Humidex calculator
Humidex is a Canadian invention and is not the American heat index. This uses the formula Environment Canada actually publishes.
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How is humidex calculated?
Humidex = air temperature + 0.5555 × (e − 10), where e is the vapour pressure in hectopascals derived from the dewpoint: e = 6.11 × exp(5417.7530 × (1/273.16 − 1/Td)) with Td in kelvin. At 30 °C with a 25 °C dewpoint the humidex is 42. It is a unitless index rather than a temperature, which is why Environment Canada quotes it as a bare number.
Humidex is Canadian, and it is not the heat index
Humidex was devised by Masterton and Richardson at Environment Canada in 1979 and is what Canadian forecasts report. The United States uses a different measure, the heat index, developed separately by Steadman.
The two are built on different assumptions and produce different numbers for identical weather. A Canadian humidex of 40 does not correspond to an American heat index of 40, and converting between them is not a matter of arithmetic.
This matters if you follow both Canadian and American forecasts, or use an app that reports whichever its data source provides without saying which.
If a figure is quoted in degrees, it is probably a heat index. Humidex is reported as a bare number.
Dewpoint, not relative humidity, drives it
The formula takes the dewpoint, because dewpoint measures the absolute amount of moisture in the air while relative humidity measures it against what the air could hold at that temperature.
That distinction matters. Ninety percent humidity at 15 °C is pleasant; ninety percent at 32 °C is dangerous. The relative figure is the same, the absolute moisture is wildly different, and it is the absolute amount that stops sweat evaporating.
This calculator accepts either. If you enter relative humidity it recovers the dewpoint first using the standard Magnus-Tetens approximation, then applies the official formula.
Where you have a choice, enter the dewpoint — it is the direct input and avoids one approximation.
Why sweating stops working
The physiology behind the index is simple enough. Your body sheds heat mainly by evaporating sweat, and evaporation depends on how much moisture the surrounding air can still absorb.
When the air is already close to saturated, sweat stops evaporating and simply runs off. You lose the water without the cooling, which is why high humidity produces both overheating and dehydration at once.
That is why still, humid heat is more dangerous than hotter dry heat, and why a fan helps in dry conditions but does very little once the dewpoint is high.
Above a humidex of 40 the practical advice is shade, water, and reduced exertion rather than air movement.
The bands describe adults at rest
Environment Canada's comfort bands run from no discomfort below 20, through some discomfort in the thirties, to dangerous above 45 and heat stroke imminent above 54.
Those describe a healthy adult, at rest, in the shade. Anyone working, exercising or in direct sun is effectively at a much higher value. Infants, older adults, people with heart or respiratory conditions, and those on certain medications are at risk well below the published thresholds.
Indoor temperature matters too. A humidex of 38 outdoors is survivable; the same conditions in an unventilated apartment overnight, with no chance to cool down, is where heat deaths actually happen.
Check on neighbours who live alone during a heat event. That is consistently the highest-value thing anyone can do.
The formula
Vapour pressure first, then a linear correction to the air temperature.
e = 6.11 × exp( 5417.7530 × (1/273.16 − 1/Td) )Td = dewpoint in kelvin (°C + 273.15)e = vapour pressure in hectopascalshumidex = Ta + 0.5555 × (e − 10)Ta = air temperature in °Cfrom relative humidity, recover the dewpoint first (Magnus-Tetens):γ = (17.27·Ta)/(237.7 + Ta) + ln(RH/100)Td = (237.7·γ)/(17.27 − γ)Environment Canada comfort bands:under 20 no discomfort 30-39 some discomfort20-29 comfortable 40-45 great discomfort, avoid exertion46-53 dangerous 54+ heat stroke imminent
The 5417.7530 constant is the latent heat of vaporisation of water divided by the gas constant for water vapour, and 273.16 K is the triple point of water. The expression is the Clausius-Clapeyron relation, which is why it appears in this shape rather than as a fitted polynomial.
Environment Canada does not report a humidex below about 20 °C, or where the correction would be negative — at those temperatures humidity is not the relevant factor.
Worked example: a humid Ontario afternoon
An air temperature of 30 °C with a dewpoint of 25 °C.
- Td in kelvin = 25 + 273.15298.15 K
- 5417.7530 × (1/273.16 − 1/298.15)1.6625
- e = 6.11 × exp(1.6625)32.22 hPa
- 0.5555 × (32.22 − 10)12.34
- Humidex = 30 + 12.3442
- Comfort bandGreat discomfort — avoid exertion
A humidex of 42, matching the Environment Canada table for those conditions.
The air is 30 °C. The humidity is adding the equivalent of a further 12 degrees of discomfort, which is why the index is reported separately rather than folded into the forecast temperature.
Staying safe in high humidex
- Move to shade or air conditioning; a fan alone does little once the dewpoint is high
- Drink before you feel thirsty — sweat is lost even when it is not evaporating
- Shift outdoor work to early morning; the dewpoint usually peaks later
- Check on older neighbours and anyone living alone
- Never leave anyone, including pets, in a parked vehicle
- Know the signs of heat stroke: confusion, hot dry skin, no sweating — call emergency services
Terms on this page
- Humidex
- Environment Canada's index combining temperature and humidity into a single unitless number. A Canadian measure, distinct from the American heat index.
- Dewpoint
- The temperature at which air becomes saturated. It measures absolute moisture, which is why the formula uses it rather than relative humidity.
- Vapour pressure
- The partial pressure of water vapour in the air, in hectopascals. Derived from the dewpoint and the direct input to the humidex correction.
- Heat index
- The American equivalent, built on different assumptions. It gives different values for identical weather and is not interchangeable with humidex.
Common questions
How is humidex calculated?
Humidex = air temperature + 0.5555 × (e − 10), where e is the vapour pressure derived from the dewpoint using e = 6.11 × exp(5417.7530 × (1/273.16 − 1/Td)), with Td in kelvin.
Is humidex the same as the heat index?
No. Humidex is Canadian, devised at Environment Canada in 1979; the heat index is American and built on different assumptions. They give different numbers for identical weather and cannot be converted between.
Why does humidex use dewpoint rather than humidity?
Because dewpoint measures the absolute moisture in the air, while relative humidity measures it against what the air could hold at that temperature. Ninety percent at 15 °C is pleasant; ninety percent at 32 °C is dangerous.
What humidex is dangerous?
Environment Canada treats 40 to 45 as great discomfort where exertion should be avoided, 46 to 53 as dangerous, and 54 or above as heat stroke imminent. Those bands describe healthy adults at rest in shade — anyone working, older, or unwell is at risk considerably lower.
Why is humidex quoted without a degree symbol?
Because it is an index, not a temperature. A humidex of 42 does not mean it feels like 42 °C in any measurable sense — it is a scale point describing combined heat and humidity discomfort.
Where these numbers come from
- Environment and Climate Change Canada — Humidex and heat The humidex formula, its origin, and the comfort bands used for heat warnings.
Last verified 2026-08-01 The method on this page is checked against the sources above at least once a year. Spotted something out of date? Tell us and we will fix it.
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