Heat stress is a serious occupational hazard. Every year, thousands of workers suffer heat-related illnesses that are entirely preventable. While the general public uses the Heat Index, safety professionals rely on Wet-Bulb Globe Temperature (WBGT) because it accounts for temperature, humidity, radiant heat, and wind. This calculator estimates WBGT from common weather data and generates actionable OSHA/NIOSH work/rest schedules.

What Is WBGT (Wet-Bulb Globe Temperature)?

WBGT is a composite temperature used to estimate the heat stress on the human body during physical activity. It is the standard used by OSHA, NIOSH, the U.S. military, and major sports organizations.

The Standard WBGT Formula

When measured with specialized instruments, WBGT is calculated as:

Outdoor WBGT = 0.7Tw + 0.2Tg + 0.1Td

Indoor WBGT = 0.7Tw + 0.3Tg

Where:

• Tw = Natural wet-bulb temperature (°F) — accounts for humidity and evaporative cooling
• Tg = Globe temperature (°F) — accounts for radiant heat from the sun or hot surfaces
• Td = Dry-bulb (ambient air) temperature (°F)

Why WBGT Is Better Than Heat Index

Because WBGT includes sun and wind, it provides a much more accurate picture of physiological danger in real working conditions.

How This Calculator Estimates WBGT

Most job sites do not have WBGT monitors. This calculator estimates WBGT from standard weather inputs using well-established psychrometric approximations:

• Air temperature (°F or °C)
• Relative humidity (%)
• Wind speed (m/s or mph)
• Solar radiation / cloud cover (full sun, partial sun, shade, or indoor)

How the Estimate Works

1. Wet-bulb temperature (Tw) is estimated with the Stull (2011) closed-form regression, which approximates the natural wet-bulb temperature from air temperature and relative humidity. Wind speed applies a small additional correction.
2. Globe temperature (Tg) is estimated from air temperature plus a solar-load offset based on sun exposure. Indoors or in shade, Tg is assumed equal to air temperature.
3. WBGT is then computed with the standard ISO 7243 formula:
   • Outdoor: WBGT = 0.7Tw + 0.2Tg + 0.1Tdb
   • Indoor: WBGT = 0.7Tw + 0.3Tg

Important: This is a field estimate. A calibrated WBGT meter at the actual work location will always be more accurate than any model.

OSHA / NIOSH Work/Rest Guidelines

NIOSH and OSHA recommend structured work/rest cycles based on WBGT and workload intensity. The following table applies to acclimatized workers wearing light clothing.

Work/Rest Schedule for Acclimatized Workers

Adjustment for Unacclimatized Workers

Workers who are new to heat, returning from leave, or recently relocated to a hot environment should follow thresholds approximately 4–5°F (2–3°C) lower than acclimatized workers. OSHA recommends a gradual acclimatization program over 7–14 days.

Clothing and PPE Adjustments

Protective clothing and equipment trap heat and reduce the body's ability to cool. NIOSH recommends adding the following adjustments to the measured or estimated WBGT:

These adjustments are added to the estimated WBGT before checking the work/rest schedule.

How to Use the Heat Stress Calculator

Step 1: Enter environmental conditions.

• Air temperature (°F or °C)
• Relative humidity (%)
• Wind speed (mph or m/s)
• Sky condition (clear, partly cloudy, overcast) or indoor setting

Step 2: Enter work factors.

• Workload intensity (light, moderate, heavy)
• Clothing/PPE type
• Worker acclimatization status

Step 3: Click Calculate. The calculator returns:

• Estimated WBGT in °F and °C
• Heat stress risk category
• Recommended work/rest schedule
• Hydration guidance
• Clothing-adjusted WBGT

Worked Examples

Example 1: Construction Site in Texas

• Air temperature: 95°F
• Relative humidity: 60%
• Wind speed: 5 mph
• Sky: Clear
• Workload: Moderate (continuous walking, moderate lifting)
• Clothing: Standard work clothes
• Estimated WBGT: 87°F (31°C)
• Recommendation: 45 minutes work / 15 minutes rest
• Hydration: 1 cup of water every 15–20 minutes

Example 2: Warehouse Without Climate Control

• Air temperature: 88°F
• Relative humidity: 70%
• Wind speed: 0 mph (indoor)
• Setting: Indoor
• Workload: Heavy (intense manual labor)
• Clothing: Standard coveralls
• Estimated WBGT: 84°F (29°C)
• Recommendation: Stop non-essential work (or 25% work / 75% rest for essential tasks)
• Hydration: Mandatory hydration breaks; 1 cup every 15–20 minutes

Example 3: Road Crew in Full Sun

• Air temperature: 92°F
• Relative humidity: 55%
• Wind speed: 2 mph
• Sky: Clear
• Workload: Heavy (asphalt work, shoveling)
• Clothing: Standard work clothes
• Estimated WBGT: 89°F (32°C)
• Recommendation: 15 minutes work / 45 minutes rest
• Hydration: Mandatory hydration breaks; consider cooling vests

Heat Illness Warning Signs

Supervisors and workers should know the signs of heat-related illness:

OSHA's Water. Rest. Shade. Campaign

OSHA's core heat illness prevention message applies to every workplace:

1. Water: Drink 1 cup (8 oz) of water every 15–20 minutes. Do not wait until thirsty.
2. Rest: Take scheduled breaks in a cool or shaded area.
3. Shade: Provide shade or air-conditioned rest areas.
4. Buddy System: Workers should monitor each other for signs of heat illness.
5. Acclimatization: Gradually increase heat exposure for new or returning workers.

Heat Stress Calculator Limitations

While this calculator provides a reliable estimate, it cannot replace on-site measurement or professional judgment:

• Microclimates: Conditions can vary significantly within a worksite (e.g., asphalt vs. grass, shaded vs. sunny areas).
• Individual factors: Age, fitness, medications, and medical conditions affect heat tolerance.
• PPE variations: Different types of protective equipment trap heat differently.
• Real-time changes: WBGT changes throughout the day. Re-measure or re-calculate periodically.

For the most accurate assessment, use a calibrated WBGT monitor at the actual work location.