WBGT Heat Stress Calculator

Frequently Asked Questions

• What is WBGT, and how is it different from the heat index?+
  WBGT — wet-bulb globe temperature — is a composite heat-stress index that combines a naturally ventilated wet wick, a black globe measuring radiant heat, and plain air temperature. The heat index is a shade comfort index computed from temperature and humidity alone; it deliberately excludes sunshine and wind. That is the crucial difference: WBGT accounts for the sun beating down on you and the breeze cooling you, and the heat index does not. Because they measure different things on different scales, their numbers must never be compared. A WBGT of 30 °C is already in restrict-activity territory, while a heat index of 40 °C sounds far more alarming but is simply a different quantity.
• Why will this tool not estimate WBGT in direct sun?+
  Because it cannot be done honestly from temperature and humidity. The globe term carries radiant load, which depends on solar irradiance, cloud cover, sun angle and wind speed — none of which are contained in a temperature and humidity reading. The common shortcut that ignores them, known as sWBGT, has been shown to overestimate real WBGT by 2 °C or more in over 30% of cases across the tropics, and the researchers who tested it recommend discontinuing its use. A fixed "add so many degrees for sun" offset is worse still: it produces the same difference in every condition, which looks like a result but is really just the assumption being read back to you.
• What is the "shade floor" this tool gives me?+
  With no sun on the globe there is no radiant source, so the globe thermometer reads close to air temperature and the no-solar-load formula collapses to 0.7 × wet-bulb + 0.3 × air temperature. That is a defensible estimate of WBGT in full shade or after dark. Because sunshine can only push the real value higher, it also serves as a floor for outdoor conditions: whatever the true sunlit WBGT is, it is at least this. That turns out to be genuinely useful, because when the shade floor alone already crosses a threshold, you have your answer without needing the sun term at all.
• How accurate is the shade estimate?+
  The wet-bulb component uses the Stull (2011) closed-form approximation, which is valid for humidity between 5% and 99% and air temperatures from −20 °C to 50 °C at sea level, with published errors from −1.0 °C to +0.65 °C and a mean absolute error under 0.3 °C. Scaled by the 0.7 weighting, that puts roughly −0.7 °C to +0.5 °C of uncertainty on the estimate, and the tool displays that band rather than hiding it. There is a second, subtler approximation: the natural wet-bulb of a real WBGT meter is not quite the psychrometric wet-bulb this formula gives, though the two agree closely in shade with light air movement.
• Where do the action thresholds come from?+
  The bands and rest breaks shown are Singapore's Ministry of Manpower framework for heat stress in outdoor work, which sets three bands: below 31 °C, 31 to below 33 °C, and 33 °C and above. It is a free, official document and it governs real workplaces here, which is why it was chosen over paywalled alternatives. Note that the rest-break requirement follows its own ladder rather than the three bands: the mandatory 10-minute hourly break for heavy work begins at 32 °C, which sits inside the middle band, and rises to 15 minutes at 33 °C.
• Do I need a WBGT meter to get a real number?+
  To get a true WBGT in sun, yes — or a reading from someone else's meter. In Singapore the National Environment Agency runs a sensor network and publishes live WBGT through the myENV app and the weather portal, free to anyone, so most people here do not need to buy anything. Singapore also requires on-site WBGT meters at construction sites with a contract sum of S$5 million or more, at shipyards, and in the process industry. If you have any of these readings, use the measured mode in this tool and you will get an exact ISO 7243 value.
• What are the three probe readings the measured mode asks for?+
  The natural wet-bulb temperature comes from a thermometer wrapped in a wetted wick and left exposed to whatever air movement is naturally present — it is not fan-aspirated, which is what makes it "natural". The globe temperature comes from a thermometer at the centre of a matt-black copper sphere, conventionally 150 mm across, which absorbs radiant heat and so registers the sun. The air temperature is an ordinary dry-bulb reading, shielded from radiation. A WBGT meter houses all three and does the arithmetic; this tool lets you check or redo it.
• Why does my tool warn that the probes might be swapped?+
  Because two of the readings have physical constraints that a swapped or mis-entered probe will violate. A wet wick cannot read hotter than the dry air around it, since evaporation only ever cools; and outdoors under solar load a black globe cannot read cooler than the air, since it is absorbing radiation. If either happens, the tool says so rather than returning a confident, wrong number. It still shows the calculation, because there are unusual conditions where these can genuinely occur, but you should check the instrument first.
• Is WBGT the same as the "wet-bulb temperature" in climate news?+
  No, and the two are widely confused. Wet-bulb temperature is a single measurement — what a wetted thermometer reads — and the much-discussed 35 °C survivability limit refers to that quantity. WBGT is a weighted composite of three separate readings, only one of which is a wet-bulb. They are different numbers with different scales, and a WBGT of 35 is not the same condition as a wet-bulb of 35. This tool shows the wet-bulb value alongside the WBGT so the distinction stays visible.
• Is the WBGT Heat Stress Calculator free?+
  Completely free, with no account and no limits. The arithmetic runs entirely in your browser, so nothing you type is uploaded or stored, and the tool keeps working offline once the page has loaded.