The present invention provides a method for determining the porosity of rock from a digital image of the rock. A three-dimensional image of a rock is obtained and segmented, and an image porosity is determined from the segmented image of the rock. A porosity correction parameter is obtained from a non-wetting liquid capillary pressure curve derived from the segmented image, and the porosity correction parameter is applied to the image porosity to obtain a corrected porosity of the rock.