Can people read my thumbnail on a phone? Free readability checker

Open your thumbnail and see it the way viewers meet it: 390 px wide in the phone feed and about 220 px in the suggested sidebar on a computer, the sizes I measured on YouTube on October 19, 2026. You also get a grayscale and a blurred squint test version, the corner the duration badge covers, a dark and light mode comparison, and plain measurements: brightness, contrast, how busy the image is and how tall your text is at phone size. Nothing is uploaded.

Updated October 19, 2026 · Built by Kevin Tabares, long-form YouTube editor at Umbrella Creators. Thumbnail specs from YouTube Help, read October 19, 2026.

Choose 1 to 3 thumbnails or drop them here
JPG or PNG work best. Choose several at once to compare them.

Your images stay on your device. They are opened and measured in this tab and never sent anywhere.

How to use the readability checker

Choose your thumbnail, or two or three versions of it. The page opens with an example I drew in the page so you can see what the results look like.
Look at the 390 px and 220 px views first. If you cannot tell what the thumbnail is about in the sidebar view, a viewer scrolling past will not either.
Check the grayscale and the squint views. A thumbnail that only works in color, or that turns into one flat tone when blurred, loses its main shape when it is small.
Measure the text. Type the words, press Draw a box around the text and drag over the text in the 390 px view. You get the height of each line on a phone and in the sidebar.
Read the list. Each line says Looks fine, Look at it or Could not check. Then see the thumbnail next to a title with the thumbnail preview tool.

The sizes YouTube shows, measured October 19, 2026

I could not find these sizes in any YouTube document, and the numbers most guides repeat (like 168 px for the sidebar) did not match what I saw. So I measured them myself: Chrome, logged out, youtube.com at common computer window widths and m.youtube.com at common phone widths, reading the size of each thumbnail and duration badge on the page.

Where	Window or phone width	Thumbnail width
Phone feed and phone search	360, 390 and 430 px phones	Full screen width: 360, 390, 430 px
Search results on a computer	1280, 1440 and 1920 px windows	500 px
Home feed on a computer	1440, 1536 and 1920 px windows	373, 405 and 396 px
Suggested sidebar on a computer	1280, 1366, 1440, 1536 and 1920 px windows	220, 235, 248, 264 and 330 px
Duration badge (bottom right)	Everywhere I measured	20 px tall, 12 px text, about 32 px wide for 0:16, 39 px for 43:48 and 48 px for 1:04:51, 8 px from the corner (4 px in the sidebar at 1280 to 1440 px windows)

I measured the mobile website (m.youtube.com); the YouTube phone app cannot be measured this way. The badge keeps the same pixel size everywhere, so it covers more of a small thumbnail: about 10% of the width and 9% of the height on a 390 px phone view, and about 18% by 16% in a 220 px sidebar. The badge zone the tool marks is approximate. An earlier run the same morning gave a slightly wider sidebar (239 to 359 px) and the computer home feed only filled in after a video was watched, so treat the computer numbers as ranges: YouTube changes these layouts.

What the measurements mean

Brightness is the average gray value. Only the extremes get a look at it: under 15% (very dark) or over 85% (very bright).
Contrast (RMS) is how spread out the gray values are, from 0 to 0.5. Under 0.12, light and dark areas are close together and the image leans on color alone.
Left after blur is the same contrast measured on the squint version. Under 0.08, the blurred image is close to one flat tone.
Fine detail is the share of pixels on a sharp edge at phone size. Over 45% the image is mostly texture, which is common in gameplay frames.
Dark and light mode compare the outer edge of the thumbnail with the page behind it (rgb 15, 15, 15 and white, read from YouTube's page styles). Under 1.3 to 1 the edge melts into the page.
Duration badge compares the detail in the bottom right corner with the rest of the image. A lot more detail there usually means a word, a face or an object is hiding under the time.

These cut-offs are my own rules of thumb for spotting problems, not YouTube rules, and none of them predicts clicks.

How I check thumbnails as an editor

Before a long-form Roblox or Minecraft video goes out, I look at its thumbnail the way a viewer meets it: small, on a phone, next to other videos. I shrink it to sidebar size, look at it in grayscale and squint at it. Gameplay frames are full of tiny blocks, particles and UI, which is why fine detail is its own measurement here: at 220 px that detail turns into noise and the character and the words have to carry the image. The other thing I always check is the bottom right corner, because the duration badge sits on top of whatever is there. This tool does those checks in one place. To pull a clean frame from the video first, use the frame grabber.

FAQ

How big is a YouTube thumbnail on a phone?

When I measured YouTube on October 19, 2026, the phone feed and phone search showed thumbnails at the full width of the screen: 360, 390 or 430 CSS pixels on common phones. On a computer, search showed them at 500 px wide, the home feed at about 373 to 405 px and the suggested sidebar at about 220 to 330 px, depending on the window width.

How do I know if my thumbnail text is readable on a phone?

Type the text in the tool and draw a box around it. The tool turns the height of each line into phone and sidebar pixels. As a yardstick, the time on YouTube's duration badge is 12 px text, with digits about 8 to 9 px tall. If your words come out smaller than that in the sidebar, they are smaller than the smallest text YouTube itself puts on a thumbnail.

What is the squint test for thumbnails?

Blurring a thumbnail, or squinting at it, removes the small detail and leaves only the big shapes and the light and dark areas. If the blurred version turns into one flat tone, nothing will stand out when the thumbnail is small either. The tool blurs every image by the same amount relative to its width and measures how much contrast is left.

Does this tool tell me my click-through rate?

No. It checks things that can be measured in the image itself: shape, size, brightness, contrast, fine detail, the duration badge corner and text size. Click-through rate also depends on the title, the topic and the audience, so the verdicts only say “looks fine” or “look at it”, never that a thumbnail will get more clicks.

What size should a YouTube thumbnail be?

YouTube Help asks for a 16:9 image at least 640 pixels wide and recommends 3840 x 2160 pixels. Upload a JPG or PNG under 2 MB from the phone app, or under 50 MB from a computer.

Is my thumbnail uploaded anywhere?

No. The images are opened and measured inside your browser on your own device. Nothing is sent to a server and nothing is saved.

Sources

YouTube Help, Add video thumbnails: 16:9, at least 640 pixels wide, 3840 x 2160 recommended, JPG or PNG, 2 MB from a phone and 50 MB from a computer. Read October 19, 2026.
Display sizes, the duration badge and the dark and light page colors: my own measurement of youtube.com and m.youtube.com in Chrome, logged out, on October 19, 2026 (table above).
Edge against the page: the contrast ratio formula from WCAG 2.1, applied to the average color of the thumbnail's outer edge.
Fine detail: a Sobel edge filter on a 390 px wide gray copy. Squint test: a blur of about 1.2% of the width (three box blur passes).

Who made this

I'm Kevin Tabares. I've edited long-form YouTube videos since 2015 (1,277 videos and 152.2M+ views tracked on YT Jobs), mostly Roblox and Minecraft gaming, plus narrated documentaries. Checking that a thumbnail still reads when it is small is part of almost every delivery, so I turned my routine into this tool. If you want an editor for your channel, get in touch.

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