Heat Stress Calculator
Heat index & "feels-like" from temperature, humidity and optional wind (with the NWS reference chart), plus occupational WBGT (ISO 7243) and a TWL risk-zone interpreter. °C/°F, fully offline.
Open Heat Stress Calculator →What is the Heat Stress Calculator?
A two-in-one heat-stress tool. The Heat Index tab turns air temperature and relative humidity into the US National Weather Service "feels-like" heat index and a risk level (from No heat stress up to Extreme Danger), with an optional wind input that adds the Bureau of Meteorology apparent temperature ("feels like with wind") and a colour-coded NWS reference chart. The Occupational tab covers the workplace indices: a WBGT (Wet Bulb Globe Temperature) calculator to ISO 7243, and a Thermal Work Limit (TWL) interpreter that reads the W/m² value from your heat-stress monitor and shows its risk zone. Everything runs in your browser, in °C or °F, and works offline.
How to use Heat Stress Calculator
- Pick a tab — Use Heat Index for the everyday "feels-like", or Occupational for workplace WBGT and TWL. You can share a link that opens straight on either tab.
- Heat index — Enter air temperature and relative humidity to read the heat index and its risk band. Optionally add a wind speed to see the wind-adjusted apparent temperature.
- Open the NWS chart — Expand the reference chart to see the full temperature-by-humidity grid; your current reading is ringed on it.
- WBGT — On the Occupational tab, enter the dry-bulb, natural wet-bulb and globe temperatures from a heat-stress meter (or estimate from humidity), choose sun or shade, and read the WBGT with screening guidance.
- TWL — Enter the TWL W/m² figure your calibrated monitor displays to see its risk zone and the controls it implies.
Frequently asked questions
Why doesn't the heat index use wind?
The NWS heat index is defined from temperature and humidity only and assumes shade with a light wind. Wind is handled by a separate model, the Australian Bureau of Meteorology apparent temperature, which the optional wind field uses to show a wind-adjusted "feels like".
What is WBGT?
Wet Bulb Globe Temperature combines the natural wet-bulb, black-globe and dry-bulb readings (ISO 7243) into a single index of environmental heat load. Outdoors it is 0.7 wet-bulb + 0.2 globe + 0.1 dry-bulb; in shade it is 0.7 wet-bulb + 0.3 globe.
Does the tool calculate TWL?
No. The Thermal Work Limit is computed by a calibrated heat-stress monitor from several measured inputs, and its validated algorithm is not publicly reproducible. To stay accurate and safe, this tool interprets the TWL value your monitor shows rather than re-deriving it.
What do the TWL zones mean?
Above 220 W/m² is Unrestricted, 140–220 is the Acclimatisation zone, 115–140 is Cautionary, and below 115 is Restricted (no solo work). TWL keeps the deep-body core temperature below 38.2 °C and sweat rate below 0.67 kg/m² per hour.
Is anything sent to a server?
No. Every calculation runs in your browser, so the tool keeps working offline and nothing you enter is uploaded.
Tips
- Add a margin for direct sun: the heat index and WBGT charts assume shade, and full sun can feel up to about 8 °C / 15 °F hotter.
- Relative humidity is clamped to 0–100%; the °C/°F toggle converts the heat-index result live.
- No wet-bulb reading? Use the estimate-from-humidity option for an indoor/shade WBGT ballpark, but rely on a measured reading for compliance.
- The WBGT band is a screening indicator — set exact work/rest ratios from your ACGIH TLV or ISO 7243 programme by metabolic rate and acclimatisation.