Dear saurav,

there is no doubt in the concept mentioned by the Gert Nolze but now a days software calculate all these things and give overall results in the form of scale bar unit (1/nm) of SAED pattern which is the reciprocal equivalent of distance. In your case, as you have uploaded the SAED image, if you see the scale bar mentioned in your image has the unit 1/nm. Measure the radius of the ring which has the unit 1/nm and reciprocal of radius  of SAED ring is d spacing of that ring.

dhkl can be derived using Braggs law, similar to Debye-Scherrer images. You need to know the wavelength of the electrons and the distance between sample and screen (so-called) camera length.

First conclusions: your materials is nanocrystalline (forms rings) but you also have some bigger grains in a suitable orientation which form the white spots.

Dear saurav,

there is no doubt in the concept mentioned by the Gert Nolze but now a days software calculate all these things and give overall results in the form of scale bar unit (1/nm) of SAED pattern which is the reciprocal equivalent of distance. In your case, as you have uploaded the SAED image, if you see the scale bar mentioned in your image has the unit 1/nm. Measure the radius of the ring which has the unit 1/nm and reciprocal of radius  of SAED ring is d spacing of that ring.

Dear sourav, please use software DigitalMicrograph to caculate d. Gupta's suggestion is useful.

For easy and effective SAED calculations you can explore this link,

http://www.instanano.com/characterization/d-value-calculation/saed-d-value-calculation/