Culverts are encountered throughout irrigation systems. Many of these culverts operate under free surface flow conditions. Usually, the flow depths in the culvert are governed by downstream flow conditions. This particular flow condition can be described as free surface outlet control. Only an approximate solution for determining discharge has been available for this situation. A 12-inch diameter corrugated metal pipe has been used in the laboratory to test the validity of the submerged flow analysis employed with flow measuring flumes and weirs in describing free surface outlet control in culverts. Various slopes and culvert lengths were studied. This study has shown that discharge ratings under free surface control flow conditions can be developed. Such ratings can be developed. Such ratings can be related to the relationships for inlet control and submerged outlet control. Thus, culverts can be used as flow measurement structures in irrigation systems.