Accuracy of k means clustering

How can i calculate the accuracy of the diagram given below

[Adam]

Sum of the distance of each point from the class centre it is assigned to is one obvious method.

[Reetu Singh]

@Adam is there any code or syntax in matlab for this purpose.

[Adam]

Possibly, but it's so trivial I just get the data and do the maths personally. I often can't be bothered searching for a builtin function if I know what needs to be done anyway.