Humidex calculator
At 30 °C with a dew point of 15 °C, the humidex comes out nearly 4 degrees above the thermometer reading, so the day feels noticeably hotter. Environment Canada’s formula, with the vapour pressure shown. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Creativity
- 30
- Dew point
- 15
- Vapour pressure hpa
- 17.1561
- Humidex
- 33.9752
Humidex is the Canadian comfort index that combines air temperature with humidity expressed through the dew point, so a single number tells people how hot it feels outdoors. Weather desks, school science labs, outdoor-event planners, and HVAC briefs all need the same Environment Canada formula rather than an ad-hoc heat-index mix-up. This humidex calculator accepts air temperature and dew point in degrees Celsius, evaluates the official vapour-pressure exponential, and returns the humidex value together with the intermediate vapour pressure in hectopascals.
What humidex measures and how the formula is built
Humidex was designed for Canadian summer conditions so the public can compare sticky days without reading separate temperature and humidity charts. Unlike the US National Weather Service heat index, which is built from air temperature in Fahrenheit and relative humidity, humidex is defined from air temperature T in Celsius and dew point Td in Celsius through the saturation vapour pressure implied by that dew point.
Environment Canada publishes the closed form H equals T plus zero point five five five five times open parenthesis e minus ten close parenthesis, where e equals six point one one times e to the power of five thousand four hundred seventeen point seven five three zero times open parenthesis one over two hundred seventy-three point one six minus one over open parenthesis Td plus two hundred seventy-three point one six close parenthesis close parenthesis.
The constant six point one one is vapour pressure in hectopascals at a convenient reference, and the large coefficient inside the exponential comes from the Clausius–Clapeyron style fit used in the official guidance. Because dew point cannot physically exceed air temperature for ambient air at equilibrium, any Td greater than T is rejected before the exponential runs. Equal temperature and dew point is allowed and corresponds to one hundred percent relative humidity at that temperature.
What you can do with it
Summer comfort dashboards
Convert paired station temperature and dew point into Canadian humidex for public heat-safety maps and community weather pages.
Classroom and science labs
Let students verify the Environment Canada humidex formula on lab sensors and reject impossible dew points above air temperature.
Event and workplace heat briefings
Batch-evaluate forecast grids of T and Td into humidex for outdoor sports, construction, and festival safety checklists.
FAQ
What happens if dew point is higher than air temperature?
In free air the dew point cannot exceed dry-bulb temperature; invalid pairs are not billed.
Is this the same as the US heat index?
No. Heat index uses Fahrenheit temperature and relative humidity with a different polynomial. Humidex is the Canadian index from Celsius temperature and dew point.
Can I enter temperatures in Fahrenheit?
No. Both the air temperature and the dew point must be in degrees Celsius, because the constants in the official formula assume Celsius. Convert Fahrenheit readings first, then enter them here.
What if I only know the relative humidity?
This calculator needs the dew point and does not convert relative humidity into it. Use a relative humidity converter to get the dew point from your temperature and humidity first, then come back with both values.
Is the humidex a real temperature?
No. It is a comfort value that people read like degrees Celsius to judge how hot it feels, but it is not a thermometer reading. Equal air temperature and dew point mean 100 % relative humidity and are accepted.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/weather/humidex-calculate \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"temperature":30,"dew_point":15}'const res = await fetch("https://api.kit.forhosting.com/weather/humidex-calculate", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"temperature": 30,
"dew_point": 15
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/weather/humidex-calculate",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"temperature": 30,
"dew_point": 15
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/weather/humidex-calculate", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"temperature":30,"dew_point":15}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"temperature":30,"dew_point":15}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/weather/humidex-calculate", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"temperature": 30,
"dew_point": 15
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "weather.humidex_calculate",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
When the dew point is higher than the air temperature the physical state is impossible for free air, so the task fails with invalid_input instead of inventing a number.
What humidex measures and how the formula is built
The result is dimensionless in the same sense as a comfort temperature: people read it as how hot it feels in Celsius-like units even though it is not a thermometer reading. This endpoint implements that formula exactly, rounds the numeric fields to four decimal places for stable cross-runtime goldens, and returns the intermediate vapour pressure so you can audit the exponential term without recomputing it yourself.
Input contract, response shape, and error behaviour
Pair the response with your own relative-humidity converter if you only have RH and dry-bulb temperature; this capability does not invert RH to dew point, it only evaluates humidex once both temperatures are known. Keep units in Celsius only—Fahrenheit inputs must be converted upstream so the exponential constants stay correct. POST a JSON body with two required finite numbers: temperature, the dry-bulb air temperature in degrees Celsius, and dew_point, the dew point also in degrees Celsius. Each field may arrive as a JSON number or as a numeric string; non-finite values, missing fields, and empty strings fail with invalid_input naming the field. Aliases temperature accepts include temp_c, air_temp, t, and temp; dew_point accepts dewpoint, td, and dew_point_c so older sensor payloads map without a rewrite layer. On success the body includes the echoed temperature and dew_point rounded to four decimals, vapour_pressure_hpa for the intermediate e term, humidex for the comfort index, and a formula string restating the Environment Canada identity for display next to the answer. Example: temperature thirty and dew_point fifteen yields a vapour pressure near seventeen hectopascals and a humidex a few degrees above thirty, reflecting modest humidity.
Where a humidex API earns its keep
Canadian municipal dashboards, campground apps, and school weather projects need a deterministic humidex oracle that matches the numbers people see on Environment Canada products, not a US heat-index polynomial with different inputs. IoT gateways that already log dry-bulb and dew-point sensors can post both values and store humidex without standing up a full meteorology library. Failed validation does not consume balance. Sports-event planners compare forecast grids of temperature and dew point against humidex thresholds for heat-safety briefings; a tiny parse API avoids embedding spreadsheet macros in every volunteer laptop. Pin the four-decimal rounded fields in tests, document that dew point above air temperature is invalid_input, and you get reproducible comfort indices for education, media, and edge workers without re-deriving the exponential constants each season.
Technical questions
How do I calculate humidex from temperature and dew point?
Send air temperature and dew_point in °C. The API returns H = T + 0.5555×(e−10) with e from the Environment Canada exponential in dew point, plus the intermediate vapour pressure in hPa.
Which units does the humidex calculator use?
Inputs are degrees Celsius only. Output humidex is the standard Canadian comfort number; vapour_pressure_hpa is in hectopascals.
What does the humidex calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.