The capacity of a grate inlet on-grade depends on its geometry and cross slope, longitudinal slope, total gutter flow, depth of flow, and pavement roughness.

The depth of water next to the curb is the major factor affecting the interception capacity of grate inlets. At low velocities, all of the water flowing in the section of gutter occupied by the grate, called frontal flow, and a small portion of the flow along the length of the grate, termed side flow, is intercepted. On steep slopes, only a portion of the frontal flow is intercepted if the velocity is high or the grate is short because a portion tends to splash over the end of the grate (“splash-over”). For grates less than 2 ft. (0.6 m) long, intercepted flow is small.

Refer to HEC-12 or HEC-22 for inlet efficiency data for various sizes and designs of grates. Additionally, safety for small wheeled vehicles (bicycles, wheelchairs, etc.) must be considered in grate selection.