Image Size Checker — Audit Image File Sizes & Formats
Audit every image on a page for file size, dimensions, format, and alt text. Get optimization recommendations to boost PageSpeed. Free image SEO checker.
What is an Image Size Checker?
The Image Size Checker fetches any URL and analyses every image on the page: file size in kilobytes, rendered dimensions versus intrinsic dimensions (to catch images loaded at a higher resolution than they are actually displayed), file format (JPEG, PNG, WebP, AVIF), and alt text presence. Images that are oversized, in an inefficient format, or missing key attributes are flagged with specific recommendations.
When Should You Use Image Size Checker?
Use this as the first step in any image performance optimisation project. Oversized images are one of the most common causes of slow LCP and poor PageSpeed scores. A 2MB PNG served where a 100KB WebP would be visually identical is a straightforward fix that can improve PageSpeed scores by 10–20 points on its own. Run this on your highest-traffic pages and your most image-heavy pages first to find the largest gains quickly.
How to Read Image Size Checker Results
Look first for images over 200KB in JPEG or PNG format — these are the highest-impact conversion candidates. An image loaded at 2000×1500px but displayed at 400×300px has an intrinsic dimension problem: the browser is downloading 25 times more pixels than it needs to render. The tool calculates potential savings for each image so you can prioritise where to invest optimisation time.
What Should You Know Before Using Image Size Checker?
Convert large JPEG and PNG files to WebP or AVIF — WebP typically reduces file size by 25–35% at equivalent visual quality, and AVIF reduces it further still. Use the picture element or srcset to serve appropriately sized images for different viewport widths rather than scaling a single large image in CSS. For WordPress sites, an image compression plugin that converts on upload removes the manual step. After optimising, re-run the Core Web Vitals Checker to confirm LCP has improved.
Frequently Asked Questions
What image format is best for SEO and performance?
WebP is the recommended format for most web images — it delivers 25–35% smaller file sizes than JPEG at equivalent visual quality and is supported by all modern browsers. AVIF offers even better compression (30–50% smaller than JPEG) but has slightly lower browser support. Use JPEG as a fallback for browsers that do not support next-gen formats.
What image file size should I aim for on a website?
As a general guideline, images above the fold should be under 100KB; images below the fold should be under 200KB. Hero images and full-width backgrounds should ideally be under 150KB after compression. Use the Image Size Checker to identify which images on your page are significantly over these thresholds and prioritise converting those first.
What is the difference between image dimensions and file size?
Image dimensions (e.g., 1200×800px) describe the pixel count — how many pixels wide and tall the image is. File size (e.g., 450KB) describes how much disk space the image occupies. An image can have high dimensions but small file size if compressed efficiently, or low dimensions but large file size if saved as uncompressed PNG. Both matter for performance.
How do oversized images affect page load speed?
Oversized images increase the amount of data the browser must download before the page can fully render. A 3MB hero image served over a mobile connection adds several seconds to page load time. The Image Size Checker identifies images whose intrinsic dimensions far exceed their rendered size — a common source of unnecessary download weight.
What is srcset and how does it improve image performance?
The srcset attribute lets you provide multiple versions of an image at different resolutions, and the browser automatically selects the most appropriate one based on the device's screen size and pixel density. This ensures mobile users download a small image while desktop users with high-resolution screens get a sharper version, reducing wasted bandwidth across all devices.
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