About ThumbCheck

ThumbCheck is a free tool that checks a YouTube thumbnail for the problems that are easy to miss on a big monitor: wrong size, low contrast, text that is too small for a phone, and detail hidden under the timestamp badge. You can also preview it in mock feeds.

How the checks work

• All analysis runs as JavaScript in your browser, on a 640×360 copy of your image. Nothing is sent to a server. See the privacy page.
• Every result shows the measured number, the threshold, and the reason. Expand "How this is measured, and its limits" under any check for the details.
• These are heuristics, not judgements of your design. The tool cannot read text and does not know what your video is about. Your eyes, and your real click-through rate in YouTube Studio, have the final say.
• We don't give an overall "score". Pixels can't predict views, and a single number would hide what you can actually fix.

Size rules used

From YouTube Help, "Add custom thumbnails", checked October 2026: 16:9 for videos; minimum width 640 px; 3840×2160 recommended; JPG or PNG; under 2 MB when uploading from the phone app and under 50 MB from desktop. 1280×720 is still a common, safe size.

Who makes it

ThumbCheck is made by a small team that also sells a (): You don't need it to use this tool, and the tool will stay free.

ThumbCheck is not affiliated with YouTube or Google. The feed previews are simplified mock-ups with grey placeholder videos.