What a capo actually does
It shortens every string at once, so the shape your left hand holds comes out higher than the shape says. Clamp it on the second fret, hold what looks like G, and the room hears A. Your fingers are in G; the song is in A. Both descriptions are true, and keeping them apart is the whole skill — the longer version is on the blog.
Which means a capo is not a transposition. The sound stays exactly where it was; only what you read changes. If you want the song to come out in a different key — because it sits wrong for your voice — that is the chord transposer instead.
Which frets win, and why
The shapes a guitar makes easy are the ones with open strings in them — C, G, D, A, E, Em, Am. A capo wins when it lands your song on one of those keys, and does nothing at all when it lands between them. That is why the useful frets are so unevenly spread: the good ones are the distances from your key to an open-string key.
It is also why the received wisdom is worth checking. “Song in F? Capo 5.” does not survive the arithmetic: the fifth fret turns F Bb C Dm into C F G Am, and the F is still a barre. The third fret gives you D G A Bm — one barre as well, but a lower capo — so the table above calls it a tie and takes the lower fret. Paste your own chords and it will do the same for them.
Ties go to the lowest fret, because a capo further up the neck shortens the instrument for nothing: the low end of the song goes with it. The table shows the names you would be reading at every position so you can check the answer rather than trust it.
Guitar and ukulele are different questions
The same chords are hard on one and simple on the other: E is a stretch on a guitar and three fingers on a ukulele, and Bb is the reverse. The switch above is not cosmetic — it changes which shapes count as easy, and often which fret wins.
Nothing leaves your browser
The arithmetic happens on your own machine, in the page you are looking at. No upload, no account, nothing kept.
One song at a time, or every song remembered
This page answers for the chords in the box. What it cannot do is remember: that this song lives on the second fret, that one on the fifth, that the third needs no capo at all — and have the sheet already showing the right shapes when you open it on stage next Friday.
That is what Strumfolio does with the same arithmetic: a capo kept per song, the sheet redrawn to match, and the chords still a tap away from their fingering. Bring the sheet in first with the ChordPro converter if it is still a text file.