Guitar Intonation Checker
Your guitar tunes perfectly on the open strings, yet chords up the neck sound out of tune? That is an intonation problem: the note at the 12th fret should be exactly one octave above the open string, and when the vibrating length between nut and saddle is too long or too short, the fretted note comes out sharp or flat. This checker measures that difference through the microphone, string by string, and tells you how many cents each one is off and which way the saddle needs to go.
How to use it
- Pick the instrument and tuning, turn on the microphone and keep the device close to the sound hole or the amp, away from background noise.
- Step 1: play the open string shown and let it ring. If it is more than 3 cents off, the checker asks you to tune first (the link opens the tuner); intonation only makes sense with the open string in tune.
- Step 2 (optional): play the 12th fret harmonic, touching the string lightly over the fret wire without pressing. It is a reference: if the harmonic matches but the fretted note does not, the problem is string height or finger pressure, not the saddle.
- Step 3: fret the string at the 12th fret with your normal playing pressure and play it. The deviation goes into the table: green up to 2 cents, yellow up to 5, red above that, with the adjustment next to it.
- Repeat for the other strings (the checker moves to the next one by itself). Use Redo string after moving the saddle and Reset all to start over; the last result is saved on your device.
Help and explanations
It works with 6-string guitar (standard, drop D and half step down), 4- and 5-string bass and ukulele (high G, low G and baritone). Every reading requires the note to hold steady for half a second and averages ten measurements, so the result does not depend on pick attack. Nothing is recorded or uploaded: the sound is analyzed on your device.
12th fret sharp or flat: what to do
There is only one rule: if the fretted note at the 12th fret is sharp (positive deviation), the string is too short and the saddle must move back, away from the nut, to lengthen the vibrating part. If it is flat (negative deviation), the string is too long and the saddle moves forward, toward the nut. Move a little at a time, retune the open string and measure again: every adjustment changes the open string pitch as well.
On electric guitar and bass each string has its own saddle on the bridge, moved by a screw at the back: tightening the screw pulls the saddle back (longer string, for a sharp 12th fret) and loosening brings it forward. On acoustic and classical guitars the saddle is a single piece of bone or plastic; a luthier files the front or back edge for each string (or fits a compensated saddle). The checker tells you exactly which strings need it and by how much.
Why intonation drifts
Pressing a string stretches it slightly and raises its pitch; thicker strings and higher action stretch more. That is why electric bridges sit at an angle and acoustic saddles are compensated. Old strings, a change of gauge, a different tuning (drop D, half step down) and changes in action all shift intonation, so it is worth measuring again every time you change strings.
A few cents of error are normal, especially on nylon-string guitar and ukulele, where finger pressure matters more. Up to 2 cents nobody hears it; between 2 and 5 cents a trained ear notices in high chords; above 5 cents, chords up the neck sound out of tune even with perfect open strings.
Tips
- Fret the 12th fret note with the same pressure you use when playing: pressing too hard makes the note sharp and fools the measurement.
- Play without vibrato and let the string ring clean; a reading is only accepted after half a second of steady pitch.
- On bass, pluck near the bridge with your finger, not too hard: low strings take a moment to settle on the note.
- Put on fresh strings and stretch them before measuring; old strings go false and give wrong intonation in any setup.
Frequently asked questions
What is guitar intonation?
It is the adjustment of each string's vibrating length, at the saddle, so that the fretted note at the 12th fret is exactly one octave above the open string. With correct intonation the instrument plays in tune all over the neck, not just on open strings.
The 12th fret is sharp: which way does the saddle go?
Back, away from the nut, to lengthen the string. On electric guitar, tighten the screw of that string's saddle; on acoustic, a luthier files the front edge of the saddle. If it is flat, do the opposite: saddle forward.
Do I need the 12th fret harmonic?
No, that step is optional. It helps separate the problem: the harmonic does not depend on finger pressure, so if it matches the open string and the fretted note does not, the error comes from string height or how hard you press.
How much error is acceptable?
Up to 2 cents is green and inaudible. Up to 5 cents is yellow: acceptable on nylon-string guitar and ukulele, worth fixing on electric. Above 5 cents, red: chords up the neck will sound out of tune.
Does it work for bass and ukulele?
Yes: 4- and 5-string bass (standard and drop D) and ukulele with high G, low G and baritone tuning. On bass the checker accepts a slightly weaker signal and waits for the note to settle before measuring.
Why does it say this is not the 12th fret note?
Because the reading was more than 200 cents away from the expected note: you played the open string again, another string, or the microphone picked up a harmonic. Play the requested note and hold it until the bar fills.
Is the audio sent anywhere?
No. Pitch detection runs in the browser, on your device; nothing is recorded or uploaded. The result is saved only in your browser, and you can share a link with the summary if you want.