Humidex Calculator

How to use this tool

Enter the air temperature, choose °C or °F, and give the humidity either as relative humidity in percent or as a dew point in the same unit. The result line shows the humidex and its Environment Canada category. Under it are the dew point (or the relative humidity, when you started from a dew point), the vapour pressure that drives the calculation, the humidity term, and the US heat index for the same air. The chart covers the full humidity range at your temperature, and Copy link gives an address that reopens the tool with the same readings.

The input people most often get wrong is the humidity. Weather apps quote the relative humidity of the current hour, and that figure falls through the day as the air warms even when the moisture in it has not changed. Pasting a morning reading of 85% next to the afternoon high overstates the humidex by several points. If you have a dew point, use it: it barely moves between morning and afternoon, and it is what the official formula works from anyway. The other slip is the unit selector. Type 86 with it on °C and the tool refuses the reading as off the scale; switch to °F first.

What the calculation is doing

The humidex is the Environment and Climate Change Canada measure of how hot humid air feels. Its formula is short: humidex = T + 0.5555 × (e − 10), where T is the air temperature in °C and e is the water vapour pressure in hectopascals. The 10 hPa is a baseline for air dry enough to add no discomfort, and every hectopascal above it adds 0.5555 to the felt temperature. That constant is 5/9, added when Canada went metric to turn what had been one Fahrenheit degree per hectopascal into Celsius; the formula itself is the 1979 work of Masterton and Richardson at the Atmospheric Environment Service. Below the baseline the term goes negative and the arithmetic gives a number under the air temperature, which is one reason Environment Canada only publishes a humidex that is at least one degree above the temperature.

The vapour pressure comes from the dew point, not from relative humidity directly: e = 6.11 × exp(5417.7530 × (1 ÷ 273.15 − 1 ÷ Td)), with the dew point Td in kelvin. When you enter a relative humidity the calculator first converts it to a dew point with the Magnus formula and the Alduchov and Eskridge constants: γ = ln(RH ÷ 100) + 17.625 T ÷ (243.04 + T), then Td = 243.04 γ ÷ (17.625 − γ). Magnus is an approximation, good to about 0.2% in saturation vapour pressure between 0 and 50 °C, which moves the humidex by less than 0.1. From a dew point the same curve runs the other way: RH = 100 × es(Td) ÷ es(T).

Environment Canada publishes whole numbers, so the tool rounds the result and shows the unrounded figure beside it. The NWS heat index is worked out separately from the Rothfusz regression on the Fahrenheit temperature and relative humidity, with the two published adjustments for dry and very humid air. It is only shown inside the range the NWS charts cover, up to 110 °F of air and a heat index of about 137 °F. Because vapour pressure at a fixed temperature is proportional to relative humidity, the humidex rises in an almost straight line with humidity: at 30 °C each extra 10 points adds about 2.4. Temperature is the stronger lever, because saturation vapour pressure climbs about 6% per degree.

Why the humidex and the heat index disagree

Both indexes claim to say how hot the air feels, and for the same air they rarely agree. At 30 °C and 70% humidity the humidex is 41 while the NWS heat index is 35 °C (95 °F). At 35 °C and 60% the pair is 49 against 45 °C. At 25 °C and 60% it is 30 against 25 °C. The humidex is almost always the larger number, and the gap widens as the air gets wetter, so a Canadian forecast and a US one for the same air mass can look very different.

The reason is what each one measures. The humidex is a vapour pressure index: a fixed 0.5555 for every hectopascal above 10, whatever the temperature, so it is really a statement about the moisture load in the air. The heat index is a regression fit to Steadman’s 1979 model of an adult walking in the shade in a light wind, with clothing, sweating and skin resistance built in, and it is expressed in °F. It stays close to the air temperature in dry air and only climbs steeply once the air is both hot and humid: at 30 °C and 30% humidity the heat index is 29 °C, below the air temperature, while the humidex is 32. Neither index is wrong. They are different definitions with categories set separately, so each should only be read against its own scale.

Two practical points follow. The humidex has no unit; it reads like a Celsius temperature, and the tool shows what it would be as °F for readers used to that scale, but the conversion is a convenience rather than a measurement. And the humidex is a warning trigger in its own right. Under Ontario’s harmonized heat warning system, southern Ontario gets a heat warning when two or more consecutive days are forecast to reach 31 °C with a minimum of 20 °C, or a humidex of 40 or more. The humidex threshold is 42 in the extreme southwest and 36 in the north. Environment Canada itself reports a humidex in a forecast only when the temperature reaches 25 °C and the humidex is at least one degree higher; current conditions use 20 °C.

