16 min read

Bit Depth

How many bits store each pixel's colour — and why 8 bits per channel gives 16.7 million colours.

Bits per Pixel

You met this idea in the Number Systems course: n bits give 2ⁿ different values .

Uncompressed image size in bytes = width \times height \times bytes per pixel . For an A3 poster at 300 ppi in 8-bit RGB (3 bytes per pixel): 3508 \times 4961 \times 3 \approx 52 million bytes \approx 52 MB . In CMYK (4 channels) it is about 70 MB, and at 16 bits per channel it doubles. Compression (next section) makes saved files much smaller.

Practice

How many colours can 24-bit colour show?

Bits per Pixel

You met this idea in the Number Systems course: n bits give 2ⁿ different values .

Bit Depth and File Size

Uncompressed image size in bytes = width \times height \times bytes per pixel . For an A3 poster at 300 ppi in 8-bit RGB (3 bytes per pixel): 3508 \times 4961 \times 3 \approx 52 million bytes \approx 52 MB . In CMYK (4 channels) it is about 70 MB, and at 16 bits per channel it doubles. Compression (next section) makes saved files much smaller.

Practice

How many colours can 24-bit colour show?