Wet Bulb Globe Temperature
Calculate the Wet Bulb Globe Temperature (WBGT), the gold standard for heat stress assessment. Provides military/OSHA flag color, work-rest cycles, and hydration guidance.
About this calculator
Wet Bulb Globe Temperature is a weighted composite of three separate thermometer readings, and this calculator applies the standard outdoor formula: WBGT = 0.7 x Wet Bulb Temperature + 0.2 x Globe Temperature + 0.1 x Dry Bulb Temperature. Wet Bulb Temperature carries the heaviest weight (70%) because it reflects how effectively sweat can evaporate to cool the body -- in high humidity, evaporation slows even if the air temperature itself isn't extreme, which is exactly the condition WBGT is designed to catch that a simple air-temperature reading misses. Globe Temperature (20% weight) captures radiant heat load from direct sun and hot surfaces, and Dry Bulb Temperature (10% weight) is ordinary shaded air temperature, the smallest contributor of the three.
WBGT (Indoor) drops the dry-bulb term entirely (0.7 x Wet Bulb + 0.3 x Globe) since indoor settings without direct solar load don't need a separate ambient-air correction. Heat Flag Color sorts the outdoor WBGT into the widely published US military heat-category chart (Green 80-84.9F, Yellow 85-87.9F, Red 88-89.9F, Black 90F and above, converted here to Celsius) -- this specific color chart originates with the military rather than a distinct OSHA numeric standard, though OSHA, NIOSH, and ACGIH all endorse WBGT itself as the preferred field measurement for occupational heat stress. Work/Rest Cycle and Water Intake give simplified, single-track guidance at each flag level; real ACGIH and military work-rest tables vary further by workload intensity (light/moderate/heavy) and worker acclimatization, which this calculator does not ask for, so treat the guidance here as a conservative starting point rather than a substitute for a full workload-adjusted heat stress program.
Inputs
Results
WBGT (Outdoor)
31.5 °C
Heat Flag Color
Red
Figures current as of 2026. Source: Occupational Safety and Health Administration, "Heat - Hazards and Solutions": "OSHA recommends the use of wet bulb globe temperature (WBGT) monitor to measure workplace environmental heat... NIOSH, ACGIH, the U.S. military, and many athletic organizations recommend WBGT for measurement of heat stress."
How to Use This Calculator
- Measure or obtain Dry Bulb Temperature (°C) — standard air temperature in the shade.
- Measure Wet Bulb Temperature (°C) using a sling psychrometer or whirling hygrometer.
- Measure Globe Temperature (°C) using a black globe thermometer in direct sunlight.
- Review WBGT (Outdoor) and Flag Color: Green = low risk, Yellow = moderate, Red = high, Black = extreme.
- Use Work/Rest Cycle and Water per Hour (liters) to set activity guidelines for outdoor workers or athletes.
How the result changes with Wet Bulb Temperature
| Wet Bulb Temperature | WBGT (Outdoor) | Heat Flag Color |
|---|---|---|
| 14 | 21.7 °C | Green |
| 21 | 26.6 °C | Green |
| 40 | 39.9 °C | Black |
What each input means
- Dry Bulb Temperature
- Standard air temperature measured by a shaded thermometer in °C. This is the ordinary ambient temperature.
- Wet Bulb Temperature
- Temperature from a thermometer wrapped in a wet wick. Reflects evaporative cooling capacity — the dominant WBGT factor (70% weight).
- Globe Temperature
- Temperature measured inside a 150mm black copper globe. Captures radiant heat from the sun and surroundings (20% weight).
How this is calculated
Worked example, using the default values
- Identify Input Parameters3 parametersDry Bulb Temperature = 35, Wet Bulb Temperature = 28, Globe Temperature = 42 = 3 input(s) provided
- Calculate WBGT (Outdoor)WBGT (Outdoor)31.5 = 31.5
- Calculate Heat Flag ColorHeat Flag ColorRed = Red
- Calculate WBGT (Indoor)WBGT (Indoor)32.2 = 32.2
- Calculate WBGT (Fahrenheit)WBGT (Fahrenheit)88.7 = 88.7
Figures and sources
- OSHA-recommended WBGT heat-stress measurement (2026) — Occupational Safety and Health Administration, "Heat - Hazards and Solutions": "OSHA recommends the use of wet bulb globe temperature (WBGT) monitor to measure workplace environmental heat... NIOSH, ACGIH, the U.S. military, and many athletic organizations recommend WBGT for measurement of heat stress."
Engine last updated . Checked against 4 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does Wet Bulb Temperature affect WBGT more than Dry Bulb Temperature?
The standard outdoor WBGT formula weights Wet Bulb Temperature at 70% versus Dry Bulb Temperature's 10%, because wet bulb reflects how well sweat can evaporate to cool the body -- the mechanism most directly tied to real heat-stress risk. In humid conditions, evaporative cooling slows even when the plain air temperature (dry bulb) isn't extreme, which is exactly the risk a simple thermometer reading would miss.
Why is there a separate WBGT (Indoor) figure, and why is it usually different from WBGT (Outdoor)?
WBGT (Indoor) drops the Dry Bulb Temperature term used outdoors and reweights Globe Temperature to 30%, since indoor spaces without direct sunlight don't need the ambient-air correction the outdoor formula includes for solar-exposed settings. The two figures will generally differ because they're computed with different formula weights, not because one is more or less accurate than the other for its intended setting.
Is the Heat Flag Color an official OSHA standard?
The specific Green/Yellow/Red/Black flag chart and its temperature cutoffs originate with the US military and are widely reused across athletic and occupational heat-safety programs, rather than being a distinct numeric OSHA regulation. OSHA, NIOSH, and ACGIH do all recommend WBGT itself as the preferred heat-stress measurement, but treat the specific color-coded thresholds here as the commonly used military-derived chart, not a citation to a specific OSHA rule.
Do the Work/Rest Cycle and Water Intake figures account for how hard the work is?
No -- they give one simplified guidance track per flag color regardless of workload. Real ACGIH and military heat-stress tables vary the recommended work/rest ratio and water intake further by workload intensity (light, moderate, or heavy labor) and by whether workers are heat-acclimatized, neither of which this calculator asks for. Treat the figures here as a conservative starting point, not a substitute for a full workload-adjusted heat-stress program.
Can I measure WBGT with a regular thermometer?
Not accurately -- Wet Bulb Temperature requires a thermometer with a wetted wick exposed to airflow (a sling psychrometer or aspirated psychrometer), and Globe Temperature requires a black globe thermometer that captures radiant heat, both of which respond differently than an ordinary shaded thermometer. A standard thermometer only gives you the Dry Bulb Temperature input, which alone carries just 10% of the outdoor WBGT calculation.
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