Find the "feels like" temperature from air temperature and humidity
The heat index — commonly called the "apparent temperature" or "feels like" temperature — tells you how hot it actually feels to the human body when humidity is factored in alongside the air temperature. While a thermometer measures the temperature of the air alone, your body cools itself by sweating. When relative humidity is high, that sweat cannot evaporate efficiently, making the air feel far hotter than the thermometer reading suggests. At 90°F (32°C) with 70% relative humidity, the heat index can exceed 105°F (41°C) — deep in the Danger zone even though the actual air temperature seems manageable. This calculator uses the official NOAA Rothfusz regression formula, the same equation used by the National Weather Service (NWS) for all official U.S. heat advisories. You can enter temperature in either Fahrenheit or Celsius, and provide humidity as a percentage or as a dew point temperature — whichever is more convenient. The tool automatically applies the NOAA correction factors for unusually low humidity and for the high-humidity, moderate-temperature edge case, matching the precision of professional meteorological tools. The result includes the heat index in both °F and °C, a visual segmented gauge showing where your value falls on the Safe → Extreme Danger spectrum, a risk category label based on NOAA classifications, detailed health effects for your risk level, and actionable safety tips. The calculator also computes the Canadian Humidex — the parallel metric used across Canada and parts of Europe — so you can compare both approaches side by side. Understanding heat index is not just an academic exercise. Heat-related illness is one of the most preventable causes of weather-related death. The U.S. Centers for Disease Control and Prevention (CDC) reports that extreme heat causes more deaths each year than hurricanes, tornadoes, and floods combined. Knowing your heat index before outdoor activity, athletic events, or manual labour can mean the difference between staying safe and risking heat exhaustion or heat stroke. The formula has some well-known boundaries: it is most accurate when the air temperature is 80°F (26.7°C) or above and relative humidity is at least 40%. At lower temperatures or very low humidity, the simple linear formula gives a more reliable estimate, and this tool will flag that situation with a notice. Direct sunlight — not accounted for in the formula — can raise the apparent temperature by an additional 10–15°F (6–8°C), so users should treat the result as a minimum when working or recreating in full sun.
Understanding the Heat Index
What Is the Heat Index?
The heat index is a single-number measure of how hot it feels to the human body when both air temperature and relative humidity are considered together. The concept dates back to Robert G. Steadman's 1979 biometeorological research, which was later adopted and refined by NOAA. The index assumes a standard adult in a shady location with light wind; it is not adjusted for individual differences in body composition, fitness, or acclimatisation. Values below 80°F (27°C) generally indicate no unusual thermal stress, while values above 103°F (40°C) begin to pose real danger for sustained outdoor activity.
How Is the Heat Index Calculated?
NOAA uses a two-step approach. First, a simple linear formula — HI_simple = 0.5 × {T + 61 + [(T − 68) × 1.2] + (RH × 0.094)} — is averaged with the actual temperature. If that average is below 80°F, the simple result is used directly. If it is 80°F or above, the full Rothfusz regression with nine terms (including T², RH², and cross-products) is applied. Two correction factors adjust the result for very low humidity (RH < 13% at 80–112°F, where the index is reduced) and for high humidity at moderate temperatures (RH > 85% at 80–87°F, where it is increased). Temperature is always converted to Fahrenheit for the calculation, then converted back to the display unit.
Why Does the Heat Index Matter?
High heat indices dramatically accelerate the onset of heat-related illness. At heat index values in the Extreme Caution range (91–103°F / 33–39°C), heat cramps and heat exhaustion become possible with prolonged exposure or physical activity. In the Danger range (104–124°F / 40–51°C), these conditions are likely, and heat stroke — a life-threatening emergency — becomes possible. In the Extreme Danger range (≥ 125°F / ≥ 52°C), heat stroke is highly likely. Children, the elderly, and people with heart disease, obesity, or diabetes face elevated risk at any category above Safe because their thermoregulatory systems are less efficient.
Limitations and Caveats
The Rothfusz formula is most accurate between 80–105°F (27–41°C) with humidity between 40–80%. It was designed for a standard adult at rest in the shade with light wind; physically active individuals or those in direct sunlight will experience significantly higher apparent temperatures — the NWS estimates direct sunshine can add 10–15°F (6–8°C) to the result. The formula is not valid below 80°F (26.7°C); this tool will display a notice and use the simpler linear estimate in those cases. For extreme values (above ~130°F / 54°C), the regression begins to lose accuracy. The Canadian Humidex uses a different methodology (dew point vapour pressure rather than relative humidity), so Humidex and Heat Index values are not directly comparable on the same scale.
