Online guitar tuner

Standard guitar tuning, low to high: E2 A2 D3 G3 B3 E4. The open strings sit at 82.41, 110.00, 146.83, 196.00, 246.94 and 329.63 Hz with A4 at 440. Bass is E1 A1 D2 G2, ukulele is G4 C4 E4 A4 (re-entrant, so the top string is the highest one), and the violin family is G D A E. Pick a string, pluck it once and let it ring, then read the needle.

It is chromatic, so a note that falls between two named pitches still gets a reading, which is what a fiddle or a brass instrument needs. The cents scale is 100 cents per semitone; inside about five cents is close enough that nobody in the room will hear it. If the reading jumps between two notes, the string is too quiet or something else is ringing: give the tuner one clear, steady note for a second.

Detection works the way the app does it. Autocorrelation slides the waveform against itself and looks for the distance at which it repeats, then refines that distance. That is why it follows a real pitch on a plucked string instead of just reporting whichever frequency happens to be loudest, and it is the same maths as the pitch detector in the iOS app.

A phone tuner holds up better in a loud room and can stay calibrated while you move around a stage. This page is for the moment you do not have one to hand.