Every bp has a specific molecular weight, in average each bp ~  0.65 kilodaltons. Therefore, if your plasmid was 3,000 bp long, the approx. molecular weight would be 3,000 bp x 0.65 kDa per bp = 1,950 kDa.  

If you have ordered a synthetic gene subcloned in our standard vector pEX-A2 (2,450 bp) and your gene is 1,000 bp long, the total length of the plasmid would be 3,450 bp and the molceular weight 2,243 kDa.