Equation
D = f / N
A 35 mm f/3 imaging lens has an entrance pupil about 11.67 mm across. The clear diameter of the front glass additionally depends on how off-axis bundles pass through the system.
When to use it
The entrance pupil is the image of the aperture stop seen from object space. The relation uses the nominal f-number at infinity focus. At close focus the working f-number may differ.