All antenna lengths depend on various factors. Some of these factors are:the height above ground, the diameter of the wire, nearby structures, the effects of other antennas in the area and even the conductivity of the soil. (Because of the artificial ground provided by the radials, the height above ground should have little effect on this type of antenna.)

This page allows you to calculate the length for a quarter wave ground plane antenna. It uses the standard formula, 71.3232 / f MHz to calculate the element lengths. If you've experimented with ground planes before and know of a better formula for your QTH, feel free to change the formula to suit.

Enter the formula for the antenna calculation

Your ground plane's vertical length is

Each of the 4 radials of the ground plane is also