Heat Index Calculator
The 'feels like' temperature from heat and humidity with a risk level.
Heat index
108.3 °F
feels like (42.4 °C)
Risk level
Danger
In Celsius
42.4 °C
Air temp
92 °F
Humidity
65%
AI Breakdown & Smart Takeaway
Plain-English insight on your numbers
Get a personalized explanation of what these results mean — and how to improve them.
How the Heat Index Calculator works
The Heat Index Calculator tells you what hot, humid air actually feels like on your skin — the so-called 'feels like' temperature — and pairs that number with a color-coded risk level so you know whether to slow down, hydrate aggressively, or stay indoors entirely. It's built for athletes, outdoor workers, parents, event planners, and anyone who needs to make a safety-informed decision about heat exposure.
The heat index is rooted in human physiology, not just physics. When your body overheats, it sweats, and that sweat evaporates to carry heat away from your skin. High relative humidity slows that evaporation because the surrounding air is already saturated with water vapor. The result is that your body retains heat it would otherwise shed, making a 95 °F (35 °C) day feel like 107 °F (42 °C) or more when humidity climbs above 60%. The calculator takes your measured air temperature and relative humidity and runs them through the Rothfusz regression equation — the same formula used by NOAA and the National Weather Service — to produce a single apparent temperature that reflects this physiological reality.
The Rothfusz equation is a multi-variable polynomial regression fitted to the original Steadman (1979) human-comfort model. It uses nine coefficients applied to temperature (T), relative humidity (RH), their squares (T², RH²), and their cross-product (T × RH). Because the regression is tuned to temperatures at or above 80 °F (27 °C) and humidity levels of 40% or higher, the calculator automatically falls back to a simpler averaged formula for cooler or drier conditions where the full equation produces less reliable results. This is a subtlety most online tools skip, which is why two calculators can give you different answers for the same inputs.
Once the apparent temperature is computed, the calculator maps it onto the four NOAA risk tiers: Caution (80–90 °F feels-like), Extreme Caution (91–103 °F), Danger (103–124 °F), and Extreme Danger (above 124 °F). These thresholds are based on documented onset points for heat-related illness in the general population — heat cramps, heat exhaustion, and heat stroke. Importantly, the index assumes the person is in the shade with a light wind; direct sunlight can add 10–15 °F (5–8 °C) to the apparent temperature, so the calculator's output is already a conservative floor, not a ceiling.
A common mistake is treating the heat index as a fixed number for the entire day. Both temperature and humidity shift by the hour — humidity often peaks in the early morning while temperature peaks mid-afternoon, creating different but potentially dangerous combinations throughout the day. Another error is ignoring personal risk multipliers: the elderly, infants, people on diuretics or beta-blockers, and those unacclimatized to heat face serious health risks at heat index values that an acclimatized adult athlete might tolerate. Use the calculator's risk level as a baseline and apply your own context on top of it.
Formula
NWS Rothfusz regression of temperature and relative humidity
Pro tips
- Add 10–15 °F to the calculator's output if you or the people you're assessing will be in direct sunlight — the heat index is defined for shaded conditions, and solar radiation is a significant additional load.
- Check the heat index in the morning before outdoor exercise: high overnight humidity paired with a temperature rising through the 80s can push the apparent temperature into the Extreme Caution zone well before the afternoon peak.
- Use the calculator alongside a local dew point reading for a faster gut-check: a dew point above 65 °F generally signals uncomfortable humidity, and above 75 °F the heat index will climb into dangerous territory regardless of air temperature.
- When planning outdoor events or worksites, run the calculator for the forecast temperature and humidity at the hottest hour (typically 2–4 PM local time) rather than the daily average to ensure you're designing for the worst-case exposure window.
- Remember that the heat index applies to healthy adults at rest in the shade; for children, elderly individuals, or people doing strenuous physical work, enforce rest breaks and hydration at risk levels one tier lower than you would for a healthy resting adult.
Key terms
- Heat Index
- — The apparent or 'feels like' temperature that combines air temperature and relative humidity to express how hot the air actually feels to the human body.
- Relative Humidity (RH)
- — The percentage of water vapor present in the air relative to the maximum amount the air could hold at that temperature; high RH impairs evaporative cooling.
- Apparent Temperature
- — A general term for any adjusted temperature metric — including heat index and wind chill — that accounts for environmental factors beyond dry-bulb air temperature alone.
- Rothfusz Regression
- — The NOAA-standard polynomial equation, derived from Steadman's 1979 physiological model, used to calculate heat index from temperature and humidity inputs.
- Evaporative Cooling
- — The body's primary heat-dissipation mechanism in hot weather, in which sweat absorbs skin heat as it evaporates; high humidity reduces evaporation rate and therefore cooling efficiency.
- Heat Stroke
- — A life-threatening medical emergency occurring when core body temperature exceeds 104 °F (40 °C), typically associated with heat index values above 103 °F and often requiring emergency cooling and hospitalization.
