The heat category chart is the most prescriptive heat reference in common use. It does not offer guidance to interpret; it gives a duty officer a number of minutes and a number of quarts for a given category and workload.
That prescriptiveness is why it travels so well outside the military. Industrial and emergency services operations frequently adopt it directly, because translating a number into "twenty minutes on, forty off, one quart an hour" is exactly the translation a supervisor needs and most charts leave out.
The five categories
- Easy and moderate work: no limit. Hard work: 40 minutes on, 20 off.
- Water: ½ to ¾ quart per hour.
- Extreme caution still applies for unacclimatized personnel.
- Easy work: no limit. Moderate: 50/10. Hard: 30/30.
- Water: ½ to 1 quart per hour.
- Strenuous exercise limited for unacclimatized personnel.
- Easy work: no limit. Moderate: 40/20. Hard: 30/30.
- Water: ¾ to 1 quart per hour.
- Strenuous exercise and classes in the sun limited to one hour.
- Easy work: no limit. Moderate: 30/30. Hard: 20/40.
- Water: ¾ to 1 quart per hour.
- Strenuous exercise limited; unacclimatized personnel should not conduct hard work.
- Easy work: 50/10. Moderate: 20/40. Hard: 10/50.
- Water: 1 quart per hour.
- Nonessential physical training suspended. Essential mission activity only, with controls.
Heat categories, work-rest cycles and water
The military heat category table ties each category to a work-rest cycle and an hourly water intake for easy, moderate and hard work. These are the figures the flag colors exist to communicate.
| Category | WBGT | Easy work | Moderate work | Hard work |
|---|---|---|---|---|
| 1 (White) | 78 to 81.9°F | No limit, ½ qt | No limit, ¾ qt | 40/20, ¾ qt |
| 2 (Green) | 82 to 84.9°F | No limit, ½ qt | 50/10, ¾ qt | 30/30, 1 qt |
| 3 (Yellow) | 85 to 87.9°F | No limit, ¾ qt | 40/20, ¾ qt | 30/30, 1 qt |
| 4 (Red) | 88 to 89.9°F | No limit, ¾ qt | 30/30, ¾ qt | 20/40, 1 qt |
| 5 (Black) | 90°F and above | 50/10, 1 qt | 20/40, 1 qt | 10/50, 1 qt |
Work-rest figures are minutes of work and rest within each hour. Rest means minimal physical activity, in shade where possible.
Hourly fluid intake should not exceed 1½ quarts, and daily intake should not exceed 12 quarts. Overdrinking causes hyponatremia, which presents much like heat illness and is made considerably worse by the instinctive response to it.
What counts as easy, moderate and hard work
| Category | Examples |
|---|---|
| Easy | Weapon maintenance, walking on hard surface at 2.5 mph with under 30 lb load, marksmanship training, drill and ceremony |
| Moderate | Walking in sand at 2.5 mph with no load, patrolling, individual movement techniques, defensive position construction, calisthenics |
| Hard | Walking in sand at 2.5 mph with a load, field assaults, walking on hard surface at 3.5 mph with over 40 lb load |
Heat category compared with athletic flag conditions
The ranges line up closely, which is not a coincidence. Athletics adopted the military system. What differs is the response attached to each band.
| Band | Military WBGT | Athletic WBGT | Difference in response |
|---|---|---|---|
| White | 78 to 81.9°F | Under 80°F | Military specifies water and work-rest even in category 1 |
| Green | 82 to 84.9°F | 80 to 84.9°F | Broadly equivalent |
| Yellow | 85 to 87.9°F | 85 to 87.9°F | Identical range; athletics adds equipment limits |
| Red | 88 to 89.9°F | 88 to 89.9°F | Identical range; athletics removes protective equipment |
| Black | 90°F and above | 90°F and above | Military permits essential mission activity; athletics cancels |
The meaningful difference is at the top. Athletics can cancel practice; a military unit may have an essential mission, so category 5 specifies how to work rather than stopping entirely.
Clothing adjustments
| Worn | Add |
|---|---|
| Standard combat uniform | 0°F |
| Uniform with body armor | about 5°F |
| MOPP 4 | about 10°F |
Apply the adjustment to the measured WBGT, then read the category. A measured 80°F in MOPP 4 is category 5, not category 1.
Warning signs of heat illness
Heat stress shows up before it becomes an emergency. Anyone on site should speak up immediately on any of these:
When in doubt, cool first and call for help. Collapse or confusion in the heat should be treated as possible exertional heat stroke: begin cold-water immersion immediately and call 911. Cooling comes before transport.
The chart, applied to live conditions
Military mode in the app reads the current category for your location and shows the work-rest and water guidance that goes with it, with a forecast so training can be scheduled around the categories rather than reacting to them.
- Real-time WBGT for your location or any city
- Hourly and 14-day WBGT forecasts to plan ahead
- Color-coded flag levels with mode-specific guidance
- Humidity, wind, sun and cloud cover behind every reading
WBGT is estimated from weather data with the peer-reviewed Liljegren model. Where a policy requires on-site measurement, use the app for planning and as a cross-check alongside your instrument.
Common questions
What WBGT is heat category 5?
Heat category 5, black flag, is 90°F WBGT and above. Nonessential physical training is suspended, and essential activity runs on 50/10, 20/40 or 10/50 work-rest cycles for easy, moderate and hard work with 1 quart of water per hour.
What is the difference between heat category and flag condition?
They describe the same WBGT ranges. The military numbers them 1 through 5 and attaches specific work-rest cycles and water intake; the flag colors are the visual shorthand that athletics and industry adopted.
What does black flag mean in the military?
Category 5, 90°F WBGT or above. Nonessential physical training is suspended. Essential mission activity continues under strict work-rest cycles with active monitoring and cooling available.
How much water per hour in heat category 4?
Three quarters to one quart per hour depending on workload, with work-rest cycles of no limit for easy work, 30/30 for moderate and 20/40 for hard work. Do not exceed 1½ quarts in any hour.
Can civilians use the military heat category chart?
Yes, and many industrial and emergency services operations do. It is more prescriptive than most occupational charts, giving explicit work-rest minutes and water amounts. Check it against any standard that governs you, such as the ACGIH TLVs.
Related WBGT guides
Know the real heat number
Real-time WBGT, flag levels and forecasts. Free on iPhone.
