Convert pixels to micron

I started using imageJ a few months ago, so I am not an expert on it yet. My doubt is: I have a quantity of images which express cells. I used the default settings on imageJ to calculate the surface area of these cells and then I figured out that I should have measured these in squared microns instead squared pixels.
I must transform the surfaces, perimeter and mean diameter of Feret of these cells of pixels into micrometers in the table excel.
Attached is the image processed by imageJ (1024x768 pixels) ready with magnification 10 and an image for the micrometer slide ready at the same magnification.
if at all possible, can you help me convert these parameters in squared micron instead squared pixels?
if no, how can I calibrate imageJ before processing this image?
Thank you

Please study the ImageJ User Guide and especially section 30.8 dealing with “Set Scale”:

If you want to do the conversion post hoc in Excel you simply need to know the µm/pixel relation that you can get from the imaged scale you’ve shown before.



Hi i haven’t understand how to do it? can you explain it for the below image?

The User Guide tells us:

“Before using this command [Set Scale…], use the straight line selection tool to make a line selection that corresponds to a known distance. Then, bring up the Set Scale…↑ dialog, enter the Known Distance and unit of measurement, then click ‘OK’. The Distance in Pixels field will be automatically filled in based on the length of the line selection.”

After setting the scale as described above, make a rectangular selection according to the arrow-lines in the image.

The dimensions of this selection are displayed in the status area of the ImageJ main window:
width ≈ 0.53µm
height ≈ 2.85µm

Thanks a ton for help, is their any method to stitch different part of whole image to form one complete picture in this application?. Sorry but i am very new user at imageJ. For example i have attached image to understand very clearly. In this image i have stiched image but this is not perfect. I want to do perfect stitching with auto option, so software can do it by itself. Thanks in advance…!!