Heat Index Calculator
Calculate the heat index (feels-like temperature) from air temperature and relative humidity using the NOAA Rothfusz formula. Get risk categories and heat safety guidance for India, US, and UK.
About This Calculator
The Heat Index Calculator uses the official NOAA Rothfusz regression formula to calculate the apparent temperature, or how hot it really feels when humidity is taken into account. Unlike a standard thermometer that only measures air temperature, this calculator accounts for the body's reduced ability to cool itself through sweat evaporation in humid conditions.
This tool is essential for athletes, outdoor workers, parents, coaches, event planners, and anyone who needs to assess heat-related health risks. It supports both Celsius and Fahrenheit units and provides heat index risk categories from Lower Risk to Extreme Danger based on NOAA guidelines.
How the Heat Index Formula Works
The calculator uses two formulas depending on the temperature. For temperatures at or above 80°F (26.7°C), it applies the full Rothfusz regression: HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T² - 0.05481717R² + 0.00122874T²R + 0.00085282TR² - 0.00000199T²R², where T is temperature in °F and R is relative humidity in percent. For temperatures below 80°F, a simpler linear approximation is used: HI = 0.5 × (T + 61.0 + (T - 68.0) × 1.2 + R × 0.094).
Regional Notes
India: The India Meteorological Department (IMD) issues heat warnings when temperatures exceed 40°C (104°F) in plains and 30°C (86°F) in hilly regions. High humidity in coastal cities like Mumbai, Chennai, and Kolkata can significantly elevate the heat index. The heat index is particularly relevant during April-June when pre-monsoon humidity is high.
United States: The National Weather Service (NWS) uses the heat index for heat advisories and excessive heat warnings. Heat advisories are typically issued when the heat index reaches 105°F (41°C) for two consecutive days. The NWS heat index chart is widely used in weather forecasts across the country.
United Kingdom: The UK Met Office issues heat-health warnings when temperatures are high enough to affect health. Though UK summers are milder, heat indices can still reach dangerous levels during heatwaves, especially in urban areas where the urban heat island effect intensifies conditions.
Frequently Asked Questions
What is heat index?
Heat index combines air temperature and humidity to measure how hot it feels. High humidity reduces the body's ability to cool itself through sweating, making the perceived temperature higher than the actual air temperature.
How is heat index calculated?
The heat index is calculated using the NOAA Rothfusz regression formula, which takes into account air temperature and relative humidity. For temperatures below 80°F, a simpler linear approximation is used. The formula was developed by R.G. Steadman and considers over 20 factors including walking speed, height, weight, wind speed, and solar angle.
What is the difference between heat index and temperature?
Temperature measures the actual thermal energy in the air, while heat index measures how hot it feels to the human body. Heat index accounts for humidity, which affects the evaporation of sweat. On a humid day, the heat index can be 10-15°F (5-8°C) higher than the actual temperature.
At what heat index should I be concerned?
NOAA advises caution when the heat index reaches 80°F (27°C). At 91-103°F (33-39°C), fatigue and heat cramps are possible with prolonged exposure. At 103-124°F (39-51°C), heat cramps and heat exhaustion are likely, and heat stroke is possible. Above 125°F (52°C), heat stroke is highly likely with prolonged exposure.
Is the heat index the same as feels-like temperature?
Yes, heat index is often called the feels-like temperature or apparent temperature. It combines temperature and humidity. In cold weather, wind chill is used instead. The heat index only applies to shaded areas; direct sunlight can increase the feels-like temperature by up to 15°F (8°C).
Can I use dew point instead of humidity for heat index?
This calculator uses relative humidity, but dew point can be converted using standard meteorological formulas. Higher dew points indicate more moisture in the air and contribute to a higher heat index. A dew point above 60°F (15°C) feels humid, and above 70°F (21°C) feels oppressive.
How can I stay safe during high heat index days?
During high heat index conditions, stay hydrated, avoid strenuous outdoor activities during peak heat hours (10 AM to 4 PM), wear lightweight and light-colored clothing, take frequent breaks in air-conditioned or shaded areas, and never leave children or pets in parked vehicles.
What is the difference between heat index in the US, UK, and India?
The NOAA heat index formula is used worldwide, including in the UK and India. In the US, heat index is reported in Fahrenheit as part of NWS heat advisories. In the UK, the Met Office uses a similar apparent temperature model. In India, the India Meteorological Department (IMD) issues heat warnings based on both temperature and humidity, with thresholds adjusted for local acclimatization.