The difference in pressure between the inlet and outlet of a column measured in pounds per square inch. Governed by the following approximate equation for a column packed with spherical particles of typical internal porosity (0.5): Δp = 3000Lη/t_Md_p2, where L is the column length in centimeters, η is the mobile-phase viscosity in centipoise, t_M the column holdup time in minutes, and d_p is the particle diameter in micrometers. Pressure can be expressed in pounds per square inch, bars, atmospheres, or pascals.