The height of packing required in a distillation column or an absorption column that is able to provide the same change in composition in the liquid or vapour phase as one theoretical plate. It can be found from the total height of the packed bed divided by the height of a theoretical stage. This is a device that has perfect contact between the liquid and vapour phases such that both streams leave in equilibrium. The HETP effectively divides the column into equal portions whereas the height of a transfer unit (HTU) is a continuous function.