Every musical note in the Western system has a precise frequency in hertz (Hz), determined by the equal-temperament tuning system. This calculator converts any note name and octave into its exact frequency, plus the MIDI note number and the sound wavelength in air — useful for synth programming, instrument building, acoustics, and audio engineering.
The equal-temperament formula
12-tone equal temperament divides each octave into 12 equal semitone steps. Because doubling the frequency raises pitch by exactly one octave, each semitone step multiplies frequency by the twelfth root of 2 (≈ 1.05946). Starting from A4 as the anchor:
f = ref × 2^((n − 69) / 12)where n is the MIDI note number (A4 = 69, C4 = 60, B4 = 71) and ref is the A4 reference frequency (standard: 440 Hz). The exponent changes by 1 for each semitone, so 12 semitones = exponent of +1 = exactly double the frequency.
The A4 reference and why it matters
The 440 Hz standard (ISO 16) has been widely adopted since the 1940s, but it is not universal:
- Many European orchestras tune A4 to 442–443 Hz, producing a brighter sound.
- Baroque and historically informed performance ensembles often use 415 Hz (roughly a semitone flat) or 392 Hz.
- Some modern orchestras tune as high as 444 Hz.
Adjusting the reference in this calculator recalculates every note simultaneously, so you can check what any note sounds like at any tuning standard.
MIDI note numbers
MIDI encodes pitch as integers from 0 to 127. Middle C is C4 = MIDI 60. Each semitone up adds one; each semitone down subtracts one. A4 = 69. These numbers are used everywhere in digital audio: synthesisers, DAWs, sequencers, and VST plugins all speak MIDI pitch.
Wavelength
The wavelength shown is calculated from the speed of sound at approximately 20°C:
wavelength (m) = 343 ÷ frequency (Hz)For A4 at 440 Hz the wavelength is about 0.78 metres. At C2 (65.4 Hz) it is about 5.25 metres — longer than a grand piano. These figures matter for speaker placement, room acoustics, and standing-wave calculations.
Quick reference: common notes
| Note | MIDI | Frequency (440 Hz ref) | Wavelength |
|---|---|---|---|
| C4 (middle C) | 60 | 261.6 Hz | 1.31 m |
| A4 (concert A) | 69 | 440.0 Hz | 0.78 m |
| A5 | 81 | 880.0 Hz | 0.39 m |
| G♯4 / A♭4 | 68 | 415.3 Hz | 0.83 m |
| C1 (low bass) | 24 | 32.7 Hz | 10.5 m |
Practical applications
- Synthesiser programming — oscillator frequencies in many synths are set in Hz. Knowing that middle C is 261.6 Hz and A4 is 440 Hz lets you tune patch settings precisely.
- Sample pitch shifting — to pitch-shift a sample by an interval, calculate the source and target frequencies; the ratio gives the playback speed multiplier.
- Room acoustics — wavelengths determine where standing waves form in a room. A bass frequency at 40 Hz has a wavelength of about 8.6 metres — roughly the length of a small room — explaining why bass traps are needed in the corners.
- Instrument building — fork lengths for xylophone bars, speaking-pipe lengths for organs, and string tensions for harps are all derived from target frequencies.
- Tuning verification — if you measure a tone at 437 Hz and want to confirm it is A4 flat by 3 Hz, entering A4 at your reference immediately shows the expected 440 Hz.
All calculations run locally in your browser.