How PNG and WebP Differ
PNG (Portable Network Graphics) is a lossless format. Every pixel is stored exactly as it was captured, with support for full 8‑bit alpha transparency – meaning 256 levels of opacity per pixel. This makes PNG the go‑to format for logos, screenshots, line art, and any image where pixel‑perfect reproduction or a transparent background is required. The trade‑off is file size: photographs stored as PNG are considerably larger than the same image in a lossy format like JPEG, because PNG’s compression algorithm (deflate) is not optimised for smooth colour gradients.
WebP is a modern image format developed by Google that supports both lossy and lossless compression, alpha transparency, and animation. The conversion tool on this page uses lossy WebP compression, controlled by a quality slider. In lossy mode, WebP typically produces files 25–35% smaller than a JPEG of comparable visual quality. Because the input is lossless PNG, converting to lossy WebP introduces irreversible quality loss – unless you set quality to 100%, which is still technically lossy (it discards data, though the difference may be imperceptible). The tool does not offer lossless WebP; this is a deliberate design choice aimed at maximum file‑size reduction for web use.
Another key difference: WebP’s transparency support is equally capable (8‑bit alpha channel) but the compression handles transparent regions more efficiently. Where a PNG might store every transparent pixel explicitly, WebP can encode large transparent areas with little data. This often leads to dramatic savings for icons or UI elements on transparent backgrounds.
The Quality Slider and Its Effect
The slider ranges from 40% to 100% and starts at 92%. This value directly controls the compression aggressiveness of the WebP encoder:
- 40% – maximum compression, smallest file size, noticeable artefacts (blocking, blurring, colour banding). Best for photographs converted from PNG where file size is critical and some quality loss is acceptable.
- 70% – a common balance. Most viewers won’t see obvious defects in photographs, and simple graphics retain sharp edges.
- 100% – the highest quality setting. The encoder still applies lossy compression but with very low quantisation. For many images, the difference from the original PNG is visually identical, but the file size will be larger than at lower settings. At 100%, WebP can sometimes be larger than the source PNG for simple images (solid colours, text) – this is expected and shown as “Larger” in the output.
Important: even at 100%, the conversion is not lossless. A pixel‑perfect comparison between the PNG and the 100% quality WebP will reveal differences, especially in high‑frequency regions like sharp text edges. If you must preserve exact pixel data, keep the PNG.
What Happens During Conversion
All processing occurs inside your browser. No images are uploaded to any server – this is a fully client‑side operation. After you select or drag up to 20 PNG files, each image is decoded, then re‑encoded as WebP with the chosen quality setting.
For every converted file, the tool displays:
- Output filename (usually the original name with
.webp) - Dimensions (unchanged from input)
- Original size (file size of the PNG)
- Converted size (file size of the WebP)
- A Saved label if the WebP is smaller, or Larger if it is bigger.
Below the list of individual results, a summary line shows Total: ‹original total› -> ‹converted total›. When you convert two or more images, a Download all (.zip) button appears, bundling every WebP into a single archive created in your browser. You can also download each image individually.
If your browser does not support writing WebP files (very old browsers or specialized environments), the tool automatically falls back to converting to PNG and labels the result: “Your browser doesn’t support saving this format — used PNG instead.” This ensures you still get a usable file, albeit one that is larger and not WebP.
Edge Cases and Error Handling
The tool is designed to catch common problems cleanly:
- Unsupported file type – if you drop a file that isn’t a PNG (this page converts PNG only), the message appears: “This file type isn’t supported.”
- Corrupted or non‑image files – “This file could not be opened as an image. It may be corrupted, empty, or not actually an image.”
- Conversion failure – the generic fallback is “Conversion failed. Try a different image or format.”
- Batch limit exceeded – the tool converts up to 20 images at a time; select more and the message appears: “Choose up to ‹max› images at a time.”
- No images selected – with an empty queue the convert button stays disabled, so an empty batch cannot be started.
- Partial failures – when some files cannot be processed, the summary shows “‹count› file(s) could not be processed.”
All these messages are client‑side and appear in the interface, not as console logs.
When to Use This Tool – and When to Keep PNG
The primary audience is anyone who currently serves PNG images on the web and wants to reduce page weight without losing transparency or visual quality. Typical scenarios:
- A web developer converting a set of PNG icons or logos to WebP for a production site. The quality slider lets them test trade‑offs between file size and sharpness.
- A graphic designer preparing a batch of screenshots for a website mockup that must load quickly.
- An archivist with a collection of lossless PNG scans testing whether lossy WebP can save disk space without unacceptable visual degradation.
- Anyone who needs to reduce image file sizes before upload or storage, and values client‑side processing for privacy – sensitive PNG files never leave the device.
You should not convert to WebP when:
- The image must be pixel‑identical to the original (e.g., medical imaging, archival preservation, or legal evidence). Lossy WebP destroys that guarantee.
- You need to support very old software that cannot open WebP (e.g., some email clients, legacy desktop applications, or embedded systems). In that case, serve PNG with fallback or convert only for modern contexts.
- The WebP output is larger than the PNG. This happens frequently with simple graphics, text, or diagrams – the “Larger” label will appear. At that point, you are better off keeping the PNG.
Frequently Asked Questions
Q: Why is my WebP file larger than the original PNG?
Lossy compression works best on complex, photographic content. PNG is already efficient for simple shapes, flat colours, and text. When you convert such an image to lossy WebP, the compression overhead can exceed the data savings, producing a larger file. The tool labels these cases as “Larger” so you can decide.
Q: Does 100% quality on the slider mean lossless?
No. The slider controls lossy WebP encoding; even at 100%, the encoder discards some information. The visual difference is often imperceptible, but a pixel‑wise comparison will show deviations. For truly lossless output you would need WebP lossless mode, which this tool does not provide because its focus is size reduction.
Q: Can I convert multiple PNGs at once?
Yes – the tool accepts up to 20 images in a single batch. When you convert two or more, you can download a single ZIP containing all WebP files.
Q: Is my data safe? (privacy)
Completely. All conversion happens in your browser using JavaScript. The tool never uploads your images to any server. This makes it suitable for private or sensitive content.
Q: What do I do if my browser can’t save the WebP format?
The tool automatically switches to saving as PNG and displays a message: “Your browser doesn’t support saving this format — used PNG instead.” You will still receive a usable image, though it will not benefit from WebP compression.
Q: Why does the conversion fail for some images?
Possible causes: the file is corrupted, it is not actually a PNG (e.g., a renamed file), or the WebP encoder encounters an unsupported feature. The tool reports the specific error message for each failed file.