Frost porosity is a very important property to estimate the density and thermal conductivity of the frost layer. Having the real-time frost porosity information can be very helpful for the frost growth prediction and precise defrost control strategy development. In this work, a capacitance sensing approach has been developed to detect the frost growth in real-time. An interdigital electrode pair was designed and fabricated to sense the porosity changes during the frost growth based on the fringing effect. Frost growth under different conditions was observed. The averaged approach by measuring the frost mass and volume was also adopted for comparison. Result shows that the frost porosity measured by the capacitance sensing approach agrees well with the averaged measurement approach with a maximum difference of 3.4% except for the data at the first 5 min. The temporal variation of frost porosity can reflect the frost growth period in great details. Frost porosity increases quickly at the start of the frosting process and then maintains a high value for a certain time, after that it keeps decreasing during the observed one hour of frost growth. Air velocity affects the duration of the frost porosity increase-maintain-decrease variation and the frost porosity reaches to the decreasing stage much faster under a high air velocity. Frost porosity measured with different configurations of the electrode was also compared under the same condition and the deviation between different configurations is within 3.0%. This work has been the first to demonstrate a real-time precise frost porosity detection. (C) 2019 Elsevier Ltd. All rights reserved.