Celsius Fahrenheit Kelvin converter
Kelvin is Celsius plus 273.15, so 100 °C is 373.15 K. Convert a temperature between Celsius, Fahrenheit and Kelvin, and only those three, when a weather app, an oven probe or a lab notebook shows the wrong scale. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Value
- 100
- From
- Celsius
- To
- Fahrenheit
- Result
- 212 °F
- Input
- 100
- Kelvin
- 373.15
This Celsius, Fahrenheit and Kelvin converter takes a finite temperature, a source scale, and a target scale, then returns the same thermodynamic state on the requested scale. The closed set is Celsius, Fahrenheit, and Kelvin: the three names people type when a lab notebook, a weather readout, a kitchen probe, or a science worksheet disagrees with the unit they need next. Rankine, Réaumur, and Delisle are out of scope on this landing. Conversion always goes through kelvin so every pair shares one identity instead of a drifting pairwise matrix: kelvin equals Celsius plus 273.15, and Celsius to Fahrenheit is the familiar nine-fifths plus thirty-two.
The three scales and the kelvin identities behind every pair
Celsius, Fahrenheit, and Kelvin are the three temperature names people actually type into a converter when a weather app, a lab notebook, a cooking probe, or a science worksheet disagrees with the unit they need next. This landing is that closed set: it does not ask the visitor to pick Rankine, Réaumur, Delisle, Newton, or Rømer, because those are different searches with different pages. Every pair is reduced to kelvin with definitional offsets, then mapped to the target scale, so there is no pairwise matrix that can drift.
Celsius to kelvin adds 273.15. Fahrenheit to kelvin uses (F plus 459.67) times five ninths, which is the same as converting to Celsius with (F minus 32) times five ninths and then adding 273.15. Kelvin is already the SI thermodynamic scale, so a kelvin input is the pivot unchanged.
Going back, Celsius is kelvin minus 273.15, and Fahrenheit is kelvin times nine fifths minus 459.67, which matches the classroom identity F equals C times nine fifths plus 32. Zero on any source scale is a real temperature: 0 °C is the ice point, 0 °F is a cold winter day, and 0 K is absolute zero, each mapping to a different number on the other two scales. Signed values above absolute zero are accepted.
Prefer this path when both scales are in the three-name set.
Where a three-scale temperature converter earns its keep
Weather desks convert a Celsius forecast into Fahrenheit for a US bulletin, then into kelvin when the same reading feeds a scientific model. Cooking and food-safety teams convert oven probes quoted in Fahrenheit into Celsius for a metric recipe, and lab techs convert ice-point checks at 0 °C into kelvin for an instrument log. Teachers and exam generators need a deterministic oracle that rejects an unrecognized unit name instead of silently returning NaN a buggy grader might treat as success.
Firmware and IoT pipelines convert sensor kelvin into Celsius for a dashboard and into Fahrenheit for a consumer display from one call. This landing targets people searching for a Celsius Fahrenheit Kelvin converter with the three everyday scientific scales. It is not a Rankine path, not a Réaumur path, and not a mixed-string parser for values such as one hundred degrees C.
Prefer this capability when both units are already in the three-name set and you want one call; if you must convert Rankine, use a broader four-scale hub instead. Stay at or above absolute zero so the kelvin pivot cannot go negative.
What you can do with it
Weather, travel, and public bulletins
Convert a Celsius forecast into Fahrenheit for a US bulletin, or a Fahrenheit reading into Celsius and kelvin when the same number must feed a metric model.
Labs, ovens, and instrument logs
Convert ice-point 0 °C into kelvin for a calibration log, or an oven probe in Fahrenheit into Celsius for a metric recipe card.
Homework, firmware, and IoT dashboards
Check textbook C↔F↔K conversions with exact 273.15 / 9/5 identities, and reject unrecognized scale names instead of returning NaN.
FAQ
What formulas does this Celsius Fahrenheit Kelvin converter use?
Kelvin is the pivot: K = C + 273.15 and K = (F + 459.67) × 5/9. Back-conversion is C = K − 273.15 and F = K × 9/5 − 459.67, which matches F = C × 9/5 + 32.
What happens if the unit name is unrecognized?
Missing fields, NaN, Infinity, non-numeric strings, and unknown names such as rankine or reaumur are rejected and not charged.
How do I convert 100 Celsius to Fahrenheit?
The result is 212, plus the intermediate temperature 373.15 kelvin.
Does it convert Rankine or Réaumur?
No. This converter covers only Celsius, Fahrenheit and Kelvin, the three scales people usually need. Rankine, Réaumur and Delisle are different searches and are deliberately left out.
Is 0 °F the same temperature as 0 °C?
No. Zero means something different on each scale: 0 °C is the ice point, 0 °F is a cold winter day and 0 K is absolute zero, and each maps to a different number on the other two scales.
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/conv/temperature-converter \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"value":100,"from":"celsius","to":"fahrenheit"}'const res = await fetch("https://api.kit.forhosting.com/conv/temperature-converter", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"value": 100,
"from": "celsius",
"to": "fahrenheit"
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/conv/temperature-converter",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"value": 100,
"from": "celsius",
"to": "fahrenheit"
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/conv/temperature-converter", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"value":100,"from":"celsius","to":"fahrenheit"}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"value":100,"from":"celsius","to":"fahrenheit"}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/conv/temperature-converter", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"value": 100,
"from": "celsius",
"to": "fahrenheit"
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "conv.temperature_converter",
"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
Unrecognized unit names fail with invalid_input instead of inventing a temperature.
The three scales and the kelvin identities behind every pair
The response echoes the parsed input, the canonical from and to names, the converted result, the target unit symbol, the intermediate temperature in kelvin, a formula string restating those identities, and a short English message you can paste into a worksheet.
Input contract, rounding, aliases, and error behaviour
The documented names are celsius, fahrenheit, and kelvin. On success the body includes input (the parsed source number), from and to as canonical names, result (the converted temperature), unit (the target symbol °C, °F, or K), kelvin (the SI pivot), formula, and message. Round-trip checks are easy: one hundred celsius is two hundred twelve fahrenheit and 373.15 kelvin; zero celsius is 273.15 kelvin; thirty-two fahrenheit is zero celsius; zero kelvin is −273.15 celsius and −459.67 fahrenheit. Converting a scale to itself returns the same number, which is useful in tests that pin the identity path. POST a JSON body with three required fields: value, a finite number or a strict numeric string; from, the source scale; and to, the target scale. The solver also accepts the compact aliases C, F, and K, plus degree-marked forms such as °C and °F and the synonym centigrade, then normalizes them to the canonical lowercase names before returning. Failed validation—missing fields, empty strings, NaN, Infinity, non-numeric phrases, unrecognized unit names such as rankine or reaumur, or a source temperature below absolute zero—returns invalid_input naming the field and is never charged. Pricing is a flat $0.002 per successful request with unit item.
Where a three-scale temperature converter earns its keep
Failed tasks that return invalid_input do not consume prepaid balance, which keeps exploratory scripts cheap when learners deliberately omit value or pass a misspelled unit. Successful API calls bill as one item at $0.002.
Technical questions
Which temperature units can I convert?
Celsius, Fahrenheit, and Kelvin. Aliases C, F, K, °C, °F and the synonym centigrade are accepted and normalized to those canonical names.
What does the Celsius Fahrenheit Kelvin converter cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.