M stands for Mode Field diameter and C stands for Cut off wavelength. MAC value is the ratio of Mode field diameter to the cut off wavelength. If Mode field diameter is 9.0 micrometers, that is 9000 nanometers and cut off wavelength is 1260 nanometers for a singlemode fiber, then MAC value is 9000/1260 = 7.143. Since it is a ratio, there is no unit for MAC value. As you can see the unit of mode field diameter is micrometer and in our calculation we converted it to nanometers to match with the unit of cut off wavelength.
But I have a doubt; what is A in MAC?