How this instrument works
Concert pitch A4, tuned to 440 Hz, is the internationally adopted reference frequency (formalised as ISO 16) that orchestras, instrument makers, and tuning apps calibrate to. Every other note's frequency can be derived from it by counting semitones away from A4 and applying the equal-temperament formula: frequency = 440 x 2 raised to the power of (semitones / 12).
This works in both directions — positive semitone values move upward from A4, negative values move downward — because 12-tone equal temperament splits every octave into 12 equal ratio steps rather than 12 equal frequency amounts. That's why doubling a frequency is always exactly 12 semitones away, whether you start from A4, C4, or any other note: the ratio between adjacent semitones is constant everywhere on the keyboard.
This instrument accepts any semitone value, not just whole numbers — fractional values let you find frequencies for pitches slightly off the standard equal-tempered grid, useful for microtonal work, cents-based pitch analysis, or checking how far a slightly-out-of-tune note actually sits from its nearest reference pitch.
- Enter Semitones from A4 (440 Hz) — a positive number for notes above A4, negative for notes below.
- Read Frequency (Hz) directly beneath the input.
- Use whole numbers for standard 12-tone equal-tempered notes; decimals work for microtonal or cents-based offsets.
- To go the other direction — finding how many semitones separate two known frequencies — use this site's semitone interval calculator instead.
Worked example — 0 semitones from A4
Enter Semitones from A4 (440 Hz) = 0. That's 440 x 2^(0/12) = 440 x 1 = 440.0 Hz exactly. Frequency reads 440.0.
A semitone offset of zero is A4 itself, so the formula returns the reference frequency unchanged — a useful sanity check confirming the instrument is anchored to the standard 440 Hz concert pitch before you move up or down from it.
Questions
Why is A4 the reference pitch instead of C?
It's a historical and practical convention rather than an acoustic necessity — A was widely used as the tuning-fork reference note well before international standardisation, and 440 Hz for A4 was formally adopted as ISO 16 in the 20th century. Any note could in principle serve as the anchor; A440 is simply the one the world settled on.
What semitone value do I enter for middle C (C4)?
Enter -9. Middle C sits 9 semitones below A4, and 440 x 2^(-9/12) works out to approximately 261.6256 Hz — the standard 12-tone-equal-temperament frequency for C4 used across electronic tuners, synthesizers, and MIDI-to-frequency conversions.
What does entering 12 give me?
Exactly one octave above A4: 440 x 2^(12/12) = 440 x 2 = 880.0 Hz, which is A5. Every 12-semitone step doubles the frequency, so entering 24 would give 1760 Hz (A6), and -12 would give 220 Hz (A3).
Can I enter negative or decimal semitone values?
Yes to both. Negative values move below A4 (down the keyboard), and decimal values let you compute frequencies for microtonal pitches or fine cents-based offsets that fall between the standard equal-tempered notes — the underlying power-of-two formula works continuously, not just at whole-semitone steps.
Is 440 Hz the only correct tuning standard?
It's the internationally adopted standard (ISO 16) and the default this instrument uses, but it isn't universal in practice — some orchestras tune slightly sharper, commonly around 442–443 Hz, by house convention. If you need a different reference frequency, scale this instrument's output by the ratio of your reference to 440.