Humidex grid from 20 to 45 °C against 20% to 100% humidity

Every cell comes from the same formula as the tool, at 1 °C and 10 point steps. The colours are the Environment Canada categories, and grey figures are less than one degree above the air temperature, so Environment Canada would not publish them. Read across a row for the effect of humidity at a fixed temperature, and down a column for the effect of temperature, which is the larger of the two. At 70% humidity the step from 20 to 25 °C adds 8 points; the step from 40 to 45 °C adds 14.

A worked example you can check on paper

Take the tool’s defaults, 30 °C and 70% relative humidity. Magnus term: γ = ln(0.70) + 17.625 × 30 ÷ (243.04 + 30) = −0.35667 + 1.93653 = 1.57985. Dew point: Td = 243.04 × 1.57985 ÷ (17.625 − 1.57985) = 23.93 °C, which is 297.08 K. Exponent: 5417.7530 × (1 ÷ 273.15 − 1 ÷ 297.08) = 1.5977. Vapour pressure: e = 6.11 × exp(1.5977) = 30.19 hPa. Humidity term: h = 0.5555 × (30.19 − 10) = 11.22. Humidex: 30 + 11.22 = 41.22, reported as 41, which is “great discomfort, avoid exertion”. The canada.ca humidex table and the CCOHS page both give 41 for 30 °C at 70%.

Starting from a dew point instead: 30 °C with a 22 °C dew point. Vapour pressure e = 6.11 × exp(5417.7530 × (1 ÷ 273.15 − 1 ÷ 295.15)) = 26.80 hPa, h = 0.5555 × 16.80 = 9.33, humidex = 39.33, reported as 39. The relative humidity that goes with it is 100 × es(22) ÷ es(30) = 100 × 26.39 ÷ 42.37 = 62.3%. For comparison, the NWS heat index for 86 °F and 70% humidity is 95.1 °F, or 35 °C, six points under the humidex for the same air.

Mistakes that catch people out

Pairing a morning humidity with an afternoon temperature. Relative humidity falls as the air warms even when the moisture in it is unchanged. At 30 °C, 85% gives a humidex of 45 and 60% gives 39. Use the reading from the same hour, or better, use the dew point, which hardly changes through the day.

Typing 0.7 for 70%. The humidity box takes a percentage. The calculator refuses anything under 1%, so the slip shows up as an error rather than a wrong answer, but a spreadsheet using the same formula will not warn you.

Reading a humidex against the heat index categories. A heat index of 41 °C is “danger” on the NWS scale, while a humidex of 41 is “great discomfort” and a heat index of 35 °C describes the same air. The scales were set separately and the numbers are not interchangeable.

Expecting a humidex from cool or dry air. Below a dew point of about 7 °C the vapour pressure is under 10 hPa, the humidity term is negative, and the formula returns a number below the air temperature. Environment Canada does not publish those, and neither should you quote them; the tool tells you when a result fails the reporting rule.

Treating it as a full measure of heat stress. The humidex uses temperature and humidity only. It assumes shade and light wind, and knows nothing about direct sun, radiant heat from asphalt or equipment, work rate or clothing. A humidex of 38 on a shaded deck and the same 38 on a paving crew at noon are not the same risk, which is why occupational guidance adds its own margins.

Popular calculations

Each address opens the calculator with those readings filled in, and you can change them once it is open.

Celsius readings

Hot and dry, hot and humid

Fahrenheit readings

From the dew point

How this was checked

The engine was run against the complete humidex table on the canada.ca humidex page, 291 cells from 21 to 43 °C and 20% to 100% relative humidity. It reproduces the published whole number in 274 of them. In the other 17 the unrounded result sits within 0.08 of a rounding boundary, for example 46.50 where the table says 46 at 34 °C and 60%. The largest difference anywhere in the grid is 0.58, so Environment Canada evidently used a slightly different route from relative humidity to dew point, and no cell is a whole point out. The published examples also reproduce: 21 °C at 100% gives 29, as the canada.ca page says; 30 °C at 70% gives 41, quoted by canada.ca and by the Canadian Centre for Occupational Health and Safety; and 30 °C with dew points of 15 °C and 25 °C give 34 and 42.

