How a capo works
A capo is a clamp that presses all six strings against one fret. It acts as a new nut: every string is shortened, so every open string sounds higher. Each fret raises the pitch by one semitone, the smallest step on the guitar, and twelve frets make an octave.
Your hands form the same shapes as before, but every chord comes out three semitones higher. A G shape sounds as B♭, a C shape as E♭, and an Em shape as Gm.
That is the whole reason to use one. Keys such as B♭ and E♭ are full of barre chords that are tiring to hold and hard to keep clean. With a capo you keep playing friendly open shapes such as G, C, D and Em while the song sounds in the key you need.
How to read the chart
Each row is a chord shape that you finger. Each column is a capo fret. Each cell is the chord you actually hear. Find G in the left column, run along to column 3, and the cell says B♭.
You can read it in both directions. If a lead sheet says "capo 3, play G", read across from the G row. If a song is in B♭ and you would like to play G shapes, look along the G row until you find B♭, then read the fret from the column heading: 3.
Shapes marked with an orange dot are the open ones that need no barre: C, A, G, E and D in major, and Am, Em and Dm in minor. A good capo position lets you play a whole song from those rows. Most songs use several chords, so the song helper checks them all at once.
Choosing a capo position for your voice
A capo can only move a song up, never down, so start from the position on the original recording or chord sheet and adjust from there.
Because each fret is one semitone, one or two frets is usually enough to fit a voice. Sing the highest and lowest notes of the melody before you decide. Frets above 7 are rarely worth it: the spaces between frets get narrow, and the guitar starts to sound thin. If the position that suits your voice forces awkward chords, type the song's chords into the song helper and it will tell you which capo fret gives the easiest shapes.
Sharps versus flats
A♯ and B♭ are the same note on the same fret. The name depends on the key being written. B♭ major has two flats, while A♯ major would need ten sharps, so musicians write B♭. By default the chart names major keys C, D♭, D, E♭, E, F, F♯, G, A♭, A, B♭ and B. Minor keys use sharps for C♯, F♯ and G♯, and flats for E♭ and B♭.
Other charts mix the two, which is why you may see A♯ on one sheet and B♭ on another. Nothing about what you play changes. Use the ♭ and ♯ buttons in the calculator to switch every chord on the page.
Capo on ukulele
The maths is identical: each fret raises every string by a semitone, so a C shape with the capo on fret 2 sounds as D on a ukulele just as on a guitar. The note names in the chart are right for any instrument.
What differs is which shapes are easy. On a ukulele in standard tuning, C, F, Am and G7 are open shapes that take one to three fingers, and F is no barre at all. The orange dots and the song helper's ranking are tuned for guitar, so on a ukulele read the chart for the note names and ignore the dots. Use a capo made for ukulele, because a guitar capo is usually too wide for the neck.