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Wavelength is the physical spacing between adjacent peaks or nulls in the electromagnetic field of a radio wave. Represented by the lower case Greek lambda (µ), it's inversely proportional to operating frequency (f). The generic formula for calculating wavelength is:

µ = 300/f_MHz where wavelength is in meters
µ = 984/f_MHz where wavelength is in feet

At microwave and high-UHF frequencies, the dimensions are very small, and smaller units are more appropriate:

µ = 300,000/f_MHz where wavelength is in millimeters (mm)
µ = 11,808/f_MHz where wavelength is in inches

The most common antenna lengths are fractions of a wavelength, specifically µ/2 and µ/4. When building the antenna with wire, tubing, pc-board copper, or any other conductor, the actual physical length varies, depending on various factors like closeness of adjacent objects, dielectric constant of any base, and conductor thickness. In all cases, the actual physical length is shorter than the computed value, and the optimum length is almost always determined experimentally.