WBGT Calculator – Wet Bulb Globe Temperature
Estimate Wet Bulb Globe Temperature (WBGT) indoors or outdoors from air temperature, humidity, globe temperature and wind speed. Based on ISO 7243-style formulas for quick heat stress screening.
WBGT Calculator
Outdoor: WBGT = 0.7·Tnwb + 0.2·Tg + 0.1·Ta. Indoor: WBGT = 0.7·Tnwb + 0.3·Tg.
Use direct mode if you have Ta, Tnwb and Tg from an instrument. Use meteo mode if you only have standard weather data.
What is Wet Bulb Globe Temperature (WBGT)?
Wet Bulb Globe Temperature (WBGT) is a composite heat stress index that combines:
- Air temperature (dry-bulb, Ta)
- Humidity (via natural wet-bulb temperature Tnwb)
- Radiant heat (via globe temperature Tg)
- Air movement (affecting both Tnwb and Tg)
It is widely used in occupational safety, sports, and the military to decide when to limit or stop work and training in hot environments.
WBGT formulas (ISO 7243 style)
Outdoors with solar load:
\[ \text{WBGT}_{\text{out}} = 0.7\,T_{nwb} + 0.2\,T_g + 0.1\,T_a \]
Indoors or outdoors without sun:
\[ \text{WBGT}_{\text{in}} = 0.7\,T_{nwb} + 0.3\,T_g \]
where:
- \(T_{nwb}\) = natural wet-bulb temperature (°C)
- \(T_g\) = globe temperature (°C)
- \(T_a\) = air (dry-bulb) temperature (°C)
Estimating wet-bulb and globe temperature from weather data
If you do not have a WBGT meter, this calculator can estimate:
- Natural wet-bulb temperature from air temperature and relative humidity using a psychrometric approximation.
- Globe temperature from air temperature, solar radiation and wind speed using a simplified radiative–convective balance.
These estimates are useful for planning and screening, but for regulatory compliance you should use calibrated instruments and follow the full ISO 7243 procedure.
Interpreting WBGT values
Exact limits depend on acclimatization, clothing, workload and national standards. The table below gives a simplified overview for moderately acclimatized adults in light–moderate work:
| WBGT (°C) | Risk level | Typical guidance |
|---|---|---|
| Below 25 | Low | Normal work with basic hydration and breaks. |
| 25 – 28 | Moderate | Increase monitoring, schedule extra breaks, encourage frequent drinking. |
| 28 – 31 | High | Introduce work–rest cycles, reduce workload, closely monitor vulnerable workers. |
| Above 31 | Very high / extreme | Only very light work or stop work; emergency plans and medical supervision recommended. |
Always compare your results with the specific limits in your country (e.g. ISO 7243, ACGIH TLV, national occupational safety regulations, or sports federation guidelines).
Worked example
Suppose you measure outdoors in the sun:
- Air temperature \(T_a = 32\,^\circ\text{C}\)
- Natural wet-bulb \(T_{nwb} = 27\,^\circ\text{C}\)
- Globe temperature \(T_g = 45\,^\circ\text{C}\)
For outdoor conditions with solar load:
\[ \text{WBGT} = 0.7 \cdot 27 + 0.2 \cdot 45 + 0.1 \cdot 32 \]
\[ \text{WBGT} = 18.9 + 9.0 + 3.2 = 31.1\,^\circ\text{C} \]
A WBGT of about 31 °C indicates very high heat stress for most workers, and only very light work or rest would typically be recommended.
Best practices for using WBGT
- Measure at worker height and in representative locations (not only in the shade).
- Repeat measurements during the day, especially at the hottest hours.
- Consider clothing and PPE (impermeable clothing increases risk).
- Adjust limits for unacclimatized workers, older people, or those with medical conditions.
- Combine WBGT with training, hydration plans, and emergency procedures.
Frequently asked questions about WBGT
Can I use weather app data instead of a WBGT meter?
Yes, for a first screening. This calculator can estimate WBGT from air temperature, humidity, solar radiation and wind speed. However, weather stations may be far from your site and may not reflect local radiant heat (e.g. dark surfaces, machinery). For formal risk assessment, use a calibrated WBGT instrument on site.
What units does this WBGT calculator support?
You can enter temperatures in °C or °F, wind speed in m/s, km/h or mph, and solar radiation in W/m². Results are shown in °C and °F.
How often should I recalculate WBGT during the day?
In stable indoor environments, measuring every few hours may be sufficient. Outdoors in summer, conditions can change quickly; many guidelines recommend checking at least every hour, and more often during heat waves or when work intensity changes.
Does WBGT replace other indices like Heat Index or Humidex?
No. Heat Index and Humidex consider only air temperature and humidity in the shade. WBGT is more suitable for workplaces and sports because it also accounts for solar radiation and air movement. Many occupational standards are based specifically on WBGT.