Digital micrograph software can be used for this purpose. Many toturials can be found on YouTube, google it please.

Dear Abou,

since you are observing something which seems to be a poly-crystalline material you should measure the ring radii and calculate the reciprocal number of these values. I feel a bit strange about the fact that the scale-bar indicates 20 nm while we are looking at the reciprocal space, so it should be something like: n 1/nm.

You can perform the measurement in many ways, if you want to use some software, you can go for DM as Mohammed suggests or also ImageJ. You can easily find both on the internet.

Good work and good luck.

Answers from both Vigliaturo and Obeid is recomended. Their suggestions are completely help to get solutions to your question