Entropy converter
One calorie per kelvin is exactly 4.184 joules per kelvin. Convert entropy between joules, kilojoules, calories and kilocalories per kelvin, BTU per Rankine and ergs per kelvin, so steam tables, lab books and textbook answers line up. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Value
- 1
- From
- J/K
- To
- cal/K
- Result
- 0.2390057361 cal/K
- Converted
- 0.2390057361
- Value j per k
- 1
- Factor
- 0.2390057361
An entropy converter turns one thermodynamic entropy reading into another unit without spreadsheet macros or hand-written factor tables. Enter a finite value plus a source unit and a target unit among joules per kelvin, kilojoules per kelvin, thermochemical calories or kilocalories per kelvin, British thermal units per Rankine, and ergs per kelvin; the tool returns the converted entropy, the SI intermediate in J/K, the exact unit factor applied, and step-by-step substitution lines you can paste into a lab report. Process engineers reconciling SI and customary steam tables, chemistry students converting textbook entropy changes, HVAC analysts reading mixed property charts, and coding tests that need fixed conversion goldens all need the same deterministic path.
Which entropy units are supported and how conversion works
This capability converts among six everyday thermodynamic entropy units that dominate chemistry lab books, steam tables, physics homework, and engineering datasheets. Joules per kelvin (J/K) are the SI coherent unit of entropy and the internal hub: every conversion multiplies the input by how many joules per kelvin equal one source unit, then divides by how many joules per kelvin equal one target unit. Because a kelvin temperature interval equals a celsius interval, J/°C is treated as identical to J/K for conversion purposes.
Kilojoules per kelvin scale by exactly one thousand. Thermochemical calories per kelvin use the exact factor four point one eight four joules per calorie; kilocalories per kelvin use four thousand one hundred eighty-four. British thermal units per Rankine use the International Table BTU of one thousand fifty-five point zero five five eight five two six two joules divided by the Rankine interval of five ninths of a kelvin, which is the same arithmetic as multiplying by nine fifths.
Ergs per kelvin use the exact SI relation one erg equals ten to the minus seven joules. Same-unit requests succeed and return the input magnitude with a short message that no change of unit was required.
Practical tips, limits, and when to use this landing
Prefer sending bare numbers for value when you control the client; string numbers are accepted only when they match a strict finite numeric pattern without currency symbols or thousand separators. Round-trip checks are straightforward: convert A to B and B back to A and confirm you recover the original within the published twelve-decimal rounding.
When teaching unit analysis, pair classic examples such as four point one eight four joules per kelvin equaling one calorie per kelvin, one thousand joules per kelvin equaling one kilojoule per kelvin, and one BTU per Rankine converting to about one thousand eight hundred ninety-nine point one joules per kelvin. Stay inside the absolute range so intermediate SI values cannot overflow into a non-finite result that the solver must reject.
This landing targets people searching for an entropy converter, J/K to cal/K, BTU per Rankine to joules per kelvin, or thermodynamic entropy unit conversion online; it is not a specific-heat capacity converter (J/(kg·K)), not an information-theory entropy tool in bits or nats, and not a full Gibbs free-energy calculator.
What you can do with it
Steam-table and process data alignment
Engineers convert absolute or delta-S values from customary BTU/R tables into SI J/K for consistent energy-balance models and plant data historians.
Chemistry and thermodynamics homework
Students convert textbook entropy changes between cal/K, kcal/K, and J/K and check hand work against deterministic steps before submitting.
FAQ
What happens if I send an unknown unit name?
Unknown names are never silently mapped to a guessed factor.
Does this convert specific entropy like J/(kg·K)?
No. This tool converts absolute entropy (energy per temperature). For mass-specific heat capacity use the specific-heat capacity converter.
Is the entropy converter free?
The browser tool is free and runs locally.
How many J/K is one BTU per Rankine?
About 1899.1 J/K. The International Table BTU is divided by the Rankine interval, which is five ninths of a kelvin, so the BTU value is effectively multiplied by nine fifths.
Is J/°C the same as J/K here?
Yes. A temperature step of one kelvin is the same size as a step of one degree Celsius, so an entropy in J/°C converts exactly like one in J/K.
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/entropy \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"value":1,"from":"J/K","to":"cal/K"}'const res = await fetch("https://api.kit.forhosting.com/conv/entropy", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"value": 1,
"from": "J/K",
"to": "cal/K"
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/conv/entropy",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"value": 1,
"from": "J/K",
"to": "cal/K"
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/conv/entropy", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"value":1,"from":"J/K","to":"cal/K"}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"value":1,"from":"J/K","to":"cal/K"}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/conv/entropy", 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": 1,
"from": "J/K",
"to": "cal/K"
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "conv.entropy",
"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.
Limits
max_abs | 1000000000000 |
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
Unknown unit names fail with a clear invalid_input error instead of silent wrong factors.
Which entropy units are supported and how conversion works
Common aliases are accepted case-insensitively: J/K, joules per kelvin, and J/C for J/K; cal/K and calories per kelvin for cal/K; kcal/K for kcal/K; kJ/K for kJ/K; BTU/R, BTU per Rankine, and British thermal unit per Rankine for BTU/R; and erg/K for erg/K. If the name is not on that list the solver rejects the request with invalid_input naming the bad field and listing supported units, so clients never guess whether a typo produced a wrong number. Absolute magnitude must stay within the published one-trillion limit so intermediate products remain finite in IEEE double precision on the Worker and in the browser.
How to call the API and read the response
Value must be a finite number or a strict numeric string within absolute limits of one trillion. Wire this into dashboards, mobile clients, or batch jobs that already hold an absolute or change-of-entropy reading and only need a second unit for display, datasheet comparison, or regulatory reporting without shipping floating conversion tables in every app binary. Prefer numbers over pre-formatted display strings when you control the client so thousand separators and currency symbols cannot fail the finite-number parser. Send a JSON body with three required fields: value (aliases s, entropy, amount, and n), from (aliases from_unit, unit_from, and source), and to (aliases to_unit, unit_to, and target). From and to must resolve to supported unit names after alias normalization. On success the response echoes the rounded input value, canonical from and to labels, result and converted set to the same rounded target magnitude, value_j_per_k as the SI intermediate, unit as the target label, factor as the from-to multiplier, formula as the short equation string, message as an English summary, steps as ordered substitution lines, and supported_units as the six canonical identifiers. Failed validation—missing fields, non-finite numbers, values outside absolute limits, or an unknown unit name—returns invalid_input and is never charged. Pricing is a flat $0.002 per successful request with unit item and zero marginal unit price.
Practical tips, limits, and when to use this landing
Normalize display labels on your side if you need locale-specific spellings—this API always returns the short canonical tokens J/K, kJ/K, cal/K, kcal/K, BTU/R, and erg/K so automated tests stay stable. Prefer this capability when both endpoints are among the six supported absolute entropy units and you need deterministic factors and step strings; if you only need mass-specific heat capacity, use the dedicated specific-heat capacity converter instead. Failed tasks that return invalid_input do not consume prepaid balance, which keeps exploratory scripts cheap when clients deliberately probe unknown unit names during integration.
Integration use cases
Cross-standard lab reporting
Labs publish entropy results in SI while instrument software still records calorie-based units; one API call keeps LIMS and reports in sync.
Technical questions
Which entropy units does this converter support?
J/K, kJ/K, cal/K, kcal/K, BTU/R, and erg/K, plus common aliases such as joules per kelvin, calories per kelvin, and BTU per Rankine.
Is cal/°C the same as cal/K here?
Yes for temperature intervals: a celsius degree has the same size as a kelvin, so cal/C aliases resolve to cal/K.