The military calculation is the most mechanical of the heat frameworks, and that is its virtue. There is no interpretation step. Measure, adjust for what people are wearing, read the category, read the cycle.
The step people skip is the adjustment, and it is the step that matters most, because protective equipment can move the effective reading by ten degrees.
The calculation
- Measure WBGT at the training area, or compute it with the WBGT calculator.
- Add the clothing adjustment: about 5°F for body armor, about 10°F for MOPP 4.
- Read the heat category from the adjusted WBGT.
- Classify the workload as easy, moderate or hard.
- Read the work-rest cycle and hourly water intake from the table.
- Step more conservative for unacclimatized personnel and anyone with a prior heat injury.
Clothing adjustment
| Worn | Add |
|---|---|
| Standard combat uniform | 0°F |
| Uniform with body armor | about 5°F |
| MOPP 4 protective posture | about 10°F |
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 |
Worked example
A measured 83°F WBGT, which reads as category 2 and is not.
| Posture | Adjusted WBGT | Category | Hard work cycle |
|---|---|---|---|
| Combat uniform | 83°F | Category 2 (Green) | 30 minutes work, 30 rest, 1 qt/hr |
| Uniform and body armor | 88°F | Category 4 (Red) | 20 minutes work, 40 rest, 1 qt/hr |
| MOPP 4 | 93°F | Category 5 (Black) | 10 minutes work, 50 rest, 1 qt/hr |
One reading, one morning, three different training plans. Skipping the adjustment on the third row would mean running hard work at a 30/30 cycle in conditions that call for 10/50.
Limits of the calculation
The table assumes acclimatized personnel in good health. It does not account for prior heat injury, recent illness, sleep debt, dehydration carried from the day before, sickle cell trait, or the medications that reduce heat tolerance.
It also cannot see the training area. A reading taken at the headquarters building does not describe a range with no shade, a hardstand in full sun, or a confined space with running equipment in it. Measure where the training happens.
Finally, the cycle only works if the rest half is real: shade where possible, minimal activity, and fluids in hand. A 20/40 cycle whose forty minutes are spent standing in the sun delivers the recovery of no cycle at all.
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.
Category and cycle, computed live
Military mode reads the current category for your location and shows the work-rest and water guidance beside it, with an hourly forecast for building a training schedule.
- 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
How do you calculate the military heat category?
Measure WBGT, add the clothing adjustment of about 5°F for body armor or 10°F for MOPP 4, then read the category from the adjusted value: category 1 at 78 to 81.9°F, rising to category 5 at 90°F and above.
What is the work rest cycle for hard work in category 4?
Twenty minutes of work and forty minutes of rest within each hour, with one quart of water per hour. Rest means minimal physical activity, in shade where available.
How much does MOPP 4 add to WBGT?
About 10°F. A measured 80°F WBGT in MOPP 4 behaves like 90°F, which is category 5. This is why protective posture training is generally scheduled for the coolest hours available.
Does the calculation change for unacclimatized personnel?
Yes. The table assumes acclimatized personnel, and unacclimatized ones need more conservative cycles and closer supervision. Acclimatization takes 10 to 14 days of progressive exposure, with most adaptation in the first week.
What is the maximum water intake?
One and a half quarts per hour and 12 quarts per day. Exceeding these risks hyponatremia, which presents much like heat illness and gets worse if treated by drinking more water.
Related WBGT guides
Know the real heat number
Real-time WBGT, flag levels and forecasts. Free on iPhone.
