There are many detectors for the least significant bit (LSB) steganography which is broadly used in hiding information in the digital images. The length of the hidden information is one of the most important parameters in detecting steganographic information. Using 2-D gradient of a pixel and the distance between variables the proposed method gives the length of hidden information in natural grayscale images without original image. Extensive experimental results show good performance even at low embedding rate compared with other methods. Furthermore, the proposed method also works well disregarding the status of the embedded information.