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FAQ: Why does the file size of my raster image change when I export or create a new image?

Question

Why does the file size of my raster image change when I export or create a new image?

Answer

When creating new images or exporting existing images the bit-depth may change. This results in a change in the amount of disk space the new image requires. This happens for a number of reasons, discussed in articles in the Related Information, below.

The reason is due to the amount of bits required to store each individual pixel (cell) in a raster image. When working with 8 bit images, 1 byte (8 bits) is required to store each pixel in the image. When working with 16 bit images, 2 bytes are required, and with 32 bit images, 4 bytes are required and so on.

An easy way to determine the approximate size of the image is to use the formula below:

Rows x Columns x number of bands x pixel depth (8 bits = 1 byte)

For example:

8-bit image:
100 rows x 100 columns x 3 bands x 1 = an output raster that is approximately 30,000 bytes in size.

32-bit image:
1000 rows x 1000 columns x 1 band x 4 = an output raster that is approximately 4,000,000 bytes.

Related Information