1> the estimate value=W*L*(square capacitance), you can find the square capacitance (ff/um^2) in process file from process supplier.
2> also, you can simulate the MOSCAP and an idear cap in same condition by Hspice, compare their I/V curve, the same I/V curve means the same capacitor value.
You can obtain the equivelent capacitance of MOSCAP by HSPICE transient simulation. caculate the total charge (Q) stored in the CAP in fixed voltage range (Vr), then the C=Q/Vr .
U can simply do an AC simulation with a voltage source connected in series with the mos cap, by setting the AC voltage to 1 and w=1 ,u can get the value of the capacitance as it is simply the current coming out of the ac source.
i=V/Zc
=jVwc ==> c=i/wV (for V=1, w=1) ==> c=i
The capacitor values depend on the voltage applied to the Gate of the transistor which decides its region.
Ex: Take Vgs is negative then it is in accumulation region and the capacitance is Cox now on if u increased Vgs and the depletion region is formed in that case the capacitance will be Cox+Cdepletion again u increased Vgs till the inversion takes place below the oxide and that time the capacitance becomes Cox. This are the three regions of operation called as
1.Accumulation,2. Weak Inversion,3.Strong Inversion and I think this might help u to get the capacitance of the NMOS. If anything else needed let me know.