The present invention provides capacitor-based fluid sensing units. The capacitor-based fluid sensing unit comprises a substrate, a first electrode configured on the substrate, a sensing layer configured on the first electrode, a second electrode configured on the sensing layer. More particularly, the second electrode is a porous electrode, while the sensing layer is made of a porous dielectric material and has a thickness between 50 nm and 5 mm. Permittivity of the sensing layer changes as fluid permeates from the second electrode to the sensing layer. The subsequent change in capacitance of the capacitor-based fluid sensing unit is used to determine the volume of the fluid.