Heat Stress Calculator: Wet Bulb Globe Temperature (WBGT)

The WBGT heat stress index from your temperature readings, compared with the reference value from your own heat stress procedure.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Air temperature alone does not show how hot a place feels to a worker. Humidity stops sweat evaporating, and heat from the sun and from hot surfaces adds to the load. The wet bulb globe temperature, WBGT, combines the three into one number.

Outdoors in direct sun the index is 0.7 times the natural wet-bulb temperature, plus 0.2 times the globe temperature, plus 0.1 times the air temperature. Indoors, or outdoors with no direct sun, it is 0.7 times the natural wet-bulb temperature plus 0.3 times the globe temperature. These are the formulas of ISO 7243.

How high a WBGT is acceptable depends on the work rate, the clothing and whether workers are used to the heat, so the reference value is an input that you take from your own procedure. The calculator shows the margin to it, and what to consider when the margin is small or negative.

Formulas used

  • WBGT_{sun} = 0.7\,T_{nwb} + 0.2\,T_g + 0.1\,T_a
  • WBGT_{no\;sun} = 0.7\,T_{nwb} + 0.3\,T_g

Worked example

With the default numbers in the calculator above:

  1. Formula for these conditions: outdoors in sun
  2. Wet bulb globe temperature: 28.00 C
  3. Margin to the reference value: 3.5527e-15 C

Change the numbers in the calculator to see the full working, every step, for your own case.

A second worked example

A different case, with these numbers: Conditions Indoors, or outdoors with no direct sun.

  1. Formula for these conditions: indoors or no sun
  2. Wet bulb globe temperature: 28.80 C
  3. Margin to the reference value: -0.80 C

Common questions

Is WBGT the same as the temperature on the weather report?

No. It includes humidity and radiant heat, so it can be well below or above what a normal thermometer shows. A humid, sunny day with a mild air temperature can still give a high WBGT.

What WBGT is safe?

There is no single number. It depends on the work rate, clothing and acclimatisation. Use the reference in your heat stress procedure, and treat a small margin as a warning.

How do I measure the three temperatures?

With a natural wet-bulb thermometer, a black globe thermometer and a shaded air thermometer, or with a WBGT meter that reads all three.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: ISO 7243 (hot environments, WBGT index; the standard itself is licensed); Safe Work Australia guidance on managing the risk of heat at work; Managing the work environment and facilities Code of Practice (Queensland).

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