Porosity measurement is a key factor to identify the hydraulic performance of low permeable porous materials (e.g. rock or concrete). Porosimetry tests such as Mercury Intrusion Porosimetry (MIP), Nuclear Magnetic Resonance (NMR), or Gas Expansion (GE) are cost-prohibitive, use hazardous materials, or are incapable of accessing all inter-connected pores. An alternative Gas Expansion Induced Water Intrusion Porosimetry (GEIWIP) method was developed to measure the porosity of a low permeable porous medium using a gas/water intrusion apparatus. This method overcomes the previously mentioned porosimetry drawbacks by using distilled de-aired water (DDW) as a hazard-free liquid which is a wetting fluid to intrude the porous structure and fill the pores. As the DDW has the tendency to fill all inter-connected pores, no back-up pressure is required. This method has lower cost and needs less preparation time comparing to MIP test. Additionally, the GEIWIP set-up, the gas/water intrusion apparatus, can be moved to the field site and provide mobile measurement feasibility. The reliability of the test results was obtained by a repetitive testing process. The porosity of concrete samples with different mixtures was obtained and compared to those of MIP and NMR tests.