Heat Stress (WBGT) Calculator

What is Wet Bulb Globe Temperature (WBGT)?

WBGT is a composite heat stress index that combines the effects of air temperature, humidity, wind speed, and radiant heat into a single value. Developed by the U.S. military in the 1950s, it is now the international standard (ISO 7243) for assessing heat stress risk in occupational settings.

The index uses three temperature measurements: natural wet bulb temperature (Tnwb, reflecting humidity and evaporative cooling), globe temperature (Tg, reflecting radiant heat), and dry bulb temperature (Tdb, air temperature). Outdoors with solar load, WBGT = 0.7×Tnwb + 0.2×Tg + 0.1×Tdb. Indoors without solar load, the dry bulb component is omitted: WBGT = 0.7×Tnwb + 0.3×Tg.

ISO 7243 (Annex A, Table A.1) defines reference WBGT values for five workload classes — resting, light, moderate, heavy, and very heavy — each with a lower value for unacclimatized workers. For example, acclimatized workers performing moderate work have a reference value of 28°C, while unacclimatized workers have a lower threshold of 26°C. ACGIH publishes a comparable scheme as Threshold Limit Values (TLVs).

Formula: Outdoor: WBGT = 0.7 × Twb + 0.2 × Tg + 0.1 × Tdb Indoor: WBGT = 0.7 × Twb + 0.3 × Tg

Example Calculation

Outdoor conditions: dry bulb 35°C, wet bulb 28°C, globe temperature 42°C. WBGT = 0.7×28 + 0.2×42 + 0.1×35 = 19.6 + 8.4 + 3.5 = 31.5°C. For moderate work by an acclimatized worker (ISO 7243 reference value = 28°C), this WBGT exceeds the threshold — work-rest cycling or additional controls are required.

When to Use This Calculator

• Safety managers determining work-rest schedules for outdoor workers during summer heat events or in hot process environments
• Construction site supervisors evaluating whether environmental conditions require heat stress controls before starting work shifts
• Occupational hygienists conducting heat exposure assessments in foundries, laundries, kitchens, or other high-radiant-heat workplaces
• Military and athletic training staff assessing whether training intensity should be reduced based on current WBGT readings
• Choosing between heat-stress and comfort assessment — use this ISO 7243 WBGT index for hot environments and work-rest planning; for comfort in moderate indoor conditions, use the thermal comfort (PMV/PPD) calculator (ISO 7730)

Common Mistakes to Avoid

• Using dry bulb temperature alone instead of WBGT — a 30°C day at 90% humidity is far more dangerous than 38°C at 20% humidity
• Applying indoor WBGT formula outdoors — the outdoor formula includes a solar radiation term (0.1×Tdb) that accounts for direct sun exposure
• Classifying workload incorrectly — overestimating as 'light' when workers are performing moderate or heavy tasks leads to inadequate protection; use metabolic rate tables
• Ignoring acclimatization status — new workers or those returning from absence need 7-14 days of gradual exposure; use the lower unacclimatized thresholds during this period

How to Interpret Results

• If WBGT is below the threshold for the workload and acclimatization status, continuous work is acceptable with standard hydration
• If WBGT is within 1-2°C of the threshold, implement a 75% work / 25% rest schedule per hour and ensure water is available within easy reach
• If WBGT exceeds the threshold, mandatory work-rest cycling is required — 50/50 or 25/75 ratios depending on how far above threshold; consider rescheduling to cooler hours
• If WBGT exceeds 33°C for any workload, all non-essential outdoor work should be suspended regardless of acclimatization

Related Standards & References

• ISO 7243 — Ergonomics of the thermal environment — Assessment of heat stress using the WBGT index
• ACGIH TLV for Heat Stress — Threshold Limit Values based on WBGT and workload
• OSHA Technical Manual Section III:4 — Heat Stress
• NIOSH Criteria Document — Occupational Exposure to Heat and Hot Environments (2016)

WBGT reference values by workload

Every workload class ISO 7243:2017 defines, calculated by this tool's own threshold lookup for both acclimatization states.

Frequently Asked Questions

What is the difference between acclimatized and unacclimatized workers?

Acclimatization is a physiological adaptation to heat that develops over 7-14 days of gradual exposure. Acclimatized workers sweat more efficiently, maintain lower core temperatures, and have improved cardiovascular stability. ISO 7243 provides separate, lower WBGT reference values for unacclimatized workers (ACGIH's TLVs do the same) because they are significantly more susceptible to heat illness.

Why is WBGT preferred over simple air temperature?

Air temperature alone does not account for humidity, radiant heat, or air movement — all critical factors in heat stress. A 30°C day with 90% humidity is far more dangerous than 35°C with 30% humidity because sweat cannot evaporate. WBGT integrates all these factors, making it a much more reliable predictor of heat-related illness risk.