Note frequency calculator
A4 sounds at 440 Hz and is MIDI note 69: enter a note name and an octave, change the reference A if your ensemble tunes elsewhere, and get the exact frequency and MIDI number. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Scientific pitch octave
- 4
- Scientific name
- A4
- Pitch class
- 9
- Midi
- 69
- Semitones from a4
- 0
Musicians, synthesizer builders, and audio engineers often need the exact frequency of a written pitch when they change concert A or when they map notes onto oscillators, filters, and sample rates. Twelve-tone equal temperament places every semitone at a constant frequency ratio of the twelfth root of two, so once you fix A4 in hertz the entire keyboard is determined by a single closed-form power of two. This capability takes a note name, a scientific pitch octave, and an optional reference A4 frequency, then returns the equal-temperament frequency in hertz together with MIDI number and semitone offset for pipelines that already speak MIDI. Unrecognized note names fail with a clear invalid input error so bad spellings never become silent wrong pitches.
How equal temperament turns a note name into hertz
Scientific pitch notation pairs a pitch class such as A or C sharp with an octave integer so that A4 is the familiar concert reference and C4 is middle C. In twelve-tone equal temperament every adjacent semitone multiplies frequency by two to the power of one twelfth, which means twelve steps double the pitch exactly and octaves stay pure while all keys share the same interval sizes.
This calculator maps the note name to a pitch class from zero for C through eleven for B, including common enharmonic spellings such as D flat for C sharp and B flat for A sharp. It then builds the MIDI note number used worldwide in sequencers: MIDI equals open parenthesis octave plus one close parenthesis times twelve plus the pitch class, so A4 is sixty-nine. Frequency follows the standard formula f equals A4 times two raised to the power of open parenthesis MIDI minus sixty-nine close parenthesis over twelve.
Send note as a short English string, octave as an integer, and optionally a4 when your ensemble tunes to four hundred thirty-two, four hundred fifteen for historical pitch, or four hundred forty-two for a brighter modern orchestra.
What you can do with it
Synth and sample mapping
Map keyboard notes to oscillator frequencies or root pitches for multisamples when the project concert A is 440, 432, or another documented reference.
Ear training and music theory tools
Generate exact target frequencies for pure-tone drills, interval trainers, and homework checkers that must reject invalid note spellings.
FAQ
How do I get the frequency of A4?
Override a4 when your ensemble uses a different concert pitch.
Which note names are accepted?
English pitch classes C through B with optional sharp (#) or flat (b), including common enharmonics such as C#/Db and A#/Bb.
Is the note frequency calculator free?
The browser tool runs free on your device.
Can I use baroque pitch or 432 Hz?
Yes. Pass a4 with the reference frequency in hertz (for example 415 or 432). Every note scales from that A4 under equal temperament.
What MIDI number does A4 have?
A4 is MIDI note 69 and, with the default reference, sounds at 440 Hz. Every note you enter also gets its MIDI number, so you can use the result directly in a sequencer or synth.
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/hobby/note-frequency \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"note":"A","octave":4}'const res = await fetch("https://api.kit.forhosting.com/hobby/note-frequency", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"note": "A",
"octave": 4
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/hobby/note-frequency",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"note": "A",
"octave": 4
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/hobby/note-frequency", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"note":"A","octave":4}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"note":"A","octave":4}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/hobby/note-frequency", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"note": "A",
"octave": 4
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "hobby.note_frequency",
"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
How equal temperament turns a note name into hertz
Missing fields, non-integer octaves outside the published range, non-positive A4 values, and unrecognized note names all raise invalid_input and are not charged.
Reading the response fields and choosing A4
Default A4 is four hundred forty hertz when you omit the field; override it only when your project documents a different concert pitch, because changing A4 scales every frequency on the keyboard by the same ratio. Extremely low or high octaves still obey the formula within the allowed range of negative one through nine, covering the practical MIDI keyboard without inventing notes outside that span. On success the response echoes the normalized note display name, the octave you sent, and scientific_name such as A4 or F sharp five for labels in user interfaces. The pitch_class integer and midi number let you wire the same result into MIDI routers without recomputing the map.
API usage, pricing, studio workflows, and practical limits
Call the free in-browser note frequency calculator when you are checking a score, programming a modular synth, or verifying a homework answer without installing a DAW. Source inputs follow the same retention rules as other KIT parse items. Send a JSON body with required string note and integer octave, plus optional number a4 for the reference pitch in hertz. Use the prepaid API when a web instrument, sample mapper, ear-training app, or firmware tool needs a stable contract under KIT balance at $0.002 per successful item; validation failures never consume balance after the engine rejects the request. Pair this endpoint with wave-speed or filter-cutoff tools when you are designing physical models that combine pitch with electronics, or keep it alone when you only need a single number for an oscillator. Authors shipping SDKs can map frequency_hz into UI meters and midi into note-on messages without reimplementing the twelfth-root table in every language.
Integration use cases
Audio firmware and DAW utilities
Embed a stable prepaid endpoint or isomorphic library path so plugins and microcontrollers share one equal-temperament definition with MIDI numbers.
Technical questions
What formula does the calculator use?
Twelve-tone equal temperament: f = a4 * 2^((midi - 69) / 12), where midi is the scientific-pitch MIDI number for the note and octave (A4 = 69).