PMT function - does anyone know the formula?

Hi,

I am trying to code the equivalent to the EXCEL PMT function.

in VB, the formula looks like this:

PMT = ((PV - FV) * rate/ (1 - (1 + rate) ^ -(nper)))

This is great for PMT calcs where Type =0 (i.e. payments occur at the end of the period)

However, I'm trying to code for a Type 1 scenario (i.e. payments occur at the start of the period).

For example, using =PMT(6.25/12,36,15000,0,0) gives 458.03 (the same as my formula) whereas =PMT(6.25/12,36,15000,0,1) gives 455.66

Any math wizards out there who can tell me how to modify my formula?

All the best,

M

Okay, so I've been a bit dense here as I'v now discovered that the EXCEL PMT function is available in Visual Basic.

This means I can code a solution; however, I would still be interested in how the formula actually differs so please post if you know the answer.

M