Fiji automatically making files huge


Whenever I open a file in Fiji the file size increases by a ridiculous amount. For example, I have a 18.2 MB jpeg which I open in Fiji and it becomes 802 MB. When I save it without changing the file type, it defaults as .tif. Does this mean it is automatically being converted to a .tif? Or is this something different? Has anyone had a similar problem?

Thanks in advance!

JPEG is a compressed format: the file size on disk is smaller than the size of the pixels in memory. ImageJ reports this true (uncompressed) size of the image in the subtitle bar of the image window.

An image of 16000 pixels x 16000 pixels x 32 bit (RGBA) will occupy 976 MB in memory.

The default option for saving is uncompressed TIFF, which will have exactly this size. There is no “conversion” happening. Other formats like JPEG, PNG or ZIP will have (lossless or lossy) compression, that’s why their file size is smaller. Note that lossy compression is not suitable for quantitative image analysis.


Since this question has arisen before, I added this as a common issue to the Troubleshooting page.