WBGT Calculator (From Your Three Measured Temperatures)
This calculator turns the three temperatures a WBGT meter reads — natural wet-bulb, black globe and dry-bulb — into a wet bulb globe temperature, using the outdoor and indoor equations published in OSHA's Technical Manual. It adds the clothing adjustment factor OSHA publishes and compares the result with OSHA's workload-based limits.
Wet bulb globe temperature
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Results are estimates for planning purposes and do not replace a competent person's evaluation or an engineer's design. Follow the OSHA standards that apply to your work and your employer's written program.
The three sensors, and what each one is telling you
| Sensor | What it measures | Weight outdoors | Weight indoors |
|---|---|---|---|
| Natural wet-bulb (Tnwb) | How well sweat can evaporate — humidity and air movement together | 0.7 | 0.7 |
| Black globe (Tg) | Radiant heat: direct sun, and hot machinery, furnaces or structures nearby | 0.2 | 0.3 |
| Dry-bulb (Tdb) | Plain air temperature, shielded from radiant heat | 0.1 | not used |
Indoors the dry bulb is dropped because, with no radiant load, the globe and the dry bulb read the same. The globe stays in the equation because it still catches non-solar radiant sources. Source: OSHA Technical Manual, Section III, Chapter 4.
How to take the reading so the number means something
- Measure at the work area, or as close to it as you can get. A reading from the parking lot describes the parking lot.
- Let the globe stabilize. A black globe takes time to come to equilibrium after it is moved into or out of the sun; a reading taken too early reads low.
- Wet the natural wet-bulb sensor with distilled water and leave it in the moving air of the area. It is not the same instrument as a sling psychrometer.
- Measure at least hourly during the hottest part of the shift, during the hottest months, and whenever a heat wave hits or is forecast — OSHA cites the 2016 NIOSH criteria document for this.
- Crews that move between areas — sun to shade, outdoors to a boiler room — need a reading for each condition, then a time-weighted average.
The published limits, and what they are not
OSHA publishes limits by workload on its heat hazard recognition page, adapted from NIOSH guidelines (2016). They are compared against the effective WBGT — the measured value plus the clothing adjustment factor.
| Workload | Action limit — unacclimatized | Threshold limit value — acclimatized |
|---|---|---|
| Light | 82.4 °F (28 °C) | 86 °F (30 °C) |
| Moderate | 77 °F (25 °C) | 82.4 °F (28 °C) |
| Heavy | 73.4 °F (23 °C) | 78.8 °F (26 °C) |
| Very heavy | 69.8 °F (21 °C) | 77 °F (25 °C) |
| Clothing | Add to measured WBGT |
|---|---|
| Work clothing (baseline) | 0 |
| Cloth coveralls | 0 |
| Spunbound melt-blown synthetic (SMS) coveralls | 0.9 °F (0.5 °C) |
| Polyolefin coveralls | 1.8 °F (1 °C) |
| Double-layer cloth clothing | 5.4 °F (3 °C) |
| Limited-use vapor-barrier coveralls | 19.8 °F (11 °C) |
What this page does not give you: work/rest cycles in minutes per hour. Those come from ACGIH threshold limit value documentation, which is copyrighted — OSHA's own Technical Manual states the ACGIH heat stress information is copyrighted and not publicly available. Crossing a limit above is a signal to act on the work, not a countdown.
Which workload category is this job?
| Category | Examples | Typical rate |
|---|---|---|
| Rest | Sitting, thinking | 115 W |
| Light | Sitting with minimal hand and arm work, writing, driving a car, occasional or slow walking, stooping or kneeling, standing watch | 180 W |
| Moderate | Hammering nails, pushing light carts, continuous normal walking, operating mobile equipment, raking, scraping or painting, general carpentry, hand tools, machining, packaging | 300 W |
| Heavy | Carrying loads, shoveling, sawing or heavy carpentry, roofing, pushing heavy wheelbarrows, fast walking over 4 mph, landscaping, welding, grinding and cutting, drilling rock or concrete, mixing cement | 415 W |
| Very heavy | Any activity at near maximum pace, climbing stairs or ladders, using an axe, intense shoveling or digging, sledgehammer use, brick or stone masonry, firefighting | 520 W |
Categories and examples: OSHA — Heat Hazard Recognition. Pick the category for the work being done now, not the average of the shift: the same crew moves between moderate and very heavy within an hour.
