A certain capacitor has rectangular plates 56cm by 20cm and the gap width is .21mm. what is it's capacitance? A certain capacitor has rectangular plates 56 cm by 20 cm, and the gap width is 0.21 mm. What is its capacitance? We see that typical capacitances are very small when measured in farads. A one-farad capacitor is quite extraordinary! Apparently it has a very large area A (all wrapped up in a small package), and a very small gap s.