How is this possible to calculate the variance of a gray scale image,I want to get it as a single number?

I loaded an RGB image & converted it into gray scale using rgb2gray() command, also it was modified into an N-by-N matrix by zero appending. After these I tried to get a single value for the variance but the was an 1D array of elements. What can I do to get a single value. Please help me. Thanks to all who tries to help me.