Why preview a thumbnail in context
Thumbnails are designed large and seen small, surrounded by other videos. A layout preview answers a simple question before you publish: is the thumbnail still understandable at the size and in the context where people actually see it?
Dark mode matters too. Many viewers use YouTube's dark theme, where thumbnails with dark edges can blend into the background, and light thumbnails stand out more.
How it works
Your image is displayed inside four layouts modelled on common YouTube placements, using typical CSS widths: a home feed grid (about 360 px per card), a search result row, the suggested-video sidebar (about 168 px wide) and two phone layouts (a full-width feed and a compact list at about 160 px).
You can edit the title, channel name, views and duration to see how your real metadata wraps next to the image, and zoom between 75% and 150%. The image is shown from memory in your browser and is never uploaded.
Example
A thumbnail with a small face on the right and a three-word caption might look clear in the home feed at roughly 350 px, but in the suggested sidebar at 168 px the face may be a few pixels tall. Switching to “Suggested” shows this immediately. The Readability Tester then measures how much detail survives at each size.
Limitations
- Layouts are simplified mock-ups built from typical proportions, not pixel-perfect copies of YouTube.
- YouTube's layouts change often and differ across devices, apps and window sizes.
- A preview can't tell you whether a thumbnail will perform well — only how it looks at a given size.