We use terahertz transmission through sedimentary rock samples to assess the macro and micro porosity. We exploit the notable water absorption in terahertz spectrum to interact with the pores that are two orders of magnitude smaller than the terahertz wavelength. Terahertz water sensitivity provides us with the dehydration profile of the rock samples. The results show that there is a correlation between such a profile and distribution of micro to macro porosity of the rock. The study further estimates the absolute value of total porosity based on diffusion theory. We compare our results with mercury injection capillary pressure as a benchmark to confirm our analytic framework. This porosimetry method can set a foundation for a more affordable, less invasive porosimetry that can be used in geological studies and in other industries without the need for hazardous mercury or ionizing radiation.