Note Frequency Calculator
Select a note and octave to get its exact frequency in hertz, plus the wavelength, period, and MIDI note number. Switch the A4 reference between 440 Hz (ISO standard), 432 Hz (alternative tuning), 442 Hz (orchestral), and 444 Hz (slightly bright). For the reverse direction, enter any frequency to find the closest note and how many cents off it sits. All math is shown step by step.
How musical note frequencies work
Western music divides each octave into 12 equal steps called semitones. In twelve-tone equal temperament, every semitone is a ratio of 2^(1/12), approximately 1.05946. That means each semitone step multiplies the frequency by that same ratio. An octave, which is 12 semitones, therefore doubles the frequency: A4 = 440 Hz, A5 = 880 Hz, A3 = 220 Hz. This logarithmic spacing is what makes music feel evenly spaced to the human ear, even though the raw Hz values grow exponentially. The global standard for concert pitch is A4 = 440 Hz, codified by ISO 16 in 1975, though many orchestras tune slightly higher (442 Hz) and some musicians prefer 432 Hz for a warmer sound.
The note frequency formula
Every note frequency is derived from a single formula: f = f_A4 x 2^((n - 69) / 12), where n is the MIDI note number (A4 = 69, middle C = C4 = 60) and f_A4 is the chosen reference frequency for A4. To go the other direction (frequency to note), rearrange to n = 69 + 12 x log2(f / f_A4). The fractional part of n, multiplied by 100, gives the cent deviation: how far the frequency sits above or below the nearest in-tune note. One cent is 1/100 of a semitone, so a deviation under about 5 cents is imperceptible, while anything over 25 cents sounds noticeably out of tune.
Wavelength, period, and MIDI number
A sound wave in air at 20 degrees Celsius travels at roughly 343 m/s. The wavelength of a note is simply the speed of sound divided by the frequency: wavelength = 343 / f. Middle C at 261.63 Hz has a wavelength of about 1.31 m; the highest audible A (A8, around 7040 Hz) has one of only about 4.9 cm. The period is the duration of one cycle: 1 / f, often expressed in milliseconds. MIDI note numbers are the integers used by digital audio workstations and synthesizers to address notes: C-1 = 0, C4 = 60, A4 = 69, B8 = 119. Knowing the MIDI number lets you map any frequency directly to a synthesizer key or a piano-roll position.
Cents deviation and tuning
Cents are the standard unit for measuring tiny pitch differences. There are 100 cents in a semitone and 1200 in an octave. When you enter an arbitrary frequency in the reverse-lookup mode, the calculator tells you the nearest equal-tempered note and how many cents sharp or flat the entered frequency is. A string instrument tuned a few cents sharp will clash noticeably with a keyboard tuned to A4 = 440 Hz. Cents are also how electronic tuners report accuracy: a reading of +8 cents means you are slightly sharp and should lower the pitch; -15 cents means you are noticeably flat. Professional players typically aim for within 5 cents of concert pitch.
Note frequencies at A4 = 440 Hz (octave 4)
| Note | Frequency (Hz) | MIDI number | Wavelength (m) |
|---|---|---|---|
| C4 (Middle C) | 261.63 | 60 | 1.311 |
| C#4 / Db4 | 277.18 | 61 | 1.238 |
| D4 | 293.66 | 62 | 1.168 |
| D#4 / Eb4 | 311.13 | 63 | 1.102 |
| E4 | 329.63 | 64 | 1.040 |
| F4 | 349.23 | 65 | 0.982 |
| F#4 / Gb4 | 369.99 | 66 | 0.927 |
| G4 | 392.00 | 67 | 0.875 |
| G#4 / Ab4 | 415.30 | 68 | 0.826 |
| A4 | 440.00 | 69 | 0.780 |
| A#4 / Bb4 | 466.16 | 70 | 0.736 |
| B4 | 493.88 | 71 | 0.695 |
Standard equal-tempered frequencies for all 12 chromatic notes in octave 4. Each octave doubles or halves these values.
Frequently asked questions
What frequency is A4, and why is 440 Hz the standard?
A4 is 440 Hz under the international standard ISO 16, adopted in 1975 after decades of gradually rising pitch across European orchestras. Before the 20th century, concert A ranged from about 415 Hz (Baroque) to 452 Hz in some opera houses. The 440 Hz value was a practical compromise that is now used by virtually all recorded music, tuning apps, and digital audio workstations. Some musicians and composers prefer 432 Hz, claiming it sounds warmer, though there is no scientific consensus that it is physically or psychologically superior.
How do I calculate the frequency of any note from A4?
Use f = 440 x 2^((n - 69) / 12), where n is the MIDI note number of the note you want. For C4 (middle C), n = 60, so f = 440 x 2^((60-69)/12) = 440 x 2^(-0.75) = approximately 261.63 Hz. For A5, n = 81, so f = 440 x 2^((81-69)/12) = 440 x 2^1 = 880 Hz. This calculator does all of that arithmetic for you; select the note and octave and the result appears instantly.
What is a cent, and how many cents are in a semitone?
A cent is 1/100 of a semitone, making 1200 cents in an octave. The human ear can detect differences as small as 5-10 cents in controlled conditions, though in practice differences under about 10-15 cents are difficult to hear in isolation and become noticeable mainly when two slightly mismatched pitches are played together, causing beats. Electronic tuners and DAWs express tuning accuracy in cents because the unit is fine-grained enough to describe practical intonation errors.
What is a MIDI note number?
A MIDI note number is an integer from 0 to 127 used in the MIDI protocol to address piano keys and synthesizer pitches. Middle C (C4) is 60, concert A (A4) is 69, and the full piano range spans roughly MIDI 21 (A0) to MIDI 108 (C8). Because MIDI numbers are independent of the chosen A4 reference frequency, two instruments can share the same note number while producing slightly different frequencies if their tuning references differ.
Why does wavelength change with frequency?
Sound travels at a fixed speed in a given medium (343 m/s in air at 20 degrees Celsius). Wavelength is the distance the wave travels in one cycle: wavelength = speed / frequency. As frequency rises, each cycle gets shorter in time and therefore shorter in physical distance. A 20 Hz bass note has a wavelength of about 17 m, while a 10 kHz treble tone has a wavelength of only 3.4 cm. This is why bass frequencies pass through walls much more easily than high frequencies: their long wavelengths are not blocked by typical wall thicknesses.
Does the A4 reference affect all other note frequencies?
Yes. Every note is defined relative to A4 by the 2^(n/12) ratio, so shifting A4 from 440 Hz to 432 Hz scales all note frequencies by the ratio 432/440, approximately 0.9818. That means every note drops by about 31.8 cents. The interval relationships between notes stay exactly the same; the entire pitch system just shifts down. If one instrument is tuned to A4 = 440 Hz and another to A4 = 432 Hz, they will sound noticeably out of tune with each other, because the mismatch is about 31.8 cents, well above the threshold of audibility.