Check a Thumbnail Before You Upload It
Drop an image in and it measures the four things YouTube enforces: the resolution, the aspect ratio, the file size, and the format. You get a score, the real numbers, and a specific fix for whatever is wrong.
The image is measured on your device. Nothing is uploaded, which matters when the thumbnail belongs to a video that has not been published yet.
What YouTube Actually Requires
There are four rules, and they are not equally serious.
Resolution. 1280x720 is the recommended size, and 640 pixels is the minimum width YouTube will accept. Below that the upload is refused outright. Between the two it uploads but softens, which is most visible on a TV or a large monitor where the thumbnail is scaled up.
Aspect ratio. 16:9. Anything else gets letterboxed or cropped to fit, and you do not get to choose where the crop lands. This one does not block the upload, which is exactly why it slips through so often.
File size. Under 2MB. Over that and the upload is refused. A 1280x720 JPG at sensible quality lands well under a megabyte, so a thumbnail over 2MB is usually a PNG that should have been a JPG.
Format. JPG, PNG, GIF and BMP are accepted. WebP and HEIC are not, which catches people exporting from newer phone cameras and design tools.
Why the Score Caps When Something Is Blocked
A thumbnail that YouTube will refuse is not a good thumbnail with one flaw, it is unusable. So if the resolution is under the minimum, the file is over 2MB, or the format is not accepted, the score is capped no matter how good everything else is.
A wrong aspect ratio is treated differently, because it does not block the upload. A 4:3 image will go up, and YouTube will letterbox it. That is a quality problem and a serious one, but it is not the same as being rejected, and the score reflects that difference rather than flattening the two together.
The Mistake That Costs the Most
Uploading a thumbnail that is not 16:9 and never noticing.
It does not fail. Nothing warns you. YouTube pads or crops the image and carries on, so the thumbnail appears in search results and suggested videos with bars down the sides, or with the top of someone’s head cut off. Because it looks fine in your editor and fine in the upload dialog, the problem is only visible where it matters, which is in a feed next to competitors whose thumbnails fill the frame.
The check for this is arithmetic, which is the whole reason to run it: 1280 divided by 720 is 1.778, and if your image does not come out at that number, it will be adjusted without asking.
The Bottom Right Corner Is Not Yours
Every thumbnail gets the video’s runtime stamped over it, in a small dark label in the bottom right corner. On a live stream it is a red LIVE badge in the same place.
Nothing warns you about this, and it is not part of any size specification, so it is easy to design a thumbnail with a word or a face in that corner and then watch it get covered in every listing. The image is still 1280x720 and still passes every check here, because the problem is not the dimensions, it is where you put things inside them.
The practical rule is to treat the bottom right as reserved. Keep text, faces and anything load-bearing out of roughly the last sixth of the width and the last fifth of the height. That corner is fine for background, sky, a blurred edge, or whatever you would not miss.
It Gets Displayed Far Smaller Than You Design It
This is the reason the 16:9 rule and the resolution floor matter more than they look like they should.
You can see how small YouTube expects to go from the images it generates for itself. Alongside the 1280x720 version it keeps a 480x360, a 320x180 and a 120x90. It would not store a 120 pixel wide copy of every thumbnail on the platform unless it intended to display one.
So the design question is not whether your text is readable on the canvas in front of you at full size. It is whether it survives being scaled to the width of a sidebar entry or a phone listing. Three or four large words hold up. A sentence, a small logo, or thin lettering does not, and neither does a photograph where the subject is far from the camera.
Design at 1280x720 for the sharpness, then shrink the file on screen to something like the width of a thumb before you decide the design works.
Who Uses This
Creators checking an export before an upload, particularly after resizing a design that started at a different shape.
Designers handing thumbnails to a client who will upload them later, where a rejected file means an email and a delay.
Anyone exporting from a phone or a newer design tool, where the default format is often WebP or HEIC and the failure only appears at upload time.
What It Cannot Tell You
Whether the thumbnail is any good.
Contrast, whether the text is readable at the size it will actually be displayed, whether a face reads clearly in a small rectangle, whether it is honest about the video: none of that is measurable from pixel dimensions, and a tool that scored it would be making up a judgment it cannot support.
So this covers the mechanical half, the part where a wrong number gets your upload refused or your image quietly letterboxed. The other half is a design question, and it is yours.
Getting a Thumbnail to Check
If you need to pull the existing thumbnail off a video, whether it is yours or one you are studying, the YouTube thumbnail downloader fetches every size that video has. To work on the title and description that sit next to the thumbnail, the video SEO analyzer checks the title cutoff and keyword placement.
