In this case you need to prepare two types of plates every time with equal amount of medium and the weight of the plates. Now You just grow your target organisms in one type of plate with 5 - 8 replicates and other plates keep as control without giving any microorganism rather than medium. Now you should wrap both type of plates using same amount of enclosure (parafilm/ mocropore tape/others). Incubate of the plates in the same environment up to the date required for growing of your target microorganism. Thereafter, just weight both types of your plates in the same type of fine balance and could get the weight of your organisms by differentiate both type of plates weight.

Ujjal Kumar Nath but the media I used is liquid broth not solid agar.

Then take equal amount of medium in a simillar cultural flask.

I agree with Prof Dr. Ujjal Kumar Nath . In case of broth culture, you may grow the inoculated broth with control broth media in a mechanical shaker to reach the log growth phase of microbes. After that, centrifuge the matrix and discard the aqueous supernatant and compare the mass of the precipitated solid mass. The difference is the mass of microbial growth.

You may use a basic formula to validate the methods.

Weigh out the tube/flask before running the experiment. Then after growing the bacteria Centrifuge to obtain a pellet get rid of your supernatant and then measure the tube/flask. You can then resuspend in media and continue using it