YouTube's official spec is 1280×720 pixels at a 16:9 ratio. Here's how to hit that exactly, no matter what size your source image is.
A thumbnail that isn't exactly 16:9 gets letterboxed or oddly cropped by YouTube's player. Resizing precisely to 1280×720 before you upload means your thumbnail looks exactly the way you designed it, in every placement — search results, suggested videos, and the video page itself.
Open Squeezr and drop in your thumbnail design or photo.
Turn on Resize, unlock the aspect-ratio lock, and set width and height to 1280 × 720.
Choose Crop if your source image isn't already 16:9, so the full frame is filled with no bars.
Export as JPG — YouTube accepts JPG, PNG, GIF and BMP, but JPG keeps the file size down for faster upload.
Upload it in YouTube Studio's thumbnail slot.
Skip the manual math — open Squeezr with this preset already dialed in.
Open Squeezr preset to 1280×720 →YouTube caps thumbnail display at 1280×720 regardless of what you upload — a larger file just means slower upload and more storage for no visible benefit. 720p is sharp enough even on 4K displays, since thumbnails are shown fairly small in most placements.
YouTube's UI overlays a video duration badge in the bottom-right corner of every thumbnail, and mobile layouts crop the edges slightly tighter than desktop. If your thumbnail has text or a face near the edge, use Fit mode instead of Crop so nothing important gets trimmed off, then add your own padding around it in your design tool before resizing.