"Heat stress calculator" covers several different calculations, and the first useful step is working out which one your situation actually calls for. Picking the wrong index produces a number that is accurate and irrelevant.
Heat stress is the load the environment and the work place on the body. Heat strain is the body's response: core temperature, heart rate, sweat rate. Indices estimate stress. Only physiological monitoring measures strain, which is why every index eventually hands off to it when conditions get extreme.
Which index for which situation
| Index | Inputs | Use it when |
|---|---|---|
| WBGT | Temperature, humidity, wind, solar radiation | People are exerting outdoors, equipment is involved, or a standard requires it. The basis of ISO 7243, ACGIH and athletic thresholds. |
| Heat index | Temperature, humidity | General awareness, people at rest or light activity, indoors with no solar load. Simple to obtain, understates risk in sun. |
| Humidex | Temperature, dew point | Public comfort reporting, mainly Canada. Not an exposure limit. |
| ACGIH TLV | WBGT, workload, clothing, acclimatization | Occupational screening. Not a separate measurement; a decision framework built on WBGT. |
| ISO 7933 PHS | Full environmental, metabolic and clothing model | Detailed analysis when WBGT screening has been exceeded and you need more than a yes or no. |
Calculating WBGT
For most people asking about heat stress, WBGT is the answer, because it is what the standards use. The WBGT calculator computes it either exactly from three thermometer readings or as an estimate from temperature, humidity, sun and wind.
Outdoors with a solar load the weighting is 0.7 × natural wet bulb + 0.2 × black globe + 0.1 × dry bulb. Indoors or in shade it reduces to 0.7 × natural wet bulb + 0.3 × black globe.
A full occupational heat stress assessment
Screening is step one of a sequence, not the whole exercise. ACGIH and ISO 7243 both frame it this way.
- Screen. Measure WBGT, apply the clothing adjustment, classify the workload, and compare against the TLV or the action limit.
- If under the limit, maintain general controls: water, shade, training, acclimatization and supervision. Document the screening.
- If over, apply engineering and administrative controls and re-screen: shade, cooling, reflective barriers, scheduling, mechanization, rotation.
- If still over, move to detailed analysis with ISO 7933 predicted heat strain, or to direct physiological monitoring.
- Monitor the people. Heart rate recovery, body mass change across the shift, core temperature where feasible, and direct observation.
- Record all of it. The assessment is only demonstrable if it was written down.
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.
What counts as light, moderate and heavy work
The workload category moves the threshold by several degrees, so classifying it honestly matters more than getting the reading to a decimal place.
| Category | Roughly | Examples |
|---|---|---|
| Light | Up to 200 W | Sitting or standing to control machines, light hand or arm work, driving |
| Moderate | 200 to 350 W | Sustained hand and arm work, walking with moderate lifting, pushing a wheelbarrow |
| Heavy | 350 to 500 W | Shoveling, sledgehammer work, carrying heavy loads, continuous digging |
| Very heavy | Over 500 W | Intense sustained effort, axe work, climbing stairs or ladders with a load |
ACGIH screening limits
The ACGIH threshold limit values are the numbers industrial hygienists actually screen against. They are set by workload and by how much of each hour is spent working, and they assume acclimatized workers in light summer clothing.
| Work per hour | Light | Moderate | Heavy | Very heavy |
|---|---|---|---|---|
| 75 to 100% | 31.0°C | 28.0°C | — | — |
| 50 to 75% | 31.0°C | 29.0°C | 27.5°C | — |
| 25 to 50% | 32.0°C | 30.0°C | 29.0°C | 28.0°C |
| 0 to 25% | 32.5°C | 31.5°C | 30.5°C | 30.0°C |
For unacclimatized workers, the action limit sits roughly 2 to 3°C lower across the board. In Fahrenheit, the moderate-work continuous limit of 28.0°C is 82.4°F, which is a long way below the 90°F that people intuitively think of as dangerous.
Individual factors no calculator includes
Every index describes an environment and an average worker. Neither of those is the person in front of you.
Acclimatization state, age, body composition, cardiovascular fitness, prior heat illness, current illness or fever, sleep, hydration status, alcohol the night before, and a long list of common medications including diuretics, antihistamines, beta blockers and some antidepressants all change an individual's tolerance, sometimes considerably.
This is why supervision and a culture of reporting are not soft additions to a heat program. They are the only mechanism that catches the person for whom the index was wrong.
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.
WBGT, calculated continuously
Rather than entering conditions, the app reads them for your location and runs the full Liljegren model, with modes for athletics, military, outdoor work, aviation and OSHA so the guidance matches the situation.
- 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 is the best heat stress calculator?
For people exerting outdoors or anywhere a standard applies, WBGT is the right index, because ISO 7243, ACGIH, NATA and military guidance are all written in it. The heat index is adequate for general awareness and people at rest.
What is the difference between heat stress and heat strain?
Heat stress is the load the environment and the work place on the body. Heat strain is the body's response to it, measured as core temperature, heart rate and sweat rate. Indices estimate stress; only physiological monitoring measures strain.
How do you calculate heat stress at work?
Measure WBGT, add a clothing adjustment factor, classify the workload, and compare against the ACGIH TLV for acclimatized workers or the lower action limit for unacclimatized ones. If the limit is exceeded, apply controls and re-screen.
Is the heat index a heat stress calculator?
It is a limited one. It uses only temperature and humidity and assumes shade with light wind, so it misses solar radiation and wind entirely. OSHA advises adding roughly 15°F to the heat index in direct sunlight for that reason.
What medications increase heat risk?
Diuretics, antihistamines, beta blockers, some antipsychotics and antidepressants, and stimulants can all reduce heat tolerance by affecting sweating, blood flow or fluid balance. Workers and athletes should discuss this with a clinician rather than guessing.
Related WBGT guides
Know the real heat number
Real-time WBGT, flag levels and forecasts. Free on iPhone.
