Determining the capacity of image for data hiding is a complex task. This capacity may be influenced by many factors such as the content of image and also Human Visual System (HVS). A few works on this topic are reported. In this paper we propose a new method for calculating capacity in gray scale images. Our method is based on measuring Smoothness and Connectivity of neighbors of a specific pixel in image bitplanes. This method can produce a signed value for each pixel capacity such that the sign shows the modification in pixel value must be increasing or decreasing. Comparison with the existing methods showed that the proposed method gave better estimation for capacity of data hiding in an image