Dew point simple approximation calculator
Get the classic pocket estimate of dew point: 20 °C at 80 % relative humidity gives 16 °C, a 4-degree depression. Below 50 % humidity the rule is refused rather than giving a misleadingly cold answer. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Creativity
- 20
- Relative humidity
- 80
- Dew point
- 16 celsius
- Dew point depression
- 4
- Humidity deficit
- 20
- Accuracy threshold
- 50
The simple dew point approximation is the pocket rule many meteorologists, pilots, and HVAC technicians still evaluate in their heads: dew-point temperature equals air temperature minus one hundred minus relative humidity, all divided by five. Written Td = T − (100 − RH) / 5, with T and Td in degrees Celsius and RH in percent, the identity is accurate only near saturation. This calculator applies that exact linear estimate, returns the dew-point depression alongside Td, and errors when relative humidity is below the fifty-percent accuracy threshold instead of inventing a number that would look precise and be systematically wrong.
What the simple dew-point approximation means and when it is valid
The simple dew-point approximation used in field meteorology, aviation weather briefings, and introductory moist-air courses is a linear rule of thumb: dew-point temperature Td equals air temperature T minus the quantity one hundred minus relative humidity, all divided by five. Compactly, Td = T − (100 − RH) / 5, with T and Td in degrees Celsius and RH in percent. Each five-percentage-point drop of relative humidity below saturation cools the estimated dew point by about one degree Celsius relative to the dry-bulb temperature.
At one hundred percent relative humidity the air is saturated, the subtracted term vanishes, and dew point equals air temperature. At ninety percent the depression is two degrees; at eighty percent it is four degrees; at fifty percent—the conventional accuracy floor for this linear model—it is ten degrees. The slope is a first-order fit to the Clausius–Clapeyron saturation curve near one hundred percent humidity, not a Magnus–Tetens, Bolton, or Arden Buck inversion of saturation vapor pressure.
Those exponential formulas remain usable at low humidity; this linear rule does not. Below the accuracy threshold the curvature of the saturation curve makes the constant slope too steep, so a reported Td would sit systematically colder than the true dew point. Use it when you want the textbook pocket estimate itself, when you must show students how a mental rule compares with a full dew-point calculator, or when a script should fail closed rather than silently accept a dry-air input the approximation was never meant to cover.
What to enter and what you get back
Relative humidity must lie between zero and one hundred inclusive, but the simple approximation is accepted only at or above the accuracy threshold of fifty percent. Values below fifty percent are rejected with a message that relative humidity is below the accuracy threshold, because the linear rule is valid only near saturation. Values above one hundred percent are rejected as unphysical for this model.
Negative relative humidity is likewise rejected. Example: twenty degrees Celsius at eighty percent relative humidity yields a four-degree depression and a dew point of sixteen degrees Celsius. At one hundred percent humidity the depression is zero and dew point equals air temperature.
What you can do with it
Classroom comparison with Magnus dew point
Compute the linear Td = T − (100 − RH)/5 next to a Magnus–Tetens dew point so students see where the pocket rule holds and why humidity below fifty percent is refused.
Aviation and field walk-around checks
Apply the “one degree per five percent below saturation” briefing rule in a script or lesson page with the same arithmetic written on the whiteboard.
Near-saturation HVAC and greenhouse notes
Estimate dew point from dry-bulb temperature and high relative humidity when the space is already near saturation and a full psychrometric inversion is unnecessary.
FAQ
What formula does this dew point simple approximation calculator use?
The linear near-saturation rule Td = T − (100 − RH) / 5, with T and Td in degrees Celsius and RH in percent. Each five-percentage-point drop below saturation lowers the estimated dew point by one degree Celsius.
Why does the calculator reject relative humidity below 50%?
The simple approximation is valid only near saturation.
Is this the same as a Magnus–Tetens dew point calculator?
No. A Magnus–Tetens (or Bolton) calculator inverts saturation vapor pressure and remains usable at low humidity.
What units should I use for temperature and humidity?
Temperature is dry-bulb air temperature in degrees Celsius. Relative humidity is a percent from 0 to 100, accepted only at or above 50. The factor of five is calibrated for the Celsius scale, not Fahrenheit.
What dew point do 20 °C and 80 % humidity give?
16 °C. The humidity deficit is 100 minus 80, which is 20; dividing it by 5 gives a depression of 4 °C, and 20 minus 4 leaves a dew point of 16 °C.
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/earth/dew-point-simple \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"temperature":20,"relative_humidity":80}'const res = await fetch("https://api.kit.forhosting.com/earth/dew-point-simple", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"temperature": 20,
"relative_humidity": 80
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/earth/dew-point-simple",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"temperature": 20,
"relative_humidity": 80
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/earth/dew-point-simple", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"temperature":20,"relative_humidity":80}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"temperature":20,"relative_humidity":80}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/earth/dew-point-simple", 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": 20,
"relative_humidity": 80
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "earth.dew_point_simple",
"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
What the simple dew-point approximation means and when it is valid
This capability therefore refuses relative humidity below fifty percent with invalid_input instead of returning a polished but misleading value.
Input contract, response shape, and error behaviour
POST a JSON object with two required finite numbers. temperature is the dry-bulb air temperature T in degrees Celsius; aliases t, temp, temp_c, temperature_c, and air_temperature are accepted. relative_humidity is RH in percent; aliases rh, humidity, rh_percent, and relative_humidity_percent are accepted. Each field may arrive as a JSON number or as a numeric string; missing, blank, or non-finite values fail with invalid_input naming the field. On success the body echoes the rounded temperature and relative_humidity, returns dew_point and dew_point_depression in degrees Celsius, reports humidity_deficit as one hundred minus RH, restates accuracy_threshold as fifty, and labels unit as celsius. formula and equation strings reprint Td = T − (100 − RH) / 5 with the substituted numbers, method is simple_near_saturation, and steps walks through the deficit, the division by five, and the final subtraction for teaching UIs. Failed validation is not billed; successful analytic calls use the published $0.002 base price with no model tokens and no outbound network.
Where a simple dew-point approximation API earns its keep
Meteorology instructors need a deterministic oracle that reproduces the pocket rule exactly so lab write-ups can compare the linear estimate with a Magnus–Tetens dew point without spreadsheet drift, and so a dry-air input is marked wrong instead of quietly returning a too-cold Td. HVAC and greenhouse notebooks sometimes want the near-saturation estimate when the space is already humid and a full psychrometric inversion would be heavier than the question. Research and grading pipelines that batch thousands of (T, RH) pairs need fixed twelve-decimal rounding and a hard humidity floor they can assert in tests. Aviation and outdoor-education briefings still teach “subtract one degree for every five percent below saturation” as a walk-around check; a tiny parse API lets a lesson page or a briefing script apply the same arithmetic the instructor just wrote on the board. Keep temperature in Celsius, keep relative humidity at or above fifty percent, pin dew_point and dew_point_depression in tests, and you get a reproducible near-saturation estimate for courses, docs, and edge workers without standing up a vapor-pressure library of your own.
Technical questions
What does the simple dew point approximation calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input, including humidity below the accuracy threshold, is not billed.