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“Serve images in next-gen formats”: what it means now
Lighthouse renamed this image audit. Here is what it measures, what it estimates, and how to check the image bytes on your own page.
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“Serve images in next-gen formats” used to be a red line in Lighthouse. Since Lighthouse 13, it is called “Serve images in modern formats” inside the Improve image delivery insight.
The name changed. The job did not. Lighthouse finds older image formats and estimates what could be removed by converting them.
Google says it directly: “This audit has moved into the Improve image delivery insight as of Lighthouse 13.”
What the Lighthouse warning measures
The audit checks BMP, JPEG, and PNG images on the page. It then models the possible reduction from WebP and AVIF.
Lighthouse collects each BMP, JPEG, and PNG image on the page, converts each to WebP, and estimates the AVIF file size, reporting the potential savings based on the conversion figures.
— developer.chrome.com, the audit documentation.
The important word is estimates. The bytes your page sent are measured. The possible WebP or AVIF weight is not. Lighthouse also leaves an image out when the potential reduction is below 8 KiB, so the list focuses on larger opportunities.
What “next-gen” formats are
In this audit, they are WebP and AVIF instead of JPEG or PNG. The comparison only means something at the same perceived quality.
We tested that part ourselves: 14 images from two real stores, 336 encodes, with perceived quality equalized at SSIMULACRA2 70.
| Format | Against JPEG, median | Won on |
|---|---|---|
| WebP — the free plan's format | 21.8% lighter | 11 of 14 images |
| AVIF — the Pro plan's format | 32.1% lighter | 14 of 14 |
| AVIF against WebP | about 21% lighter still | 16 real derivatives, same width and quality |
Those are file-weight measurements, not page-load times. We did not measure how long a site took to open. Hosting, scripts, caching, and the rest of the page still matter.
Browser support matters too. Google's audit notes that WebP, AVIF, and WebM do not work on iPhones below iOS 16. Delivery has to check what each browser accepts and keep the original as the fallback.
What to do in WordPress
Google's WordPress advice is to use a plugin or service that converts uploads automatically. Automatic is the useful part. Doing a catalogue by hand never ends.
Tools take different routes. Some rewrite files in the media library and keep backups. NimboCDN changes the image address at delivery time. The originals stay untouched, and deactivating the connector restores the old delivery at once. The ShortPixel and Imagify comparison shows how two library-based tools differ.
If a product category is the slow page, start with what images can and cannot fix in WooCommerce. For the whole site, rule out the other causes of a slow WordPress site.
Check the image bytes on your page
Paste a public page below. The checker weighs every image that travels and encodes the heaviest ones for real. Today's number is measured. The possible after is estimated.
That estimate has a median error of 6.2 percentage points for AVIF and 10.2 for WebP, measured on eight stores the model had not seen. The full method is on the website image size checker.
If the site uses WordPress, the report offers the connector. For a single file, the free compress image tool works without leaving your browser. Two minutes. No account. No credit card. The home page starts shipping lighter right away.
- Reading the page
- Weighing and converting the images
Today is measured: those bytes traveled. With NimboCDN is estimated, and the label stays beside the number.
Questions
Is “Serve images in modern formats” the same audit?
Yes. Lighthouse 13 moved the old “Serve images in next-gen formats” audit into Improve image delivery and renamed it. Both names refer to sending WebP or AVIF instead of older formats such as JPEG and PNG.
Is changing format the same as compressing an image?
No. A format is the encoding used for the image. Compression controls how that encoding spends bytes. The comparison above holds perceived quality constant so the weight difference comes from the format, not from making one image look worse.
Does every browser accept AVIF?
No. Support is narrower than WebP. Delivery should check the formats a browser accepts and fall back to the original when needed. NimboCDN does both, without changing the original file.
Will fixing this make the whole site fast?
We cannot claim that from these measurements. Lighter images remove bytes, and images are often the heaviest part of a page. We did not measure page-load time, which also depends on hosting, scripts, plugins, caching, and the rest of the page.