How can I calculate the resolution of load cell?

 Let's assume that you have 5kg loadcell. 

Its output is specified for 1mV/V excitation meaning that if you have 5V excitation, you shall get 5mV output for a full-scale load of 5kgs.

Simply correlate this as 5000uV (microvolts) corresponding to 2000gm of load.

Next, your digital indicator has a full-scale input span of +/-20mV to give you a count of +/-20,000 meaning it will have a resolution of 1uV (microvolt).

        Resolution of Load Scale  = 5000 values for 5000 gm = 1gm ;