What does bit depth actually mean?
In PNG, bit depth is the number of bits used for each channel, not for the whole pixel. A channel is one component of a colour — for example the red, green, or blue part of an RGB pixel. PNG supports bit depths of 1, 2, 4, 8, and 16 bits per channel, and the depths available depend on the colour type. More bits per channel means more possible values per channel, which gives finer colour precision and smoother gradients, but also a larger file.
What are the five PNG colour types?
PNG defines five colour types. Each one decides which channels a pixel stores and whether it can carry transparency:
| Colour type | Channels | Supports alpha |
|---|---|---|
| Grayscale | 1 (grey) | No |
| Truecolour (RGB) | 3 (red, green, blue) | No |
| Indexed-colour (palette) | 1 (palette index) | Via palette |
| Grayscale with alpha | 2 (grey, alpha) | Yes |
| Truecolour with alpha (RGBA) | 4 (red, green, blue, alpha) | Yes |
Indexed-colour images store a palette of up to 256 entries at 8-bit, which is why they are limited to 256 distinct colours. Truecolour and truecolour-with-alpha are the workhorses for the web.
What is the difference between 8-bit and 16-bit?
At 8 bits per channel, a truecolour PNG is 24-bit RGB — three channels of 8 bits — giving roughly 16.7 million colours. That is plenty for almost every screen image. At 16 bits per channel, the same truecolour image becomes 48-bit colour, which holds far more precision for very smooth gradients and heavy editing. The trade-off is size: 16-bit files store twice as much data per channel and are noticeably larger, so they are used for editing masters and for scientific and medical imaging rather than everyday web graphics.
How does this relate to PNG-8, PNG-24, and PNG-32?
Those familiar labels are just shorthand for a colour type at 8 bits per channel. PNG-8 is an 8-bit indexed-colour image with a palette; PNG-24 is 8-bit truecolour (three 8-bit channels = 24 bits); and PNG-32 is 8-bit truecolour with alpha (four 8-bit channels = 32 bits). For a closer look at the first two, see PNG-8 vs PNG-24.
How do you pick a bit depth and colour type?
For most web images, choose 8-bit truecolour, or truecolour-with-alpha when you need transparency — this is what the majority of web PNGs use. Drop to 8-bit indexed-colour when an image has few colours and you want the smallest file. Reserve 16-bit for editing masters and high-precision work where extra colour data matters. If you are new to the format, start with what is a PNG.
Frequently asked questions
- Is bit depth the same as colour type?
- No. The colour type decides which channels a pixel has (grey, RGB, palette, or with alpha), while bit depth sets how many bits each channel uses. A PNG always has both.
- Do I need 16-bit PNGs for the web?
- Almost never. Browsers display 8 bits per channel, so 16-bit files just add weight without visible benefit. Keep 16-bit for editing masters and high-precision imaging.
- Why does my PNG only have 256 colours?
- It is an indexed-colour PNG. Indexed images store a palette of up to 256 entries at 8-bit, which keeps files small but limits the total number of distinct colours.
- Does higher bit depth reduce banding in gradients?
- Yes. More bits per channel give finer steps between tones, so smooth gradients show less banding — at the cost of a larger file.