Guide
How YouTube thumbnails work
A thumbnail downloader is not magic. It is a polite client of files YouTube already put on a CDN, keyed by an 11-character ID. This page is the map.
The ID is the key
Every watch, Short, embed, and youtu.be link collapses to eleven characters. Thumbnail URLs do not use the title. They use that ID. If you cannot extract it, you cannot fetch a still. The ID extractor and ID guide cover the URL shapes.
The CDN path is predictable
https://i.ytimg.com/vi/{id}/{filename} is the JPEG family. https://i.ytimg.com/vi_webp/{id}/{filename} is WebP when YouTube encoded one. img.youtube.com often aliases the same objects. Filenames are listed in sizes.
Placeholders lie if you only look at HTTP 200
Some missing maxres requests do not 404. They return a tiny 120×90 image with a broken-film graphic. A downloader that saves that file as “HD” is lying. ThumbUHD measures width and height and treats ≤120×90 as unavailable.
Titles come from oEmbed, not from the JPEG
YouTube’s public oEmbed endpoint (youtube.com/oembed) returns title, author, and a thumbnail URL for public videos. Unlisted or private videos typically refuse oEmbed. The stills may still exist; the title will not. We do not use a Data API key on this site.
What we will not fetch
Video streams. Audio. Captions. Storyboard sprite sheets (undocumented, brittle). Channel banners that require the Data API. If a still is not a public i.ytimg.com object we can measure, it is out of scope.
A request, in order
- Parse the paste for an 11-character video ID (watch, Shorts, embed, live, youtu.be, music, nocookie).
- Ask
https://www.youtube.com/oembed?url=…&format=jsonfor title and author when the video is public. - HEAD/GET each candidate on
i.ytimg.comand read real width, height, and bytes. - Hide 120×90 results on “large” filenames. Keep
default.jpg, which is supposed to be 120×90.
GET https://www.youtube.com/oembed?format=json&url=https://www.youtube.com/watch?v=VIDEO_ID
GET https://i.ytimg.com/vi/VIDEO_ID/maxresdefault.jpgFull path list: YouTube thumbnail URL format.
Why a downloader still needs to probe
You cannot know from the watch page alone whether maxres exists. Two videos on the same channel can differ: one has a custom 1280×720 still, the other lives on HQ. Guessing “always maxres” is how fake-UHD sites ship placeholders. The size checker is that probe with the numbers printed.