Significant progress has been made in the field of pore-scale imaging in the last decade which is revolutionizing various scientific disciplines, including biology, material science, and food engineering. Pore-scale imaging holds considerable potential for estimating volumetric and hydromechanical properties of rocks for a variety of geoscience applications. However, the micron-scale images, generated by the present imaging technology, are limited in resolution and thus a significant portion of rock pore volume can remain unresolved. We use the established concepts of capillary physics in rocks to infer rock porosity that sits in pores that are smaller than the image voxel without the need for higher resolution imaging that is only possible at the expense of image field of view or physical laboratory measurement. This allows us to estimate porosity of reservoir rocks using X-ray computer tomography even when a significant portion of pore volume is below image resolution. We find that the porosity inferred by our method matches the experimentally obtained total porosity for rocks. The proposed concept is also applicable to other modes of imaging, including thin-section light microscopy, scanning electron microscopy, and synchrotron source micro-CT.