WBGT calculator
This free WBGT calculator turns your heat stress meter readings into a wet bulb globe temperature. Outdoors in the sun, WBGT = 0.7 × natural wet bulb + 0.2 × globe + 0.1 × dry bulb; indoors or with no sun, WBGT = 0.7 × natural wet bulb + 0.3 × globe. Pick a work rate to compare your WBGT with the NIOSH heat stress limits.
Formula: WBGT = 0.7 × natural wet bulb + 0.2 × globe + 0.1 × dry bulb.
Enter the three readings from your heat stress meter (two if you are indoors or out of the sun). Nothing you type leaves your browser.
This calculator applies the WBGT formulas and the RAL and REL equations in NIOSH Publication 2016-106. It is a planning aid, not legal advice, and it does not replace a competent person’s judgment on site.
NIOSH limits are recommendations, not an OSHA rule. For air- and vapor-tight protective clothing, NIOSH says dry bulb (air) temperature is a better measure than WBGT.
Heat stress limits by work rate (NIOSH)
Worked out from the NIOSH equations for steady work all hour, a 70 kg (154 lb) worker and normal one-layer work clothing. Harder work means a lower WBGT limit.
| Metabolic rate | REL (used to heat) | RAL (new to heat) |
|---|---|---|
| 200 W (about 172 kcal/h) | 86.4°F / 30.2°C | 81.4°F / 27.5°C |
| 250 W (about 216 kcal/h) | 84.4°F / 29.1°C | 79.0°F / 26.1°C |
| 300 W (about 259 kcal/h) | 82.8°F / 28.2°C | 77.0°F / 25.0°C |
| 349 W (about 301 kcal/h) | 81.4°F / 27.5°C | 75.3°F / 24.0°C |
| 400 W (about 345 kcal/h) | 80.2°F / 26.8°C | 73.8°F / 23.2°C |
| 425 W (about 366 kcal/h) | 79.7°F / 26.5°C | 73.1°F / 22.8°C |
| 500 W (about 431 kcal/h) | 78.2°F / 25.7°C | 71.3°F / 21.8°C |
| 600 W (about 517 kcal/h) | 76.6°F / 24.8°C | 69.3°F / 20.7°C |
NIOSH also gives limits for 45, 30 and 15 minutes of work per hour as charts (Figures 8-1 and 8-2). Those are not in this table.
How to calculate wet bulb globe temperature
A heat stress meter reads three thermometers. The natural wet bulb shows how well sweat can cool the body (humidity and wind). The black globe shows radiant heat from the sun and hot surfaces. The dry bulb is the plain air temperature.
The WBGT calculator weights them the way NIOSH does. Outdoors in the sun it uses all three (0.7, 0.2 and 0.1). Indoors or with no sun it drops the dry bulb and uses the wet bulb and globe (0.7 and 0.3). The weights add up to 1, so the answer comes out in the unit you typed in, and we convert it to the other unit for you.
If you pick a work rate, the tool works out the NIOSH limits with RAL = 59.9 - 14.1 × log10(watts) and REL = 56.7 - 11.5 × log10(watts), in °C, and shows whether your WBGT is over them. NIOSH counts 1 kcal/h as 1.16 W. The three work levels in the list (250, 425 and 600 W) are the easy, moderate and hard work levels NIOSH uses in its fluid table.
Common questions
▸What is WBGT?
WBGT (wet bulb globe temperature) is a heat stress index. It combines humidity and air movement (the natural wet bulb reading), air temperature and radiant heat from the sun or hot surfaces (the black globe reading), and, outdoors in the sun, the air temperature itself (the dry bulb reading). It was developed in 1957 to control heat illness at military training camps.
▸How do you calculate WBGT?
Outdoors in the sun: WBGT = 0.7 × natural wet bulb + 0.2 × globe + 0.1 × dry bulb. Indoors or with no sun: WBGT = 0.7 × natural wet bulb + 0.3 × globe. For example, readings of 78°F wet bulb, 115°F globe and 92°F dry bulb in the sun give 54.6 + 23.0 + 9.2 = 86.8°F WBGT.
▸Can I get WBGT from the air temperature and humidity alone?
Not with this WBGT calculator. The natural wet bulb and globe readings depend on sun, wind and humidity at the spot where people work, so they are measured with a heat stress (WBGT) meter. The heat index from a weather report is a different number. Some state heat rules use air temperature or the heat index instead; the heat rule checker covers those.
▸What is the difference between the NIOSH RAL and REL?
The REL (Recommended Exposure Limit) is for workers who are used to the heat (acclimatized). The RAL (Recommended Alert Limit) is lower and is for workers who are new to the heat (unacclimatized). Both drop as work gets harder, because hard work makes more body heat.
▸Are the NIOSH limits an OSHA rule?
No. NIOSH is the federal research agency for worker safety, and its RAL and REL are recommendations. They are a common way to judge a WBGT reading. Your state may have its own heat rule with its own triggers, so check that as well.
▸How often should WBGT be measured?
NIOSH recommends measuring at least once an hour during the hottest part of each shift, in the hottest months, and when a heat wave occurs or is forecast. Measure at the work area, as close as you can to where people are working.
Sources
- NIOSH, Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments, Publication 2016-106: WBGT formulas (section 9.3.2), RAL and REL equations (section 8.1), measurement advice (section 1.1.2), kcal to watts (section 1.1.3), work levels (Table 8-1). https://www.cdc.gov/niosh/docs/2016-106/pdfs/2016-106.pdf
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