Heat Index Calculator

NWS heat index from temperature and relative humidity (°F or °C).

Heat Index Calculator helps you nWS heat index from temperature and relative humidity (°F or °C). Enter air temperature and relative humidity, review the breakdown, and use the guidance below to understand what the number means — and what it does not. What the Heat Index calculator does Heat Index Calculator is built for everyday questions — shopping, home, travel, hobbies, and tech. NWS heat index from temperature and relative humidity (°F or °C). You enter air temperature and relative humidity, then Simple Calculators computes the result instantly. Defaults are chosen to be easy to read — change any field to match your situation. How the math works Choose Units (°F or °C), then enter the Air temperature and Relative humidity. Metric temperatures are converted to °F, and the US National Weather Service method is applied: a simple formula when the result is under 80 °F (27 °C), and the full Rothfusz formula, with small corrections for very dry or very humid air, above that. The result is shown with its Weather Service category — Caution from 80 °F (27 °C), Extreme caution from 90 °F, Danger from 105 °F and Extreme danger from 130 °F. The formula: Heat index = −42.379 + 2.049 × T + …