Heat Index Calculator

Heat Index – Quick Cheat Sheet

Inputs: Air temperature (°F or °C) and relative humidity (%).
Formula: NWS Rothfusz regression (in °F) with two adjustments: low humidity & hot air (RH<13%, 80–112 °F) and high humidity & warm air (RH>85%, 80–87 °F).
Below 80 °F: Uses a simple approximation pathway; above that, the full regression is used.
Risk bands (approx): <80 Normal, 80–90 Caution, 90–103 Extreme Caution, 103–124 Danger, ≥125 Extreme Danger.
Tips: Hydrate, rest in shade, and reschedule intense work as HI rises. Young children, seniors, and some medical conditions increase risk.

Edge cases & notes

Wind, sun angle, clothing, and radiant heat are not included in standard HI; direct sun can add ~15 °F perceived.
RH is clamped to 0–100%; unusual lab conditions outside this range are not modeled.
Metric shown by conversion (°C); core computation remains in °F to match the official model.