How big should MP3 cover art be? I measured what each size adds to the file

Every guide says "use a square image, 600 to 1000 pixels". Nobody says what that costs, or what happens if you ignore it. So I embedded the same photograph at nine sizes, in two formats, and weighed the results.

How I measured

The starting file is a 15-second MP3 with no tag, 93,024 bytes. The picture is a 6016×6016 photograph (one of Apple's macOS wallpapers, chosen because it's a real photo with sky, texture and fine detail, not a flat logo that would compress unrealistically well). I cropped it square, scaled it to each size with a proper resampling filter, saved it as JPEG at quality 85, JPEG at quality 70, and PNG, and wrote each one into a copy of the MP3 as a front-cover APIC frame in an ID3v2.3 tag. Then I subtracted the original file size. The tools were Python with the Pillow and mutagen libraries; nothing exotic.

The numbers

Two things stand out. First, the tag adds almost nothing on top of the image: 47 bytes for the frame header, the MIME type and the description. What you embed is what you pay. Second, PNG is a disaster for photographs: at 1000 pixels it's eleven times the size of the JPEG, and you can't see the difference on a phone screen.

What size to use

A cover is displayed small. On a phone's now-playing screen it's maybe 300 to 400 points wide; on a laptop, a thumbnail in a list; in a car, a postage stamp on a low-resolution screen. A 600-pixel JPEG covers all of those with pixels to spare, and a 1000-pixel one is enough for a full-screen view on a high-density phone display. I use 800 or 1000 for albums I care about and 600 for everything else, always JPEG, always quality somewhere in the 80s.

Above 1000 you're paying for pixels no screen will show. A 3000-pixel cover on a 12-track album is 5 MB of image data, which is more than a whole track at 128 kbps, and it's there twelve times because each file carries its own copy. Apple asks record labels for 3000×3000 artwork, but that's for the store's servers to make smaller versions from, not a size to put in your files.

When PNG is right

Flat artwork: a logo, a two-colour podcast badge, text on a plain background. JPEG smears sharp edges and PNG stores them exactly, and for a simple image the PNG can even be smaller. If the cover is a photograph or a painting, JPEG, no contest.

Quality 70 versus 85

Dropping from 85 to 70 saved about 30 percent in my test and is visible if you look for it, mostly as a slight softness in fine detail. For a cover that's shown at a quarter of its size, nobody will notice. For a 1000-pixel cover viewed full screen, stay at 80 or above.

Non-square images

Players assume a square and either crop or squash anything else. A landscape photo of the band ends up as a stretched face or a centre crop with the drummer missing. Crop it square yourself before embedding so you choose what stays.

Doing this with the editor

The editor embeds whatever you upload, as-is; it doesn't resize. The small grey label next to the preview shows the format and size in kilobytes, so if it says 850 KB you know before saving that you'll want a smaller file. Any image tool will resize: on a Mac, Preview's Adjust Size; on Windows, Photos or Paint; on a phone, the built-in crop. Save as JPEG, upload, done.