Wet Bulb Globe Temperature (WBGT) Calculator
Calculate WBGT heat-stress index from temperature, humidity, and sun exposure.
Get OSHA work-rest cycles and athlete heat-safety levels for outdoor activity.
Wet Bulb Globe Temperature (WBGT)
WBGT is the gold standard for heat stress assessment in outdoor sports, military training, and OSHA compliance. Unlike heat index (which only uses dry temp + humidity), WBGT accounts for direct sun exposure and wind.
The full formula: WBGT = 0.7 × Tw + 0.2 × Tg + 0.1 × Td (outdoor, with sun)
Where:
- Tw = wet bulb temp (humidity factor)
- Tg = globe temp (radiant heat from sun)
- Td = dry bulb temp (air temp)
Indoor or shaded version: WBGT = 0.7 × Tw + 0.3 × Td
Wet bulb estimation from RH: For typical conditions, wet bulb temp can be approximated: Tw ≈ Td × atan(0.151977 × √(RH + 8.313659)) + atan(Td + RH) - atan(RH - 1.676331) + 0.00391838 × RH^1.5 × atan(0.023101 × RH) - 4.686035
(Stull formula — accurate within ±1°C in typical ranges)
WBGT thresholds (athletic safety):
| WBGT (°C) | WBGT (°F) | Risk Level | Recommendation |
|---|---|---|---|
| Below 25.6 | Below 78 | Low | Normal activity |
| 25.6-27.7 | 78-82 | Moderate | Watch hydration |
| 27.8-29.4 | 82-85 | High | 30-min rest per hour |
| 29.5-31.0 | 85-88 | Very High | 30+ min rest per hour |
| Above 31 | Above 88 | EXTREME | Cancel outdoor activity |
OSHA work-rest cycles:
| WBGT (°F) | Work / Rest Ratio | Notes |
|---|---|---|
| Below 80 | Continuous work | Normal |
| 80-84 | 50/10 (50 min work / 10 min rest) | Light work OK |
| 84-87 | 40/20 | Moderate work limit |
| 87-90 | 30/30 | Heavy work risky |
| 90-93 | 25/35 | Limit to acclimatized workers |
| Above 93 | Stop heavy work | Hydration only |
Acclimatization period: A non-acclimatized worker can experience heat illness at WBGT 4-6°F lower than an acclimatized one. 2-week graduated exposure allows the body to adapt:
- Day 1: 20% normal heat exposure
- Days 2-7: gradual increase
- Day 14+: full acclimatization, increase tolerance ~5°F WBGT
Sun vs shade adjustments:
- Direct sun: full WBGT calculation
- Light shade (50% sun): reduce WBGT estimate by 1-2°F
- Full shade: -3 to -5°F from outdoor WBGT
Wind adjustments (cooling effect):
- Calm (<3 mph): 0 reduction
- Light breeze (3-7 mph): -1°F
- Moderate (8-15 mph): -2 to -3°F
- Strong (15-25 mph): -3 to -5°F (but evaporative cooling depends on humidity)
Heat index vs WBGT — when each is used:
- Heat index (NWS): standard public-use comfort metric, indoor/shade
- WBGT: sports + occupational heat stress, accounts for sun + wind
For outdoor activity in direct sun, WBGT is 3-7°F higher than heat index for the same temp/humidity.
At-risk groups for heat illness:
- Older adults (>65)
- Children (poor temperature regulation)
- Athletes during football preseason (acclimatization issue)
- Construction / agricultural workers
- People on certain medications (diuretics, anticholinergics)
- Anyone with cardiovascular disease
Signs of heat illness (escalating severity):
- Heat cramps (muscle pain, sweating)
- Heat exhaustion (heavy sweating, weak pulse, dizziness)
- Heat stroke (high core temp, NO sweating, confusion) — MEDICAL EMERGENCY
Prevention:
- Hydrate every 15-20 minutes (8-10 oz)
- Sports drinks for activity > 1 hour (electrolytes)
- Light-colored loose clothing
- Pre-cool with cold towels or vests
- Buddy system — recognize symptoms early