Jansky flux density converter
Switch spectral flux density between radio-astronomy and physics units: 1 Jy is exactly 10⁻²⁶ W/m²/Hz, and every answer also shows millijansky, microjansky and the CGS erg form. Handy for calibration notes, catalogues and homework. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Amount
- 1
- From
- jansky
- To
- w_m2_hz
- Result
- 0
- Jansky
- 1
- Millijansky
- 1,000
- Microjansky
- 1,000,000
Spectral flux density conversion between the radio-astronomy jansky and the SI and CGS power-per-area-per-hertz units is a daily chore for observatory pipelines, interferometry calibration notes, multi-wavelength catalogue work, and astrophysics homework. People search for a jansky flux density converter when they need jansky next to watts per square meter hertz, millijansky next to microjansky, or erg per second per square centimetre per hertz next to SI, not a general multi-unit toolbox that also handles length and temperature. This capability accepts one finite amount plus from and to unit labels among jansky, millijansky, microjansky, W per square meter hertz, and erg per second per square centimetre per hertz, bridges every conversion through the jansky so results stay consistent, and returns the target value together with the same spectral flux density expressed in all five units. Unrecognized unit names are rejected with a clear validation error instead of silently guessing a nearby symbol.
How the jansky flux density converter maps SI, CGS, and radio units through jansky
Start from the jansky as the conventional radio-astronomy unit of spectral flux density S_ν and express every other unit as a fixed factor into jansky. By definition one jansky equals exactly ten to the minus twenty-six watts per square meter per hertz. One millijansky is exactly one thousandth of a jansky and one microjansky is exactly one millionth of a jansky, which covers continuum survey depths and faint source catalogues without rewriting scientific notation by hand.
The SI coherent spectral flux density is watts per square meter per hertz; the inverse of the jansky definition therefore sets one watt per square meter per hertz equal to ten to the twenty-six jansky.
The common CGS form is erg per second per square centimetre per hertz: because one watt is ten to the seven erg per second and one square meter is ten to the four square centimetres, one jansky equals ten to the minus twenty-three erg per second per square centimetre per hertz, so one erg per second per square centimetre per hertz equals ten to the twenty-three jansky. This calculator never mixes those paths ad hoc.
It multiplies the input amount by the factor of the source unit to reach jansky, then divides by the factor of the target unit to produce the result. Because every path shares the same intermediate, converting one jansky to ten to the minus twenty-six watts per square meter per hertz and back, or comparing millijansky to microjansky, stays numerically consistent within ordinary floating-point precision.
The tool does not convert magnetic flux density in tesla or gauss, does not convert heat flux without a spectral density, does not apply redshift or k-corrections, and does not fetch live telescope catalogues. It answers only the algebraic spectral flux density unit conversion people mean when they type jansky flux density converter into a search bar and expect Jy, mJy, µJy, W/m²/Hz, and erg/s/cm²/Hz.
Where dedicated spectral flux density conversion shows up in real work
Radio continuum survey papers constantly report source peaks in millijansky or microjansky while SI-native radiative transfer codes expect watts per square meter per hertz, so a single conversion pass keeps tables, figures, and model inputs aligned. Interferometry calibration notes often quote calibrator fluxes in jansky while lab test equipment or optical-infrared comparison pipelines write CGS erg spectral densities, and mixed unit strings are a common source of silent order-of-magnitude bugs.
Multi-wavelength catalogue authors merge radio Jy columns with X-ray or ultraviolet flux densities published in erg form and need an exact algebraic bridge before stacking spectral energy distributions. Data pipelines that ingest mixed FITS headers or VO tables with Jy, mJy, and SI tags can call this capability once per reading instead of reimplementing the factors in every service.
If you later need magnetic flux density in tesla and gauss, heat flux density without a frequency bandwidth, or integrated band flux without spectral density, treat those as separate search intents and landings rather than overloading this jansky flux density converter page.
What you can do with it
Radio survey catalogues
Normalize continuum source peaks among Jy, mJy, and µJy so survey tables, cutouts, and SI model inputs stay consistent.
