The medley of "Over the Rainbow" and "What a Wonderful World" by Israel Kamakawiwoʻole (IZ) is charted in the key of C on six core chords: C, Am, F, G, Em and E7, plus one D minor bar in the "What a Wonderful World" part. The island strum, down, down-up, up-down-up, fits under all of it. The "Someday" section uses only C, G, Am and F, so that's the part I'd learn first.
IZ's medley, Rainbow-only charts, and the film version
This chord map follows IZ's full medley, the version on his 1993 album Facing Future, where "Over the Rainbow" runs into "What a Wonderful World". He recorded it in a single take.
Some charts and lessons cover only the Rainbow parts and leave "What a Wonderful World" out. If that's the chart you have, skip those lines below and the rest still lines up.
The Judy Garland film version is a different arrangement, in a different key, with different harmony and no "What a Wonderful World". If that's the one you're after, look for a chart made for the film version.
Six chords, and the shapes you may not have yet
The six chords of IZ's medley are C, Am, F, G, Em and E7, plus Dm for one bar.
The shapes here are written as four numbers, one for each string in the order G, C, E, A, from the string nearest your face to the one nearest the floor. Each number is the fret you press, and 0 means the string rings open.
Four of the six are the usual first chords, C (0003), Am (2000), F (2010) and G (0232). If any is new to you, start with how to fret C, Am, F and G (https://www.jessicabenini.com/blog/quick-guide-to-ukulele-chords-for-beginners).
Three more shapes are particular to this song.
Em, 0432. Index on the A string at the 2nd fret, middle on the E string at the 3rd fret, ring on the C string at the 4th fret. Keep the ring finger clear of the open G string so it can ring. If you know G, you can also hold the G shape and add your little finger on the C string at the 4th fret.
E7, 1202. Index on the G string at the 1st fret, middle on the C string at the 2nd fret, ring on the A string at the 2nd fret. The open E string has to ring, so keep every finger off it.
Dm, 2210, for one bar. Middle on the G string at the 2nd fret, ring on the C string at the 2nd fret, index on the E string at the 1st fret, A string open. Some charts write Dm7 (2213), which adds your little finger on the A string at the 3rd fret. Plain Dm is the easier of the two and works fine.
If a chart shows G6 instead of G, that's 0202, a two-finger shape. The common teaching charts for this medley use G.
The song, section by section
Every chart of this song is a transcription by ear of IZ's single take, so small differences between charts are normal. This map follows the reading most charts share, in roughly the order you hear the sections, and flags where they part ways.
The intro
Charts don't agree on the intro, so take this as the common version, not the only one.
The "ooh" figure
This comes before the first verse in most charts and returns at the very end. It's also where you meet E7 for the first time.
The Rainbow verse
Em arrives on the very first sung line. The second Rainbow verse uses the same chords.
The "Someday" section
Only C, G, Am and F appear here, in a loop that comes round twice.
The closing Rainbow line
F and C sit close together at the start of this line.
"What a Wonderful World"
E7 comes back on the second line and, as everywhere in this song, moves straight to Am. The later verse ends F G C instead of Am F. You may also see a chart with G in place of the Em in each first line.
The bridge
The bridge starts at "The colours of the rainbow" and uses chords you already have, plus one D minor bar on the line before "I love you". Some charts write Dm7 there, and charts disagree about this line too, so a different version on yours doesn't mean either is wrong.
The ending
The medley closes with a last "Someday" section, a last Rainbow verse and the "ooh" figure. Charts disagree on the final chord, F or C. Both work, so pick one and keep it.
The strum, and the order to learn it in
The island strum fits this song well: down, down-up, up-down-up. It's the pattern I'd put under it first.
The bracketed strokes are silent: your hand still makes them, without touching the strings. For the strumming hand itself, see my strumming guide.
IZ's own part is different. The lessons that transcribe the recording hear a thumb pluck followed by strums, and they don't agree on which string gets the pluck. The island strum is a beginner's way into the song.
One teacher's rule of thumb for this song is two patterns per chord, with one per chord in the intro. Where two chords sit close together, like the F and C that open the closing Rainbow line, you split the pattern between them. Practise well below the speed of the recording, and speed up once the changes are clean.
Which part to learn first
Here's the order I'd learn it in, one new thing at a time.
The "Someday" section. It uses only C, G, Am and F. Its lines keep ending on Am to F, and that's the one change among those four where a finger stays down. The middle finger stays on the G string at the 2nd fret.
The Rainbow verse. This adds Em, the second chord of the first sung line. No finger carries over from C to Em, so it's a full reshape.
The "ooh" figure. This brings in E7. F to E7 and E7 to Am are both full reshapes, with no finger staying in place.
"What a Wonderful World". Nothing new here, only E7 again on the second line.
The D minor bar. Leave it for last, since it turns up once.
Strum steady down-ups while you learn each section, and bring in the island strum once the changes are clean. Then join the sections in order.
Singing it while you play, and where a lesson fits
A free chart and the recording cost nothing, and with this map they're enough to learn the song's changes on your own. What they can't do is watch you play, so a habit in your fretting hand can take hold without anyone pointing it out.
I teach ukulele live online, one-to-one on Zoom, to students anywhere in Canada and in the US, and in person in Victoria, BC, on the songs you actually want to play, so you can bring this song to a lesson. You'll find how it works on my page about ukulele lessons on Zoom or in Victoria, and when you're ready, tell me which part of the medley you're working on.
Singing and playing at the same time is also part of the beginner level of my group classes, which run at two levels, beginner and intermediate. If you'd like company while you learn, learning it with a group in Victoria is another way in.
Low G, the key of G, and playing only the Rainbow part
Do I need a low-G ukulele to play it?
No. The lessons that transcribe IZ's recording describe him playing in low-G tuning, which puts the G string an octave lower, but his arrangement works just as well on a standard high-G ukulele. The chord shapes and their names are the same in both tunings, so every shape the medley needs works on either.
Why are some charts for this song in the key of G?
It depends on which version the chart follows. IZ's medley is charted in C, so a chart in G may be his arrangement moved to another key. It may also be the Judy Garland film version, a different arrangement whose charts appear online in several keys, G among them. Check the title, and whether "What a Wonderful World" is in it, before you learn from it.
Can I play it without the island strum?
Yes. Steady down-ups all the way through carry IZ's medley while your fretting hand is still busy with the changes. Once you can get from F to E7 and on to Am without stopping, bring the island strum in one section at a time, starting with the "Someday" part.
Can I leave out the "What a Wonderful World" part?
Yes. Some charts and lessons cover only the Rainbow parts of IZ's medley. You'd play the intro and the "ooh" figure, the Rainbow verses, the "Someday" section and the ending, and you'd never need the D minor bar. You still need E7, though, because it turns up in the "ooh" figure.
Why do chord charts for this song disagree?
Every chart of IZ's medley is someone's transcription by ear of a recording made in a single take, so small differences are normal. They show up mostly in the intro, on the line before "I love you" and in the last chord. Pick one reading for each and keep to it.