a different "feels like" number?
Apps may use different models or inputs. Some combine heat index with wind chill into a single "feels like" figure, others use the humidex (common in Canada) or the apparent temperature model, which includes wind speed. Small differences also come from rounding, slightly different temperature or humidity data, and whether the app reports conditions in shade or sun. Differences of a few degrees between sources are normal.
What is the difference between heat index, humidex, and wet-bulb temperature?
All three measure heat stress but scale differently. The heat index compares conditions to an equivalent dry-air temperature in shade, while the humidex (used in Canada) is referenced to dew point and tends to give similar but not identical values. Wet-bulb temperature measures the theoretical cooling limit of evaporation and is the metric most used for extreme-heat safety research; roughly 95°F (35°C) wet-bulb is considered the limit of human survivability.
How do I find relative humidity if I only know the dew point?
Relative humidity can be computed from temperature and dew point using the Magnus formula: \( \mathrm{RH} = 100 \cdot e^{\frac{17.625 \cdot D}{243.04 + D}} / e^{\frac{17.625 \cdot T}{243.04 + T}} \), where \(D\) is dew point and \(T\) is air temperature in Celsius. Alternatively, note that RH is 100% when dew point equals air temperature, and drops as the gap between them widens.
Can the heat index be lower than the actual air temperature?
Yes. In dry air, typically below about 40% relative humidity and above 80°F (27°C), sweat evaporates so efficiently that the full Rothfusz regression actually returns a value below the air temperature. The NWS low-humidity adjustment partially corrects this. This is common in desert climates — a 100°F day at 10% humidity may feel closer to 95°F, though the air temperature itself is still dangerous.
Does the heat index apply in direct sunlight or strong wind?
No — the standard heat index assumes shade and a light breeze. In full sun conditions can feel up to 15°F (8°C) hotter than the calculated value. Wind has a smaller effect in hot weather than in cold: a strong breeze slightly helps sweat evaporate, but in very humid air it adds little relief, so the calculated heat index remains a good baseline for planning.