A jobsite heat calculation ends in three numbers a foreman can actually use: minutes of work, minutes of rest, and cups of water per hour. Everything before that is arithmetic in service of those three.
Start with the reading from the WBGT calculator or an on-site instrument, add the clothing adjustment, and classify the workload honestly. Then read across.
Clothing adjustment factors
Clothing that blocks evaporation raises the effective WBGT. Add the adjustment to your measured reading before comparing it with a limit.
| Clothing | Add to WBGT |
|---|---|
| Work clothes, long sleeves and trousers | 0°C |
| Cloth coveralls | 0 to 2°C |
| Double-layer woven clothing | 3°C |
| SMS polypropylene coveralls | 0.5°C |
| Polyolefin coveralls | 1°C |
| Limited-use vapor-barrier coveralls | 11°C |
That last row is not a typo. An encapsulating vapor-barrier suit shuts down evaporative cooling almost completely, and WBGT screening stops being adequate at that point. Physiological monitoring takes over.
Work rest cycles by WBGT and workload
Minutes of work and rest within each hour, for acclimatized workers. These follow the structure of the military work-rest tables, which translate well to industrial work.
| Adjusted WBGT | Light work | Moderate work | Heavy work |
|---|---|---|---|
| 78 to 81.9°F | Continuous | Continuous | 40 work / 20 rest |
| 82 to 84.9°F | Continuous | 50 / 10 | 30 / 30 |
| 85 to 87.9°F | Continuous | 40 / 20 | 30 / 30 |
| 88 to 89.9°F | Continuous | 30 / 30 | 20 / 40 |
| 90°F and above | 50 / 10 | 20 / 40 | 10 / 50 |
Rest means shade, seated if possible, with fluids. Rest spent standing in the sun waiting for a delivery is not rest, and the cycle does not work if the rest half of it is fictional.
For unacclimatized workers, use the cycle from the next band up. Their margin is substantially smaller and the first week is when most fatalities occur.
Water intake targets
Dehydration compounds heat strain, and thirst lags behind need. Build drinking into the cycle rather than leaving it to individuals.
| Adjusted WBGT | Light work | Moderate work | Heavy work |
|---|---|---|---|
| 78 to 81.9°F | about 1 cup | about 3 cups | about 3 cups |
| 82 to 87.9°F | about 2 cups | about 3 cups | about 4 cups |
| 88°F and above | about 3 cups | about 4 cups | about 4 cups |
Hourly fluid intake should generally not exceed about 6 cups, and daily intake about 12 quarts. Drinking well beyond sweat losses carries its own risk of hyponatremia, which can present a lot like heat illness and is made worse by the obvious response to it.
A worked shift
A concrete crew, moderate to heavy work, cloth coveralls, on an 85°F WBGT day.
| Step | Value |
|---|---|
| Measured WBGT at the work face | 85.0°F |
| Clothing adjustment, cloth coveralls | +3.6°F (2.0°C) |
| Adjusted WBGT | 88.6°F |
| Workload | Heavy |
| Work rest cycle, acclimatized | 20 minutes work, 40 minutes rest per hour |
| Work rest cycle, first week on the job | 10 minutes work, 50 minutes rest per hour |
| Water target | About 4 cups per hour |
| Practical response | Move the heavy pour to the first hours of the shift, rotate crews, and mechanize what can be mechanized rather than running a 20/40 cycle all afternoon. |
A 20/40 cycle is a signal, not a plan. If the controls you have leave you working one third of each hour, the realistic answer is rescheduling the work, not grinding through it.
Running it on the day
- Take the reading at the work face before the shift starts.
- Add the clothing adjustment for what the crew is actually wearing, including PPE.
- Classify the workload. Be honest; overstating light work is how people get hurt.
- Step up one band if anyone on the crew is in their first week.
- Announce the cycle, set a timer, and make sure the shade and water are at the work face before the first break.
- Re-read hourly and adjust. Log it.
What a heat illness prevention plan contains
Whether or not a specific standard applies to you, this is the structure regulators, insurers and plaintiffs' attorneys all expect to see.
- Measurement. Who takes the reading, where, how often, and how it is recorded.
- Trigger levels. What changes at each threshold, written down in advance rather than decided in the moment.
- Water. Cool potable water available at the work area, in quantity, with a target of roughly one cup every 15 to 20 minutes.
- Shade and rest. Shade available at or before the trigger, large enough for everyone on break at once, and rest that is actually taken.
- Acclimatization. A graded schedule for new workers and anyone returning after more than a week away, with closer supervision during it.
- Training. Symptoms, reporting, first aid and the emergency plan, for workers and supervisors, in a language they understand.
- Emergency response. Who calls, what the site address is, how responders reach the work area, and who starts cooling in the meantime.
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.
Get the reading without the instrument
The app computes WBGT for your location and forecasts it hour by hour, which is what you need to decide at six in the morning whether the heavy work goes first.
- 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 work rest cycles for heat?
Take the WBGT reading, add a clothing adjustment, classify the workload as light, moderate or heavy, then read the work and rest minutes per hour from a work-rest table. Step up one severity band for unacclimatized workers.
How much water should workers drink in the heat?
Roughly one cup every 15 to 20 minutes during moderate to heavy work in the heat, which is about 3 to 4 cups an hour. Keep hourly intake under about 6 cups and daily intake under about 12 quarts to avoid hyponatremia.
What is a 20/40 work rest cycle?
Twenty minutes of work and forty minutes of rest within each hour. It appears at high WBGT with heavy work, and in practice it is a signal that the work should be rescheduled or mechanized rather than run on that cycle all afternoon.
Do work rest cycles differ for unacclimatized workers?
Yes, substantially. Use the cycle from the next severity band up for anyone in their first week on the job or returning after more than a week away, since that is when most heat fatalities occur.
Does rest in the sun count?
No. Rest means shade, seated where possible, with fluids available. A cycle whose rest half is spent standing in full sun does not deliver the cooling the table assumes, and the plan stops working.
Related WBGT guides
Know the real heat number
Real-time WBGT, flag levels and forecasts. Free on iPhone.
