Free WBGT Calculator — Indoor & Outdoor Heat Stress

What WBGT Measures, and Why Three Temperatures

The Wet-Bulb Globe Temperature is not a single thermometer reading, it is a weighted combination of three measurements, each capturing a different component of the thermal environment.

The natural wet-bulb temperature (T_wet / NWB) is measured by a thermometer wrapped in a wet wick exposed to natural air movement — not forced airflow. It captures evaporative cooling: the more humid the air, the less water evaporates from the wick, and the higher the reading. High humidity means the body cannot shed heat by sweating as effectively.

The globe temperature (T_globe / GT) is measured inside a hollow black sphere, the standard is a 6-inch (150 mm) matte black copper globe. The globe absorbs radiant heat from surrounding surfaces and the sun, reaching thermal equilibrium. This is the component most sensitive to radiant heat loads, which matter enormously in foundries, near furnaces, or outdoors in direct sun.

The dry-bulb temperature (T_air / DB) — used only in the outdoor formula — is the standard air temperature measured in the shade. Its weight (0.1) is intentionally low, because convective heat from dry air is less critical than radiant and evaporative effects under sun load.

The weighting reflects physiological reality: 70% of the heat burden in most environments comes from humidity-limited sweat evaporation (NWB). Radiant heat (globe) contributes 20–30% depending on whether there is solar exposure. Convective air temperature contributes the remainder.

Where to Find WBGT Thresholds for Your Situation

WBGT thresholds are not universal. They depend on three variables that no calculator can determine for you: (1) which regulatory or advisory referential applies to your jurisdiction and industry, (2) the estimated metabolic workload of the task (light sedentary work vs. heavy sustained physical labor), and (3) whether workers are acclimatized to heat. The same WBGT value that is acceptable for a sedentary acclimatized worker may be dangerous for a non-acclimatized worker performing heavy labor.

ACGIH TLV Heat Stress and Strain (published annually in the TLV/BEI Booklet): provides WBGT action limits by workload category and acclimatization status, with a work-rest regimen table. Widely used in occupational hygiene practice in North America. Purchase or consult through your employer or professional association.

ISO 7243:2017 (Ergonomics of the thermal environment — Assessment of heat stress using the WBGT index): international standard providing reference values by metabolic rate, with and without solar radiation. Available through ISO or national standards bodies (ANSI, CSA, AFNOR, DIN).

NIOSH Publication 2016-106 (Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments): free download from CDC/NIOSH. Provides recommended exposure limits (REL) for heat stress with WBGT-based criteria tables, including separate tables for acclimatized and non-acclimatized workers.

OSHA does not currently have a specific WBGT-based heat standard, but references NIOSH and ACGIH criteria. Some US states (California, Minnesota, Washington) have state-level heat illness prevention regulations. In Quebec, the RSST specifies temperature thresholds in Annex I but these are dry-bulb based, not WBGT. Always verify which specific standard has legal or advisory authority in your jurisdiction.

WBGT Action Thresholds — ACGIH, Military, and Athletic Standards

WBGT integrates temperature, humidity, radiant heat, and air movement — making it more comprehensive than air temperature or heat index alone. While heat index is useful for general public warnings, WBGT is the standard used in occupational health and athletic event planning because it captures all four environmental heat load pathways.

ACGIH TLV thresholds for outdoor workers (moderate work intensity, acclimatized):
— Below 26 °C WBGT: no specific action required for acclimatized workers.
— 26–28 °C: continuous work permitted; rest breaks recommended for heavy work.
— 28–30 °C: one 45-minute rest period per hour for heavy work.
— 30–32 °C: one 30-minute rest period per hour for heavy work.
— Above 32 °C: heavy outdoor work not recommended; light work with continuous monitoring only.

The US Army uses a five-flag system based on WBGT. The Black Flag condition (WBGT above 32 °C) prohibits all physical training for unacclimatized soldiers and severely restricts even acclimatized personnel. This system has been adopted by many military and law enforcement training programs worldwide.

For athletes, the American College of Sports Medicine (ACSM) recommends postponing long-distance running events when WBGT exceeds 28 °C and cancellation when it exceeds 33 °C. Many road race directors and sports governing bodies now require WBGT monitoring as a condition of event permits.

Related tools: Air Changes Per Hour, TWA Calculator, and STEL Checker.

Limitations of WBGT Estimates

Indoor vs outdoor WBGT: outdoor WBGT is measured with a black globe thermometer that directly captures solar radiation load. This calculator uses the simplified indoor formula or an estimated outdoor value derived from input parameters, it is not equivalent to a direct instrument reading in the field. For compliance purposes, use a calibrated WBGT monitor.

Acclimatization: newly exposed workers are 30–40% more susceptible to heat illness than workers who have progressively adapted over 7–14 days. ACGIH thresholds are established for acclimatized workers. For individuals in their first week of heat exposure, add approximately 2 °C buffer to effective WBGT when applying threshold tables.

