Comment by GeoAtreides

> As a mathematician, I had trouble understanding how banks calculate it (as they would obviously not point me at the derivation of the formula)

This is not true? The formula is well known:

A = P*e^(rt)

where"

A = Final amount

P = Principal (initial investment/loan)

r = Annual interest rate (as a decimal)

t = Time in years

e = Euler's number