How it's calculated
- WBGT outdoors, with solar load
WBGTout = 0.7 × Tnwb + 0.2 × Tg + 0.1 × TdbNatural wet-bulb 70%, globe 20%, dry-bulb 10%. OSHA Technical Manual, Section III, Chapter 4.- WBGT indoors, or outdoors with no solar load
WBGTin = 0.7 × Tnwb + 0.3 × TgThe dry bulb is dropped because without radiant heat the globe and the dry bulb read the same; the globe still catches non-solar radiant sources.- Averaging several readings over a shift (WBGTavg)
WBGTavg = [(WBGT1 × t1) + (WBGT2 × t2) + … + (WBGTn × tn)] / (t1 + t2 + … + tn)Time-weighted average, tn in minutes. OSHA: average over 60 minutes for continuous exposure, 60 to 120 minutes for intermittent exposure or exposure at different heat levels.- Effective WBGT (clothing adjustment)
Effective WBGT = measured WBGT + clothing adjustment factorFactors published by OSHA (adapted from NIOSH 2016): work clothing and cloth coveralls 0; SMS coveralls 0.9 °F (0.5 °C); polyolefin coveralls 1.8 °F (1 °C); double-layer cloth 5.4 °F (3 °C); limited-use vapor-barrier coveralls 19.8 °F (11 °C).- Comparison limits by workload (effective WBGT)
Light 82.4 / 86 °F · Moderate 77 / 82.4 °F · Heavy 73.4 / 78.8 °F · Very heavy 69.8 / 77 °F — unacclimatized (action limit) / acclimatized (threshold limit value)Published by OSHA on its heat hazard recognition page, adapted from NIOSH guidelines (2016). This calculator compares your effective WBGT with those values; it does not print work/rest ratios in minutes per hour, which are ACGIH material.- Unit conversion
°C = (°F − 32) × 5/9 · °F = °C × 9/5 + 32The equations are weighted averages of temperatures, so the weights are the same in either unit — only the inputs and the answer change scale.
Frequently asked questions
Wet bulb globe temperature is a single number that combines humidity, air movement, radiant heat and air temperature. OSHA calls a WBGT meter the most accurate tool for measuring workplace heat, because it captures the sun beating on a roof deck or the radiant heat off a furnace, which a weather app cannot.
Outdoors with a solar load, WBGT = 0.7 Tnwb + 0.2 Tg + 0.1 Tdb. Indoors, or outdoors with no solar load, WBGT = 0.7 Tnwb + 0.3 Tg. Tnwb is the natural wet-bulb temperature, Tg the black globe temperature and Tdb the dry-bulb air temperature. Both equations are published in OSHA's Technical Manual, Section III, Chapter 4.
Evaporation of sweat is how the body sheds most of its heat, and the natural wet-bulb sensor is the one that measures how well sweat can evaporate in that air. Humidity and air movement both show up in that reading, which is why it carries most of the weight in both equations.
Not from this page. Estimating WBGT from weather data takes a model of solar irradiance and wind — OSHA points to the Argonne National Laboratory WBGT utility for that, and even then a remote weather station will not see the radiant heat or the wind shadow of your work area. This calculator uses the temperatures you actually measured.
No. It gives the effective WBGT and compares it with the workload-based limits OSHA publishes. Work/rest regimens in minutes per hour come from ACGIH threshold limit value documentation, which is copyrighted and is not reproduced here; those cycles belong in your employer's heat illness prevention plan.
Yes. A clothing adjustment factor is added to the measured WBGT before comparing it with a limit. OSHA publishes the factors, adapted from NIOSH (2016): 0 for work clothing and cloth coveralls, 0.9 °F for SMS coveralls, 1.8 °F for polyolefin coveralls, 5.4 °F for double-layer cloth clothing and 19.8 °F for limited-use vapor-barrier coveralls. This calculator adds the one you select.
OSHA cites the 2016 NIOSH criteria document: take environmental heat measurements at least hourly, during the hottest portion of each work shift, during the hottest months of the year, and when a heat wave occurs or is predicted. Measure at or as close as possible to the work area.
OSHA publishes limits by workload, adapted from NIOSH guidelines (2016), as effective WBGT: light work 82.4 °F for unacclimatized workers and 86 °F for acclimatized ones; moderate 77 and 82.4 °F; heavy 73.4 and 78.8 °F; very heavy 69.8 and 77 °F. The lower value in each pair is the action limit for unacclimatized workers, the higher one the threshold limit value for acclimatized workers.
No. Heat index uses only air temperature and relative humidity, in the shade, with light wind. WBGT adds radiant heat and air movement, which is why OSHA recommends it for workplaces. Heat index is a screening number you can get from a forecast; WBGT is a measurement you take on site.
Sources & references
- OSHA Technical Manual, Section III, Chapter 4 — Heat Stress (WBGT equations, sensors, averaging)
- OSHA — Heat Hazard Recognition (WBGT is the recommended workplace heat measurement)
- NIOSH — Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments (2016), cited by OSHA for hourly measurement
- OSHA — Heat Injury and Illness Prevention proposed rule, regulatory text
Content checked against these sources — last reviewed August 28, 2026.