Check a Thumbnail Before You Upload It
Drop an image in and it measures the four things YouTube checks: 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.
Resolution
The real pixel size, against the minimum width and full quality.
Aspect ratio
Whether it is exactly 16:9, or will be letterboxed or cropped.
File size
Against the stricter phone upload limit, so it works from any device.
Format
JPG and PNG pass. Anything YouTube may refuse is flagged.
What YouTube Actually Requires
These are YouTube’s current rules from its help page, next to what this checker does with each one.
| Rule | YouTube’s help page | What this checker does |
|---|---|---|
| Resolution | Recommends 3840 x 2160 pixels, with a minimum width of 640 pixels | Fails under 640 pixels wide, warns under 1280 pixels, passes from there up |
| Aspect ratio | 16:9 for videos, 9:16 for Shorts | Checks for 16:9, so it is built for regular videos, not Shorts |
| File size | 2 MB when uploading from a phone, 50 MB from a computer | Uses the stricter 2 MB, so a pass works from any device |
| Format | Image formats such as JPG or PNG | Passes JPG and PNG, warns on GIF and BMP, fails anything else |
Sources, checked 2026-10-12: YouTube Help: video thumbnails, Thumbnail Size Checker settings
Resolution is the softest rule
Under the minimum width the upload is refused. Between the minimum and full size it uploads but looks soft, most of all on a TV.
Aspect ratio never blocks
A wrong shape uploads without a warning, then gets letterboxed or cropped. That is why it slips through so often.
File size depends on the device
The limit from a phone is far lower than from a computer. A big PNG is usually the culprit, and a JPG fixes it.
Format trips up new phones
Newer phones and design tools often export WebP or HEIC. Re-save as JPG or PNG before you upload.
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 the 2 MB it checks against, 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.
Unusable, score capped
Too narrow, too heavy, or the wrong format. The upload can fail, so nothing else about the image matters yet.
Usable, but it shows
The wrong shape uploads and gets padded or cropped. Serious, but a quality problem rather than a rejection.
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.
Design around the corner YouTube covers
Keep it clear
Leave the last sixth of the width and fifth of the height free.
Put background there
Sky, a blurred edge, anything you would not miss.
Shrink to check
View it at the width of a phone listing before you decide.
The runtime label sits in the bottom right of almost every listing, and no size check can catch what it covers.
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 full size 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 upload, especially after resizing a design that started at a different shape.
Designers
Handing thumbnails to a client who uploads later, where a rejected file means an email and a delay.
Phone and app exporters
Anyone whose tool defaults to WebP or HEIC, where the failure only shows 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.
Good at
- Measuring resolution, shape, file size and format
- Separating blocking problems from quality ones
- Giving the exact fix for each problem
- Checking images without uploading them
Not built for
- Design quality: contrast, readability and faces are your call
- Shorts thumbnails: it checks for 16:9, not 9:16
- The runtime corner: no size check can see what it covers
- The desktop limit: it uses the stricter phone limit on purpose
Getting a Thumbnail to Check
These tools cover the steps around the check.
Pull an existing thumbnail
The YouTube thumbnail downloader fetches every size a video has.
Check the title
The video SEO analyzer checks the title cutoff and keyword placement next to the thumbnail.
