What is heat index?
The heat index combines air temperature and relative humidity into a single “how hot it feels” value. It is widely used for outdoor work planning in the United States (NWS). It is not the same as WBGT, which also includes radiant heat and is preferred for many industrial heat-stress programmes.
Enter temperature and humidity below. The calculator uses the NWS Rothfusz regression (with common adjustments) and shows indicative caution bands.
Heat Index Calculator
Heat index · NWS-style
Combine air temperature and relative humidity into a feels-like value with caution bands. For industrial radiant heat, prefer WBGT.
Formula: HI = Rothfusz regression f(T°F, RH%)
- Rothfusz NWS regression with common low/high humidity adjustments.
- Use WBGT when radiant heat or industrial exposure programmes apply.
- Bands support planning — they are not a medical diagnosis.
Indicative NWS-style bands
- 80–90°F HI — Caution: fatigue possible with prolonged exposure
- 91–103°F HI — Extreme caution: heat cramps / exhaustion possible
- 104–124°F HI — Danger: heat exhaustion likely; heat stroke possible
- ≥ 125°F HI — Extreme danger: heat stroke highly likely
Heat index vs WBGT
- Heat index — shade air temperature + humidity; simple and familiar for weather briefings.
- WBGT — wet bulb, globe, and dry bulb; better when sun, hot surfaces, or indoor radiant sources matter.
Use WBGT (or a site heat-stress procedure) for many industrial exposure decisions; use heat index for quick outdoor weather context.
Frequently asked questions
Should I enter Celsius or Fahrenheit?
Either — pick a unit. The Rothfusz equation runs in Fahrenheit internally; Celsius inputs are converted.
Does wind or sun change heat index?
The basic NWS heat index assumes shade and light wind. Full sun can feel several degrees hotter; strong wind can feel cooler. Treat bands as indicative.
When should I use WBGT instead?
When radiant heat, enclosure, or acclimatisation programmes matter — construction in sun, foundries, and many ACGIH-style assessments favour WBGT.
Is heat index a medical diagnosis tool?
No. It supports work/rest and hydration planning. Anyone with heat-illness symptoms needs first aid and medical assessment.
