The core difference: lossless versus lossy compression
JPEG uses lossy compression. It discards image information that the human eye is unlikely to notice — particularly fine color detail in shadows, gradients, and textured areas. The file is smaller because some of the original data is permanently gone. You cannot recover the original pixel values from a JPEG; what you get is a very good approximation.
PNG uses lossless compression. It stores every pixel value exactly as it was recorded, then applies a mathematical algorithm to find and eliminate redundancy without discarding any information. Decompress a PNG and you get the exact original pixel data, bit for bit.
The size difference is not a bug in PNG. It is the cost of accuracy. When file size is the priority, JPEG wins. When exact pixel accuracy is required, PNG wins. Choosing between them is about what the image contains and what you need to do with it.
PNG or JPG: which to use, by content
Size follows content. Match the format to what the image actually is:
| Image content | Best format | Why |
|---|---|---|
| Photographs | JPG or WebP | 5-10x smaller with no visible loss |
| Screenshots, text, UI | PNG | Crisp edges; often smaller than JPG here |
| Logos, icons, transparency | PNG or WebP | Alpha transparency; JPG fills it white |
| Flat-color graphics, charts | PNG | Lossless, and it compresses repetition well |
| Files you re-edit often | PNG | No quality loss on repeated saves |
Why lossless compression still produces large files for photos
PNG uses DEFLATE compression — the same algorithm inside ZIP files. DEFLATE finds repeated patterns in data and represents them compactly. It works very well when patterns exist: a large area of solid white in a diagram, a repeating texture in a logo, or runs of identical pixels in a screenshot.
Photographs have almost no such patterns. Every pixel in a photo of a forest, a face, or a city street is subtly different from its neighbors. Pixel values vary continuously in all directions. DEFLATE finds very little to compress and the output file stays nearly as large as the raw pixel data.
JPEG works precisely in the opposite conditions. It applies a transform that separates the image into frequency components and aggressively discards high-frequency color detail — exactly the kind of complex variation that photographs are full of. The result is dramatic size reduction with minimal visible quality loss on photographic content.
Screenshots and diagrams: where PNG wins on size
The comparison reverses for non-photographic content. A screenshot of a web page has large areas of flat color, crisp text edges, and hard-edged UI elements. DEFLATE compresses this kind of content very efficiently because there is abundant repetition.
Saving that same screenshot as JPEG introduces visible artifacts. JPEG's compression algorithm blurs sharp edges and introduces blocky noise around high-contrast transitions — exactly where text and interface elements are. To make the JPEG look clean requires a quality setting so high that the file ends up larger than the PNG.
The rule of thumb: PNG for screenshots, diagrams, logos, and text. JPEG for photographs.
Transparency: what PNG can do that JPEG cannot
PNG supports an alpha channel that stores per-pixel transparency. This is why logos and icons are almost always PNG: the transparent areas let the background of any page show through. JPEG has no transparency support at all. Converting a PNG with transparency to JPEG fills the transparent areas with a solid color — white by default — which is a functional change, not just a quality tradeoff.
If an image needs a transparent background, PNG is the correct format regardless of size. WebP is an alternative that supports transparency and often produces smaller files than PNG for images with varied content.
The re-save problem with JPEG
JPEG loses information every time the file is re-encoded. Open a JPEG, edit it, and save it again as JPEG — the compression artifacts from the first save are compressed again on top of the first round of compression. Do this several times and visible degradation appears: blocky patches, color banding, blurred edges. PNG does not have this problem. A PNG can be opened and saved any number of times with no change to the pixel data.
The standard workflow for serious editing: keep working files as PNG or in the original raw format, then export to JPEG only as the final step for web delivery. This preserves quality through every edit and only incurs the lossy compression once.
Typical size differences in practice
- Smartphone photo as PNG: 15–25 MB. Same photo as JPEG at high quality: 2–5 MB. At standard web quality: under 1 MB.
- Screenshot as PNG: 200–500 KB. Same screenshot as JPEG at comparable visual quality: often larger than the PNG due to artifacts requiring high quality settings.
- Logo with transparency as PNG: 30–100 KB. As JPEG with white background fill: similar size but loss of transparency feature.
When to convert PNG to JPEG
If you have PNG photos — perhaps exported from a camera application or editing software — and they do not require transparency, converting to JPEG for web publishing is almost always the right decision. A quality setting of 80–85 typically produces a file that is indistinguishable from the original PNG at normal viewing sizes but three to five times smaller.
PhotoTools converts PNG to JPG in your browser without uploading anything to a server. Adjust the quality slider, compare the preview, and download the result. For most web photographs, the JPEG version will be significantly smaller with no visible quality loss at the sizes the image will actually be displayed.