Image Optimization Checker
Check images for missing dimensions, lazy-loading mistakes, legacy formats and missing srcset.
- 100% free
- No sign-up
What is Image Optimization Checker?
Images are usually the heaviest thing on a page and the easiest to get wrong in ways that show up directly in Core Web Vitals.
Two mistakes dominate. The first is omitting width and height, which leaves the browser unable to reserve space, so everything below the image jumps when it loads — that jump is measured as Cumulative Layout Shift. The second is subtler and more common than it should be: lazy-loading the first image. loading="lazy" is good practice below the fold and actively harmful at the top, because the hero image is usually the Largest Contentful Paint element, and deferring it defers the exact thing the metric measures. This tool checks both, plus format and srcset.
How to use Image Optimization Checker
- 1
Fetch or paste
Enter a URL, or paste the HTML.
- 2
Add width and height everywhere
Even with CSS sizing the attributes let the browser compute an aspect ratio and reserve the space.
- 3
Lazy-load below the fold only
Never the first image. Use loading="eager" on it, or leave the attribute off.
- 4
Move to WebP or AVIF
Typically a third off the weight at the same visual quality, with a <picture> fallback for anything old.
Everything this tool
does for you
- Missing width/height detection, tied to CLS
- Lazy-loaded-hero detection, tied to LCP
- Legacy format detection
- srcset presence check
- Reads a live URL or pasted HTML
- Free, no signup, no daily limit
Further reading
Related free tools
Frequently asked questions
Beyond this tool
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- Finds images with no width and height — the main cause of CLS
- Catches a lazy-loaded first image, which delays LCP
- Flags JPEG and PNG where WebP or AVIF would do
- Checks for responsive srcset
- Free, no account, no daily limit