The YouTube thumbnail file size limit is two numbers, not one
The 2 MB everyone quotes is the phone limit. From a computer the ceiling is 50 MB — the same thumbnail, a different door.
The two numbers
A custom thumbnail can be up to 50 MB when it is uploaded from a computer, and up to 2 MB when it is uploaded from the YouTube app on a phone. Both numbers are on YouTube's page about custom thumbnails, which is also where the accepted formats are: JPG and PNG.
Almost every article on the subject quotes 2 MB as the thumbnail limit. It is not wrong so much as half-quoted: it is the mobile limit, and it is the one you meet if you finish a video on your phone and set the thumbnail there.
Neither number is about quality. A 400 KB JPEG and a 4 MB PNG of the same image look identical in a feed at 360 pixels wide — the limit exists for upload and storage, not for how the thumbnail is displayed.
Which limit applies to you
The one belonging to the device doing the upload, not the one belonging to the video. A video uploaded from a computer with a 30 MB thumbnail is fine. The same thumbnail, set from the app afterwards, is refused. If a file that worked yesterday is refused today, the first question is which device you are standing at.
- Uploading from a browser on a computer — 50 MB
- Setting the thumbnail in the YouTube app on a phone — 2 MB
- Either way — JPG or PNG, at least 640 pixels wide
What a thumbnail actually weighs
A 1280 × 720 JPEG exported at a sane quality is usually somewhere between 100 and 400 KB. A 3840 × 2160 JPEG is typically 0.5–2 MB. The files that run into the ceiling are almost always PNGs: a 4K PNG of a photographic image can be 15–25 MB, because PNG stores every pixel losslessly and a photograph gives it nothing to compress.
When the file is over the limit
Lower the quality before you touch the resolution. A JPEG at quality 70 is visually indistinguishable from the same image at quality 95 once it is 360 pixels wide in a feed, and it is a fraction of the weight. Cutting resolution is the last resort, because resolution is what keeps the thumbnail sharp on a phone screen.
If the file is a PNG of a photograph, exporting it as a JPEG will usually solve the problem outright. Keep PNG only where it earns its weight: flat colour, hard edges, text-heavy designs with no gradients. The compressor does the quality search for you — you give it a target, it spends quality by bisection and only scales the image down if the target is unreachable any other way.
Why YouTube re-encodes it anyway
Whatever you upload, YouTube generates its own derivatives at several sizes and serves those. This is the reason a 40 MB upload buys you nothing over a 400 KB one, and also the reason a heavily compressed file cannot be rescued later: the artefacts are baked in before the re-encode, and re-encoding does not remove them. The size guide covers what happens to the pixels on the way through.
The formats that are refused
JPG and PNG are accepted. WebP is not, and it is the modern trap: several editors and export presets now default to it, and the file looks perfect until the upload is refused. An animated GIF is refused too — a thumbnail is a still image. The size checker names the format of a file and says whether YouTube takes it.
The questions everyone asks
What is the maximum file size for a YouTube thumbnail?
50 MB from a computer, 2 MB from the YouTube app on a phone. The 2 MB figure quoted everywhere is the mobile one.
Why does YouTube say my thumbnail is too big?
Because you are over the limit for the device you are uploading from — most often the 2 MB phone limit — or the file is not a JPG or PNG.
Should a thumbnail be a JPG or a PNG?
JPG for anything photographic; PNG for flat colour, hard edges and text with no gradients. A 4K PNG of a photo is what usually hits the ceiling.
Does a bigger file mean a sharper thumbnail?
No. YouTube re-encodes what you upload into its own sizes, so past a reasonable quality the extra megabytes are discarded before anyone sees the image.