Multi-wavelength SEDs
Bridge radio jansky columns to SI W/m²/Hz or CGS erg spectral densities before stacking energy distributions.
FAQ
What happens if I send an unrecognized unit?
The error message includes Unrecognized unit and lists the supported families so you can correct the label.
How many watts per square meter per hertz is one jansky?
Exactly 10⁻²⁶ W·m⁻²·Hz⁻¹ by the standard radio-astronomy definition of the jansky.
What does it cost?
The browser tool is free and runs locally.
How do I convert jansky to erg/s/cm²/Hz?
One jansky is exactly 10⁻²³ erg per second per square centimetre per hertz. The step comes from 1 watt being 10⁷ erg per second and 1 square metre being 10⁴ square centimetres.
How many microjansky are in a millijansky?
Exactly 1000. A millijansky is one thousandth of a jansky and a microjansky is one millionth, so faint-source catalogues can switch between them without rewriting scientific notation by hand.
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/flux-density-jansky \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"amount":1,"from":"jansky","to":"w_m2_hz"}'const res = await fetch("https://api.kit.forhosting.com/conv/flux-density-jansky", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"amount": 1,
"from": "jansky",
"to": "w_m2_hz"
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/conv/flux-density-jansky",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"amount": 1,
"from": "jansky",
"to": "w_m2_hz"
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/conv/flux-density-jansky", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"amount":1,"from":"jansky","to":"w_m2_hz"}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"amount":1,"from":"jansky","to":"w_m2_hz"}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/conv/flux-density-jansky", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"amount": 1,
"from": "jansky",
"to": "w_m2_hz"
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "conv.flux_density_jansky",
"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 you send and what you get back from the API and browser form
Send three fields: amount, a finite number for the spectral flux density magnitude; from, a string naming the input unit; and to, a string naming the output unit. When a unit string is unknown the error message includes Unrecognized unit and lists the five supported families so clients can fix the label rather than guessing. Classic checks include one jansky becoming one times ten to the minus twenty-six watts per square meter per hertz, one millijansky becoming one thousand microjansky, one jansky becoming one times ten to the minus twenty-three erg per second per square centimetre per hertz, and one watt per square meter per hertz becoming ten to the twenty-six jansky. Both unit fields accept short keys and everyday words in any common casing, including jansky or Jy, millijansky or mJy or milli_jansky, microjansky or uJy or µJy or micro_jansky, w_m2_hz or W/m2/Hz or watt per square meter hertz, and erg_s_cm2_hz or erg/s/cm2/Hz or cgs spectral flux labels. Missing fields, non-numeric amount strings, NaN, Infinity, and unrecognized unit labels fail with invalid_input and are not charged. On success the response echoes the cleaned amount, normalized from and to keys, the primary result in the target unit, the same density as jansky, millijansky, microjansky, w_per_m2_hz, and erg_per_s_cm2_hz with twelve significant digits of stable precision, the bridge factors used, the formula string, and method jansky_bridge. Pricing for successful API calls is the published base of $0.002 per item; validation failures do not consume prepaid balance.
Where dedicated spectral flux density conversion shows up in real work
Because this capability is a pure parse engine, API pricing is the published base of $0.002 per successful item and failed validation does not consume balance.
Integration use cases
Homework and curriculum tools
Map exam values between Jy, mJy, W/m²/Hz, and erg/s/cm²/Hz for answer keys and interactive astrophysics demos that share one JSON contract.
Technical questions
Which spectral flux density units does this converter support?
Jansky (Jy), millijansky (mJy), microjansky (uJy, µJy), watts per square meter per hertz (W/m²/Hz, w_m2_hz), and erg per second per square centimetre per hertz (erg/s/cm²/Hz, erg_s_cm2_hz). Common aliases are accepted; the response normalizes to short keys.
How does the jansky relate to the CGS erg spectral density?
Exactly 1 Jy = 10⁻²³ erg·s⁻¹·cm⁻²·Hz⁻¹. This follows from 1 W = 10⁷ erg/s and 1 m² = 10⁴ cm² applied to the SI definition.