A theoretical calculation is needed to predict pump slippage in a rod pumped well. Slippage lubricates the pump plunger and barrel and prevents galling of metals. Additionally, an estimate of plunger slippage is necessary to calculate pump efficiency. Historical plunger slippage equations have been proven incorrect by recent testing. A new method for calculating plunger pump slippage in rod pumped wells is introduced. This method involves calculating a velocity profile for an annulus with the inner wall moving parallel to the outer wall. An average velocity is determined for the annular fluid flow, which in turn is used to calculate the fluid slippage. The results are evaluated against the historical field data and compare favorably to recent testing for smaller plunger clearances. Work remains to be done at larger clearances.