Heat Stress Calculator
The Wet-Bulb Globe Temperature (WBGT) index combines three measurements to estimate the real heat load on a worker's body: the natural wet-bulb temperature (Tnwb, which captures humidity and evaporative cooling), the black-globe temperature (Tg, which captures radiant heat from the sun and hot surfaces) and the air temperature (Ta).
Enter Values
How to use this calculator
- Enter the natural wet-bulb temperature (Tnwb) and the black-globe temperature (Tg) in °C, both measured at the work location.
- For outdoor work in direct sun, also enter the air (dry-bulb) temperature so the solar-load formula is used; leave it blank for indoor or fully shaded work.
- Read the WBGT index, the risk category and the suggested work/rest split, then apply your site's heat controls accordingly.
How it works
The Wet-Bulb Globe Temperature (WBGT) index combines three measurements to estimate the real heat load on a worker's body: the natural wet-bulb temperature (Tnwb, which captures humidity and evaporative cooling), the black-globe temperature (Tg, which captures radiant heat from the sun and hot surfaces) and the air temperature (Ta). Outdoors in direct sun the standard ISO 7243 formula is WBGT = 0.7·Tnwb + 0.2·Tg + 0.1·Ta. Indoors or in full shade there is no direct solar term, so WBGT = 0.7·Tnwb + 0.3·Tg. The wet-bulb term dominates at 70% because sweat evaporation is the body's main defence against heat.
The resulting WBGT (in °C) is compared against screening thresholds based on ACGIH-style guidance for a moderate work rate and an acclimatised worker. As WBGT climbs, the suggested proportion of work to rest each hour falls — from continuous work at low WBGT, through 75/25, 50/50 and 25/75 splits, up to suspending heavy work at extreme values. These bands are a starting point only: heavy physical work, impermeable PPE, and workers who are not acclimatised all lower the effective limit, so the real trigger point may be several degrees cooler than shown.
Worked example
Outdoor crew in direct sun. A civil crew is working in the open. The natural wet-bulb thermometer reads 25 °C, the black-globe thermometer reads 42 °C, and the air (dry-bulb) temperature is 34 °C. Because the sun is loading the workers, use the outdoor formula: WBGT = 0.7 × 25 + 0.2 × 42 + 0.1 × 34 = 17.5 + 8.4 + 3.4 = 29.3 °C. At 29.3 °C the tool returns a Moderate risk category with roughly 75% work / 25% rest each hour for acclimatised moderate work — a trigger to push water, shade breaks and closer supervision of new or unacclimatised workers.
Common mistakes
- Entering an ordinary air temperature or a standard psychrometric wet-bulb instead of the natural wet-bulb temperature (Tnwb) — WBGT specifically uses a naturally ventilated wet-bulb, so a shielded/aspirated reading gives the wrong index.
- Using the outdoor formula (with the air-temperature term) for indoor or fully shaded work. With no direct sun, leave the air-temperature field blank so the tool applies the 0.7·Tnwb + 0.3·Tg form.
- Treating the risk bands as hard limits. They assume moderate work and an acclimatised worker; heavy exertion, PPE, or new/unacclimatised staff mean you should act at a lower WBGT than the table suggests.
Frequently asked questions
What is the difference between WBGT and the heat index?
The heat index (or humidex) estimates how hot it feels to a person in shade using only air temperature and humidity. WBGT is an occupational index that also accounts for radiant heat (direct sun, hot machinery) via a black-globe reading and for air movement, which makes it the standard for assessing workplace heat stress. Use the heat index for general comfort and WBGT for work exposure.
Which formula does this calculator use?
For outdoor work with a solar load it uses the ISO 7243 form WBGT = 0.7 × Tnwb + 0.2 × Tg + 0.1 × Ta. For indoor or fully shaded work (air-temperature field left blank) it uses WBGT = 0.7 × Tnwb + 0.3 × Tg, where Tnwb is the natural wet-bulb temperature and Tg is the black-globe temperature.
Do the work/rest ratios apply to everyone?
No. The bands are screening values for a moderate work rate and an acclimatised worker. Heavy physical work, impermeable clothing or PPE, and workers who have not yet acclimatised to heat all reduce the safe exposure, so you should apply stricter controls at a lower WBGT. This tool is guidance only — follow your jurisdiction's WHS/OSH heat regulations and a competent person or occupational-health professional.
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- Decibel Exposure Time Calculator
- Manual Lifting Calculator
Explore more in Safety, Workplace Risk & Compliance Helpers.
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