How to Use the Heat Index Calculator
Select Your Temperature Unit
Choose °F or °C depending on your preference. The calculator will handle all conversions internally, and the result will display in both units regardless of your choice.
Enter the Air Temperature
Type the current outdoor air temperature — the reading from a thermometer or weather station, not the already-adjusted 'feels like' value. The formula requires the actual dry-bulb temperature.
Enter Humidity or Dew Point
Use the toggle to choose your humidity input method. If your weather app shows relative humidity (%), select that mode. If it shows a dew point temperature, select Dew Point and enter that value instead — the calculator will derive relative humidity automatically.
Read Your Heat Index and Risk Level
The result shows the heat index in °F and °C, your risk category on a color-coded gauge, health effects, and specific safety tips. Use the Export CSV button to download a full humidity lookup table for the entered temperature, or Print Results to save a clean summary.
Frequently Asked Questions
What is the difference between heat index and temperature?
Air temperature measures only the energy in the air, while the heat index (apparent temperature) combines air temperature and relative humidity to estimate how hot the human body actually feels. When humidity is high, sweat evaporates more slowly, reducing your body's ability to cool itself. At 95°F (35°C) and 60% humidity, the heat index can reach 113°F (45°C) — meaning your body experiences conditions equivalent to standing in 113°F air. Conversely, at very low humidity the heat index may actually be lower than the air temperature because evaporative cooling works efficiently.
Why does the calculator show a 'below threshold' warning?
The NOAA Rothfusz regression formula was developed specifically for conditions where the temperature is at or above 80°F (26.7°C) and humidity is fairly high. Below 80°F, the full equation can produce inaccurate results, so the NWS recommends using a simpler linear estimate instead. This calculator detects that condition and switches automatically, while showing you a notice so you understand the result comes from a different equation. The value shown is still useful but should be understood as an approximation.
What is the Canadian Humidex and how is it different from the heat index?
The Humidex is Canada's equivalent of the U.S. heat index but uses a different formula. Instead of relative humidity, it uses dew point vapour pressure as the humidity input. The two scales are not directly comparable: a Humidex of 40 does not correspond to a heat index of 40°C, and the risk thresholds differ. Canada's Meteorological Service considers Humidex values of 40 or above as 'very high' risk requiring precautions. This calculator shows the Humidex alongside the NOAA heat index for reference, but the official U.S. risk categories are based on the heat index.
How does direct sunlight affect the heat index?
The NOAA formula assumes shade conditions with light wind. Direct sunlight adds radiant heat that the formula does not account for. The NWS estimates that being in full sun can increase the apparent temperature by 10–15°F (6–8°C) above the calculated heat index. This means a calculated heat index of 95°F (35°C) can feel like 105–110°F (41–43°C) in direct sun. Always apply this solar adjustment mentally when making safety decisions outdoors on sunny days, especially for athletic events, construction work, or gardening.
Who is most at risk from high heat index values?
People at elevated risk include the elderly (whose thermoregulatory systems are less efficient), young children (who generate more body heat per kilogram and rely on caregivers to manage their environment), and individuals with cardiovascular disease, obesity, diabetes, or chronic respiratory conditions. People taking diuretics, antihistamines, or some psychiatric medications also face higher risk. Athletes and outdoor workers who are not acclimatised to heat are vulnerable even if healthy. In all Caution or higher categories, extra precautions should be taken for these groups, including more frequent breaks, extra hydration, and access to cool spaces.
Can I use the heat index to determine whether to cancel outdoor events?
Yes — the NOAA risk categories provide a widely used framework for such decisions. Many sports organizations, schools, and outdoor event planners use heat index thresholds to set activity policies. Common guidance: below 91°F (33°C) = normal activity with hydration; 91–103°F (33–39°C) = reduce intensity, increase breaks, hydrate frequently; 103–124°F (39–51°C) = strongly consider postponing strenuous activity; 125°F (52°C) or above = cancel outdoor events, move activities indoors. Always consult local health authority guidelines for official recommendations in your jurisdiction.