Flow experiments were conducted on sand-packed columns in order to determine the Reynolds number at the point of the transition to turbulent flow. Five sizes of sand grain were employed, as from 0.704 to 0.177mm. Fig. 4 shows the relationship between the permeability calculated from Darcy's law and the Reynolds number defined by Ahmed and Sunada. The results indicate that the transition to turbulent flow takes place over a range from 0.01 to 0.001 of Reynolds number. Attempts were made to correlate the coefficients involved in Forchheimer equation with the coefficients in Ergun's equation and the coefficient in Kozney's equation. It was found that there are some relations among them, as defined in Eq. (18), Eq. (19) and Eq. (22). It was experimentally shown that the coefficients in Ergun's equation are not constant and vary with grain size.