Common Vertical Antennas:  (Mobile or Base Ground Plane)

Antenna Length in (meters) = 300 / Freq (MHz) * Length Fraction

Antenna Length in (inches) = 300 / Freq (Mhz) * 3.281 (meters to ft) *12 (feet to inches) * Length Fraction

Common Length Fractions:  1/4 = .25      1/2 = .5      5/8 = .625

If you are building a ground plane antenna, such as a base, not mounted on a car, multiply the length of the vertical element as calculated above, by 1.05 which adds 5% to the length, those are your ground planes.  Bend them down at 45° angles and trim the vertical element for match.