WBGT Heat Stress Index Calculator for Indoor and Outdoor Work
Wet bulb globe temperature combines air temperature, humidity, radiant heat and (outdoors) sunshine into one heat stress number. This calculator handles the arithmetic so you can compare a reading to your own limit quickly.
Your numbers
Measured with a wetted wick, naturally ventilated.
150 mm black globe unless your instrument states otherwise.
Needed only for outdoor work with solar load.
Take from your heat program, based on work rate and acclimatization (for example ACGIH or NIOSH tables).
Your results
Planning aid. WBGT must be measured with calibrated instruments at the worker position. Limits depend on metabolic work rate, clothing and acclimatization; use your heat illness prevention program and local regulation.
WBGT as natural wet bulb rises
Sweeps Natural wet bulb temperature from 15 to 35, holding your other inputs fixed.
Connect heat sensors to exposure records
Optional review of how on-prem sensor data can support heat exposure tracking.
Optional. The tool and every export work without this. No spam.
How to use it
Take readings at the worker's location with a WBGT instrument or its separate sensors: natural wet bulb, globe and dry bulb. Choose the right setting. Indoors, and outdoors with no solar load, the dry bulb is not used. Outdoors in sun, all three terms apply. Enter the limit from your heat program, which varies with how hard the work is and whether workers are acclimatized, and read the margin. Repeat at different times of day and near furnaces, ovens or presses.
The formulas
Outdoors with solar load: WBGT = 0.7 x Tnwb + 0.2 x Tg + 0.1 x Tdb. Indoors or outdoors without solar load: WBGT = 0.7 x Tnwb + 0.3 x Tg. Natural wet bulb carries most of the weight because it captures humidity and airflow, and the globe captures radiant heat from machines and sun. Dry bulb alone, or a heat index from weather forecasts, is not a substitute, especially near radiant sources such as forges and heat treat lines.
- Natural wet bulb is not a psychrometric wet bulb
- Measure in the actual work position
- Check at the hottest part of the shift
Reading the result
Compare WBGT to a limit suited to the job. Heavier work and unacclimatized workers need lower limits, and heavy or impermeable clothing means readings need adjustment. When the margin is small, consider engineering controls first: ventilation, shielding, insulation of hot surfaces. Then use work and rest cycles, drinking water access, acclimatization schedules and buddy monitoring. The chart shows how quickly humidity moves the number when natural wet bulb climbs.
Common mistakes
A frequent error is using the standard psychrometer wet bulb instead of a naturally ventilated one, which understates the stress. Another is putting a globe in the shade for an outdoor reading. Do not compare a WBGT to a heat index chart, because the scales differ. Finally, do not apply a limit that assumes light work to a heavy task.
Frequently Asked Questions
What is WBGT?
Wet bulb globe temperature is an index of heat stress that combines natural wet bulb, globe and, outdoors in sun, dry bulb temperatures. It reflects humidity, radiant heat and air movement, which a simple thermometer reading does not.
Do I use dry bulb temperature indoors?
No. Under the usual formula, indoor and no-solar-load WBGT uses only 0.7 natural wet bulb plus 0.3 globe. Dry bulb enters the equation only outdoors with solar load.
What limit should I use?
Choose the limit from your heat illness prevention program or a recognized guideline such as ACGIH or NIOSH, based on work rate, acclimatization and clothing. The tool lets you enter it so the result matches your policy.
Can I get WBGT from a weather app?
Some forecasts estimate outdoor WBGT, but they cannot reflect radiant heat from process equipment or microclimate at the job. For compliance and for hot indoor work, measure on site.
If you want sensor data from furnaces, ovens and shop floor monitors tied to shift schedules and exposure records, we can help connect those sources on-prem.
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