Convexity is normally calculated as:
Sum(PV x t x (t + 1) ) / Sum(PV).
PV = Present Value of individual cash flows.
t = timing of cash flows.
More loosely, the terms Convexity and Modified convexity are sometimes used interchangeably.
Obviously this can lead to confusion, so it is important to clarify whether convexity or modified convexity is intended.