# YouTube thumbnail preview

https://flaghand.com/youtube-thumbnail-preview draws an image where YouTube shows a thumbnail: the home feed, search
results, Up next and the Shorts shelf on a computer, and the list and the Shorts shelf on a phone, at YouTube's own card
sizes, in light and dark. It checks the image against YouTube's rules: 3840 × 2160 recommended for a video (2160 × 3840
for a Short), at least 640 wide (a Short: 640 tall), 16:9 (a Short: 9:16), JPG or PNG, under 2 MB from the phone app
and 50 MB from a computer. Free, no sign-up. It runs in your browser: the image is never uploaded or stored.

## How to use it

1. Drop or choose the image, and type the video's title.
2. Switch between Computer and Phone, and Light and Dark. Keep faces and words clear of the length in the bottom right.

An agent can open it with the title in the link's hash, which never reaches a server:
https://flaghand.com/youtube-thumbnail-preview#title=Hello%20world

Every rule with its source: https://flaghand.com/docs/guides/youtube-thumbnail-size

## YouTube’s rules.

Size3840 × 2160 for a video, 2160 × 3840 for a Short. At least 640 wide for a video, 640 tall for a Short.

Shape16:9 for a video, 9:16 for a Short, 1:1 for a podcast playlist.

FileJPG or PNG. Under 2 MB from the phone app, 50 MB from a computer.

WhoA verified channel. A Short’s thumbnail is added in YouTube Studio on a computer.

Every rule with its source, and how a thumbnail shows on phones, computers and TVs: [YouTube thumbnail size](https://flaghand.com/docs/guides/youtube-thumbnail-size).

## Questions.

### What size is a YouTube thumbnail?

YouTube recommends 3840 × 2160 pixels for a video (16:9) and 2160 × 3840 for a Short (9:16). A video’s thumbnail must be at least 640 pixels wide, a Short’s at least 640 tall.

### How big can the file be?

Under 2 MB when you upload from the phone app, and under 50 MB from YouTube Studio on a computer. Use a JPG or PNG.

### What covers part of my thumbnail?

The video’s length, in the bottom right corner of every card: 20 pixels tall, 8 in from the edges on home and in search, 4 in Up next, as Flaghand measured them on YouTube’s pages.

### Who can add a custom thumbnail?

A channel that has verified its account. A Short’s custom thumbnail can be added only in YouTube Studio on a computer.

### Is my image uploaded?

No. It is opened and drawn in your browser. Nothing is sent anywhere or stored, and the page sets no cookies.

## Flaghand

Flaghand checks a YouTube draft's thumbnail (JPG or PNG, and a verified channel) before its owner approves it. "Schedule this with Flaghand" on the page takes the text (not the photos) on to sign-up, in the link's hash, which no server sees, and the first draft opens with it.

Flaghand is X and LinkedIn posting for AI agents: your agent drafts through Flaghand's API, CLI or MCP server, you
approve the exact preview with your passkey, and Flaghand posts exactly that.
Sign up at https://app.flaghand.com/start. In a chat app, add the connector at https://app.flaghand.com/mcp: its sign-in page creates the account. An agent with a shell can start the sign-up for its person over HTTP (https://flaghand.com/llms.txt, Quickstart).

For agents: https://flaghand.com/llms.txt (this site) and https://app.flaghand.com/llms.txt (the product's agent guide).