Analytical and computational fluid dynamics (CFD) analyses confirmed the presence of apparent slip for water flow in microchannels with equivalent hydraulic diameter, D_{h} < 10^{3}*μ*m, markedly decreasing the friction number, fRe_{in}. The determined values of the slip length, *β*, from reported measurements of pressure losses in microchannels with aspect ratio, *α* = 1, 1.74, 2, and 40, are 0.9, 3.5, 1.6, and 0.125 *μ*m, respectively. For D_{h} > 10^{3}*μ*m, the apparent slip in microchannels diminishes, and the friction number approaches the theoretical Hagen–Poiseuille with no slip. The analytical solution for fully developed flow successfully benchmarked the CFD approach, which is subsequently used to investigate fRe_{in} and the flow development length, L_{e}, for uniform inlet velocity in microchannels. For fully developed flow, the analytical and CFD values of fRe_{in} are in excellent agreement. For microchannels with D_{h} < 10^{3}*μ*m, fRe_{in} decreases below that of the theoretical Hagen–Poiseuille with no slip, almost exponentially with decreased D_{h}. The difference increases with decreased D_{h}, but increased *α* and *β*. The friction number for uniform inlet velocity is identical to that for fully developed flow when D_{h} ≤ 100 *μ*m, but is as much as 9% higher for larger D_{h}. For uniform inlet velocity, L_{e} negligibly depends on *α* and *β*, but increases with increased Re_{in}. The obtained values are correlated as: L_{e}/D_{h} = 0.068 Re_{in}.