What is the formula for calculating mechanical advantage in a pulley system?

[phrankle]

http://www.getofftheinternet.net/pulley.png

How would you go about calculating the mechanical advantage of this pulley system?

 

[Bob S]

The tension of each rope supporting the weight of the load is the same. Divide the weight of the load by the number of ropes supporting it.

 

[phrankle]

Divide the weight of the load by the number of ropes supporting it? That doesn't seem right to me. I'm looking for a quick way to identify the Ideal Mechanical Advantage (IMA) = output force / input force.