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PNG: lossless images with transparency

PNG is a lossless format: it stores every pixel exactly, and it supports smooth transparency. That makes it right for logos, icons, screenshots and anything with sharp edges — and wrong for photographs, which come out several times larger than the same picture as a JPEG.

PNG at a glance

File extensions
.png
MIME type
image/png
Compression
Lossless. Pixels are predicted from their neighbours and the remainder is zipped.
Transparency
Full alpha channel (256 levels at 8 bits), so soft edges and shadows stay smooth.
Animation
Only as APNG, an extension that current browsers play but many editors ignore.
Colour
Palette (up to 256 colours), greyscale or full colour, at up to 16 bits per channel.
Browser support
Every browser, operating system and image app.
In ImageDoctor
Opens, resizes, crops and converts PNGs, keeping transparency. PNG has no quality setting, so the compressor shrinks it by resizing or converting to WebP. Metadata is removed without re-encoding.

Lossless means exactly the same pixels

Save a PNG a hundred times and the hundredth copy is identical to the first. Compression works by predicting each pixel from the ones above and to its left, then zipping the (usually small) differences. That works brilliantly on flat colour and repeating structure — a screenshot, a diagram, a logo — because the predictions are nearly always right.

It works poorly on photographs. A photo is millions of subtly different colours with sensor noise on top, so the predictions keep missing and there is little to zip. A PNG of a phone photo is routinely several times the size of a good-quality JPEG — often five times or more — with no visible benefit.

Transparency that looks right

Each PNG pixel can carry an alpha value from fully transparent to fully opaque, in 256 steps (more in 16-bit files). That is what lets a logo's anti-aliased edge or a soft drop shadow blend into any background. GIF can only switch a pixel on or off, which is why GIF cut-outs have jagged, haloed edges and PNG ones do not.

PNG-8, PNG-24 and PNG-32

These are informal names for PNG's colour modes. PNG-8 uses a palette of up to 256 colours and is very small for simple graphics. PNG-24 is full colour; PNG-32 is full colour plus alpha. Browser-based tools, ImageDoctor included, write full-colour PNGs — so a carefully palette-optimised PNG can come out larger after any edit. That is not a fault in the edit; it is the palette optimisation being undone.

Why "compressing" a PNG does little

PNG has no quality dial, because there is nothing it is allowed to throw away. To make a PNG meaningfully smaller you have three options:

  1. Make it smaller in pixels. Size scales with pixel count; halving the width and height quarters the data. The Image Resizer does this.
  2. Convert it to WebP. WebP keeps the transparency and is usually much smaller — often around half the size for screenshots. See PNG to WebP.
  3. Reduce it to a palette with a dedicated PNG optimiser, if the image genuinely has few colours.

When to use PNG

  • Logos, icons and graphics with transparency, especially where the file must work in any software.
  • Screenshots and images of text or interfaces, where JPEG artefacts would blur the edges.
  • Working copies you will keep editing, so nothing degrades between saves.
  • Pixel art and diagrams, where every pixel is deliberate.

When not to

  • Photographs. Use JPEG to share them, or WebP/AVIF on a website.
  • Web graphics where size matters. WebP does lossless and transparency too, usually in fewer bytes. For flat logos and icons, SVG is often smaller still and stays sharp at any size.
  • Print masters from a camera. A 16-bit TIFF or the camera's raw file keeps more editing headroom.

Convert PNG files

Each converter runs in your browser with the output format already chosen. Add one file or a whole folder.

Common questions

Yes. PNG stores every pixel exactly, so saving it again never lowers the quality.

Usually because it is a photograph. Lossless compression cannot shrink photographic detail much, so a PNG photo is often several times the size of a JPEG. Convert photos to JPEG or WebP, or reduce the pixel dimensions.

The original was probably palette-optimised (PNG-8). Browsers and most editors save full-colour PNGs, which undoes that optimisation. Convert to WebP if you need a smaller file with transparency.

Only as APNG. Current browsers play animated PNGs, but many image editors and apps show just the first frame.

Tools for PNG files

Image ConverterConvert between JPG, PNG, WebP and AVIF with control over quality, transparency and background colour.Image ResizerSet exact dimensions, scale by percentage or fit inside a bounding box with contain, cover and fill modes.Image CompressorShrink JPG, PNG and WebP files by quality or to an exact target size, with a side-by-side quality check.Favicon GeneratorGenerate favicon.ico plus every PNG size a modern site needs, with the HTML snippet and manifest.

Keep reading

All image formats