Heat stress calculator: humidex and WBGT work/rest
Updated October 18, 20262 min tool
1
The conditions
Measure by
°C
%
Measure at the work, in the shade, within 10 m of the crew. A weather report is a warning, not a measurement.
The humidex plan adds 3 to 4 for direct sun between 10 a.m. and 5 p.m.
2
The work
Steady arm and leg work, normal walking, light pushing and pulling
Acclimatized to the heat
Acclimatized means working in the heat for 7 to 14 days, at least 2 hours a day. It’s lost after a week or more away. If unsure, say no.
hours
3
Work, rest and water
Work 15 minutes, rest 45 in every hour
An adjusted humidex of 44.3 is in the severe band of the humidex plan (moderate work, unacclimatized, or heavy work, acclimatized).
Humidex
39.3
dew point 20.9 °C
Adjusted
44.3
+3.5 sun, +1.5 clothing
Water an hour
1.00 L
a cup every 15 min
Over 10 hours
10.5 L
each, with 500 mL before
Each hour
Work 15 min
Rest 45 min
:00:15:30:45:60
:00 :15 :30 :45250 mL of cool water each time. Never more than 1.5 L an hour or 11 L a day.
The humidex plan, column 1
Below
< 25
Very low
25–29
Low
30–33
Some
34–37
Moderate
38–39
Significant
40–41
Severe
42–44
Extreme
45+
If feasible, work 15 minutes, rest 45 in every hour.
Provide cool (10 to 15 °C) water: at least 250 mL (1 cup) every 15 to 20 minutes.
Anyone with symptoms should get medical attention.
Never ignore anyone's symptoms, whatever the reading. Confusion, collapse or hot dry skin is heat stroke: call 911 and cool the person right away.
The rules · Alberta
No heat-specific rule: heat is a hazard to assess and control under the general duty and the hazard assessment rules. Alberta’s guidance recommends cool water, a cooling area, a work-rest schedule with rest somewhere cool, and doing heavy work in the cooler hours.
Alberta’s guidance: one cup of water every 15 to 20 minutes.
A summary to plan with. The regulation and your employer's procedures govern.
Enter the temperature and humidity at the work, or a WBGT reading, and this calculator gives the humidex, the work and rest schedule for the crew, how much water each worker needs over the shift, and the heat stress rules in your province or territory.
How the heat plan works
The humidex plan turns temperature and humidity into actions: water, alerts, then rest breaks each hour, then stopping work. It assumes moderate work by workers who aren't used to the heat; light work, or moderate work by acclimatized workers, uses a second, higher column. Heavy work by new workers, very heavy work, process heat and clothing that stops sweat evaporating all need a WBGT reading instead.
Measure at the work, in the shade, every hour once the humidex reaches 30.
Add for direct sun and for coveralls: they change the answer by a whole band or more.
New workers, and anyone back after a week away, aren't acclimatized. Build them up over 7 to 14 days.
Rest somewhere cooler, in shade, with water. Short breaks every hour beat long ones less often.
Heat stroke is an emergency
Confusion, slurred speech, collapse, a seizure, or hot skin that may have stopped sweating: call 911. Move the person somewhere cool, remove extra clothing and cool them fast with water and fanning, or ice packs at the neck, armpits and groin. Never leave them alone.
Sources: Environment and Climate Change Canada, humidex formula and comfort bands; OHCOW, Humidex Based Heat Response Plan (April 2026); WorkSafeBC, Methods for Measuring Heat Strain or Stress (2026) and Preventing Heat Stress at Work; CCOHS, Hot Environments (2026), adapted from the ACGIH TLVs; Quebec RSST Schedule V; Yukon Workplace Health Regulations s.12; the OHS regulations of each jurisdiction (sections in the rules panel). Humidex is computed from the dew point (Magnus formula); published humidex tables can read one lower.
Frequently asked questions
At what humidex should work stop?
Under the humidex heat response plan used across Canada (OHCOW’s, which WorkSafeBC also publishes), at an adjusted humidex of 45 or more only medically supervised work continues, for moderate work by workers who aren’t acclimatized. Breaks start at 38: 15 minutes’ rest an hour, then 30 minutes at 40 and 45 minutes at 42. Add 3 to 4 for direct sun and 5 to 6 for cotton coveralls.
How much water should workers drink in the heat?
About 250 mL (one cup) of cool water every 15 to 20 minutes, starting before they feel thirsty, and 500 mL before the shift. That is 0.75 to 1 litre an hour. CCOHS advises no more than 1.5 litres an hour or 11 litres a day. Salt tablets aren’t recommended: normal meals replace the salt.
What is the difference between humidex and WBGT?
Humidex combines air temperature and humidity. WBGT, the wet bulb globe temperature, also measures radiant heat and air movement, so it is the better measure for work in the sun or near hot processes, and it is what the ACGIH limits and Quebec’s and Yukon’s regulations use. There is no exact conversion between the two.
Is there a legal heat limit in Alberta or Ontario?
Neither has a heat-specific regulation: heat is a hazard the employer must assess and control under the general duty. BC, Manitoba, New Brunswick, Nova Scotia, PEI and Newfoundland and Labrador write the ACGIH heat stress limits into law, Quebec and Yukon have their own tables, and federal workplaces will need heat procedures from January 2027.
Keep heat stress plans and hourly readings with the crew
TruStar Connect holds your heat stress plan, hazard assessments, toolbox talks and inspection records on every phone on site, offline, ready for your COR audit.