An independent route was written in a separate test script: the dew point by bisection on the Magnus curve, a second vapour pressure function in a different algebraic form, and the heat index in Horner form. On 2,020 points from 0 to 50 °C and 5% to 100% humidity the two agree to twelve decimal places, and relative humidity survives the round trip through dew point and back to the same precision. The heat index reproduces the NWS examples of 121 °F for 96 °F at 65%, 124 °F for 100 °F at 55% and 96 °F for 100 °F at 15%. Both ECCC pages that publish the formula write the constant as 273.15, while some secondary sources write 273.16; the choice moves the humidex by at most 0.005, and the tool follows ECCC. Every address in the lists above was then opened and checked for a distinct title, the right answer, a chart that renders, and clean CSV, image and print output at desktop, tablet and phone widths.

Common questions

What is the humidex?

The humidex is the Environment and Climate Change Canada index that combines air temperature and humidity into one number for how hot the air feels to an average person. The term dates from 1965 and the current formula from Masterton and Richardson in 1979. It has no unit but reads like a Celsius temperature. Environment Canada groups it into little discomfort (20 to 29), some discomfort (30 to 39), great discomfort where exertion should be avoided (40 to 45) and dangerous, with heat stroke possible, at 46 and over.

How is the humidex calculated?

Humidex = T + 0.5555 × (e − 10), where T is the air temperature in °C and e is the vapour pressure in hectopascals from the dew point: e = 6.11 × exp(5417.7530 × (1 ÷ 273.15 − 1 ÷ Td)), with Td in kelvin. At 30 °C and 70% humidity the dew point is 23.9 °C and the vapour pressure 30.19 hPa, so 30 + 0.5555 × (30.19 − 10) = 41.22, published as 41. From relative humidity the dew point comes first, by the Magnus formula, which is what this calculator does.

What humidex is dangerous?

Environment Canada calls 46 and over dangerous, with heat stroke possible, and says any humidex over 40 is an extremely high reading at which exertion should be avoided. Public heat warnings use that level: southern Ontario gets a heat warning when two or more days in a row are forecast to reach a humidex of 40, with 42 the trigger in the extreme southwest and 36 in the north. Workplaces often act earlier again, because the humidex assumes shade, light wind and no exertion.

What is the difference between the humidex and the heat index?

Both say how hot humid air feels, but they are different definitions. The humidex adds a fixed 0.5555 for every hectopascal of vapour pressure above 10 hPa, so it is a direct measure of moisture load. The US heat index is a regression fit to Steadman’s model of a person walking in the shade, expressed in °F, and it climbs more slowly. For the same air the humidex is almost always higher: at 30 °C and 70% humidity it is 41 while the heat index is 35 °C (95 °F). Compare each index only with its own categories.

Why does the forecast not show a humidex today?

Environment Canada only publishes a humidex when it adds something. In a forecast the temperature has to reach 25 °C and the humidex must be at least one degree higher than the temperature; for current conditions the temperature threshold is 20 °C. Cool or dry air fails one of those tests, and when the dew point is below about 7 °C the vapour pressure is under the 10 hPa baseline, so the arithmetic gives a humidex below the air temperature and nothing is reported. This calculator still shows the number, and tells you when Environment Canada would not.

Can I calculate the humidex from the dew point?

Yes, and it is the more direct route, because the official formula works from the dew point rather than from relative humidity. Choose “dew point” in the humidity selector and enter it in the same unit as the temperature. At 30 °C with a 22 °C dew point the vapour pressure is 26.80 hPa and the humidex 39.33, reported as 39, with a matching relative humidity of 62.3%. Use the dew point when combining readings from different times of day, because it changes far less than relative humidity does.

Related Calculators and Reference

• Heat Index Calculator: the US index in full, with its own caution and danger categories, for the same temperature and humidity.
• Wind Chill Calculator: the cold weather counterpart, also an Environment Canada index, for the other end of the year.
• Temperature Converter: Celsius, Fahrenheit, kelvin and Rankine when a reading arrives in the wrong unit.
• Water Intake Calculator: how much to drink on a day when the humidex says to take it easy.
• Dew Point Calculator: dew point from temperature and humidity, or humidity from a dew point, which is the first step of this calculation.
• Environment and Climate Change Canada humidex page: the official comfort categories, reporting rules and the published humidex table.