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What intonation means
Intonation is whether your guitar plays in tune all the way up the neck, not just on the open strings. A guitar can be perfectly tuned open and still sound sharp or flat at the twelfth fret. When that happens, chords high up the neck sound sour, and no amount of tuning the open strings will fix it.
The physics is simple. The twelfth fret sits exactly halfway along the vibrating string, so it must sound exactly one octave above the open string. One octave means double the frequency. If the open string vibrates at 110 hertz, the twelfth fret must vibrate at 220 hertz. Any difference is an intonation error, measured in cents. One hundred cents is one semitone.
How this checker works
The method is stated plainly because there is no magic here. You measure two pitches: the open string and the same string pressed at the twelfth fret. The tool compares them and reports the difference in cents. Within five cents counts as in tune. More than that, and the saddle needs moving.
You can measure with your microphone. Press the capture button, play the open string steadily, then capture the twelfth fret the same way. Or you can measure with a tuner app or a pedal tuner, read the two frequencies, and type them into the manual mode. Both routes use the same maths, and both are honest measurements, not guesses.
Reading the result
If the twelfth fret is sharp, the string is effectively too short, so the saddle must move back, away from the neck, to lengthen the string. If the twelfth fret is flat, the string is too long, so the saddle moves forward, toward the neck, to shorten it. Move a little at a time, retune the open string, and measure again. Intonation work is a loop, not a single adjustment.
Small errors are normal. Strings wear, temperature changes, and a hard pick attack bends the pitch sharp for a moment. Take two or three readings and trust the average. If the error is under five cents, leave it alone. Chasing zero on a guitar is a good way to waste an afternoon.
What else affects intonation
Old strings intonate worse than new ones, so change strings before you judge a guitar. High action, where the strings sit far from the frets, pushes notes sharp when you press down, because you stretch the string further. A worn nut or a badly cut saddle can also throw things off. If the saddle is already at the end of its travel and the guitar still will not intonate, a technician should look at the neck and the frets.
Check every string separately. It is common for three strings to be fine and one to be off. Fix the one that is off. Do not move saddles on strings that already measure well.
Microphone tips
Play in a quiet room, hold the note steady for a few seconds, and keep the guitar close to the microphone. Pluck gently: a hard attack starts sharp and settles, which confuses the reading. Let the note ring and let the tool average the pitch. If the reading jumps around, the room is too noisy or the note is too short.
Your audio never leaves your device. The microphone signal is analysed live in your browser, and nothing is recorded or uploaded. If your browser blocks the microphone, the manual mode still gives you a real result from frequencies you measured yourself.
When to check
Check intonation when you change string gauge, when you change tuning, when a guitar comes back from repair, and whenever chords above the fifth fret start sounding wrong. It takes a few minutes per string, and it is the difference between a guitar that sounds good everywhere and one that only sounds good at the bottom of the neck.
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