What is the transfer function of n-cascaded RC filters?
There isn't an explicit formula for n cascaded RC low pass filters (as you have shown) because as you move from left to right, R2 and C2 load the output of R1 and C1 thus changing R1&amp;C1's response and because R3 and C3 are doing the same to R2 and C2 this in turn reflects on the loading of R1 and C1.

Using 2 port networks and matrix maths does help you solve more easily for several sections but the formulas do tend to get long after a few sections and solving for the cut-off frequency and Q is very tiresome.

The best solution is to use a simulator like LTSpice.

Given that you have an op-amp in your circuit acting as a buffer it wouldn't be unreasonable to use this to help. This sallen-key calculator is very good and gives you the response and formulas for a 2nd order low pass filter: -



Cascading 2 of these will probably get you a better response than cascading five passive RC filters and the beauty of this method is that you can cascade them without interaction between components because the op-amp acts as a buffer.