Individual risk factors: cardiovascular disease, use of diuretics, obesity, and certain medications (antihistamines, beta-blockers, anticholinergics) significantly increase heat illness risk at any WBGT level. A WBGT reading does not account for individual physiology — occupational health surveillance is required for at-risk workers.

Clothing correction: a standard cotton work uniform adds approximately 2–4 °C to the effective WBGT experienced by the worker. Impermeable protective clothing (such as Tyvek coveralls or chemical protective suits) adds 10–15 °C effective WBGT. This calculator computes environmental WBGT only — clothing correction factors must be applied separately when selecting thresholds.

Measurement accuracy: a true WBGT reading requires three calibrated instruments — a wet-bulb thermometer exposed to natural air movement, a dry-bulb thermometer shielded from radiation, and a black globe thermometer. Weather station data or smartphone-derived estimates cannot replicate these conditions, particularly in environments with localized radiant heat sources.

Frequently Asked Questions

Why doesn't this tool tell me if my workplace is too hot?

WBGT is an environmental index. Whether a given WBGT is acceptable depends on the metabolic workload of the task, the acclimatization status of the workers, and the specific referential (ACGIH, NIOSH, ISO 7243, etc.) that applies to your situation. These variables require professional judgment that a calculator cannot substitute for. The tool gives you the WBGT number; a qualified industrial hygienist or occupational health professional interprets it in context.

What's the difference between natural wet-bulb and aspirated wet-bulb?

The natural wet-bulb (NWB) is exposed to ambient air movement without forced airflow, it absorbs radiation from surroundings and depends on natural convection. The aspirated (or psychrometric) wet-bulb is shielded from radiation and measured under forced airflow, which is used in standard psychrometers to calculate relative humidity. They give different readings in the same environment. The WBGT formula (ISO 7243, ACGIH) uses the natural wet-bulb, not the aspirated one. Using an aspirated wet-bulb value in the WBGT formula would underestimate the heat burden in radiant environments.

How accurate are smartphone-based WBGT estimates?

Smartphone apps that estimate WBGT from weather data use algorithms that approximate the three components (NWB, globe, dry-bulb) from air temperature, relative humidity, solar radiation, and wind speed. These estimates can be useful for general situational awareness outdoors, but they carry significant uncertainty in industrial settings where local radiant heat sources (furnaces, hot surfaces, adjacent processes) dominate. For workplace assessment, direct measurement with a calibrated WBGT monitor (ISO-compliant globe thermometer and natural wet-bulb sensor) is required.

Does WBGT cover cold stress?

No. WBGT is a heat stress index only. It was developed by the US military in the 1950s specifically to prevent heat casualties. For cold stress assessment, different indices apply: wind chill temperature, IREQ (Required Clothing Insulation, ISO 11079), or ACGIH TLV for Cold Stress. The WBGT formula produces meaningless results in cold environments and should not be used below approximately 15 °C.

What's a globe thermometer?

A globe thermometer is a temperature sensor (thermocouple, thermistor, or mercury thermometer) inserted at the center of a hollow matte-black sphere. The ISO 7726 standard specifies a 150 mm (6-inch) copper globe painted matte black. The black surface absorbs radiant energy from all directions, reaching thermal equilibrium between radiation absorption and convective loss to air. The equilibrium temperature, the globe temperature — reflects both radiant heat load and air temperature and velocity. Smaller globes (38 mm) equilibrate faster and are used in some portable WBGT instruments but require correction factors.

How do I correctly measure the natural wet-bulb temperature?

Place a clean cotton wick over the sensor of a thermometer, soak it with distilled water, and expose the assembly to natural air movement (2–3 m/s is ideal — too still and the wick stays saturated; too windy and you get aspirated wet-bulb instead). The wick must be replaced regularly to avoid mineral buildup. NWB is NOT the same as the wet-bulb temperature reported by a digital hygrometer (which is computed from RH and dry-bulb).

What is the difference between WBGT and heat index?

Heat index (apparent temperature) combines dry-bulb temperature and relative humidity. WBGT adds two more factors: radiant heat (sun load, measured by a black globe thermometer) and air movement. For outdoor workers in direct sunlight, WBGT is a more comprehensive and accurate measure of heat stress. Heat index is commonly used in public weather forecasts; WBGT is the standard for occupational health and athletic event planning.

At what WBGT should outdoor work stop?

ACGIH recommends ceasing heavy outdoor work when WBGT exceeds 32 °C for acclimatized workers. For light to moderate work, limits are higher but still require monitoring and rest breaks. In practice, many jurisdictions require outdoor work modifications or stoppage based on WBGT readings — check your provincial/state occupational health and safety regulations, as thresholds vary.