image sem analysis provide you to make measurment , it can measure from apparent distance and calculate the actual size. if you have'nt this software you can use scale and magnéfication to mesure the real size of particle

Is there an equation for this calculation?

there is a free software called Image j, you can download it from this link

http://rsbweb.nih.gov/ij/download.html

and you can use it to analyze your image:

1- drag your image in image j

2- draw a straight line on your scale bar in a way that they both have the same length

3- go to Analyze >set scale.. and set the known distance to your known scale bar and don't forget to change the unit of length

4- now you can measure any dimension of your image by drawing a line at the distance you want to analyze and then press on Analyze> measure....

Wish i could help and let me know if you couldn't do any of these, wish you the best with your research

Thank you for your answer, I download the program but can not use it. Is there another method for this calculation , please?

i don't quite understand why couldn't you use the program, is there an issue with installing the program??? or you installed it and you cann't use it?

I install it but can not use it