yeh definitely there is
G = n (4 pi / lamda) A
This equation determines the required antenna area for the specified gain at a given wavelength.
Added after 2 minutes:
The net efficiency n is the product of the aperture taper efficiency na , which depends on the electric field distribution over the antenna aperture (it is the square of the average divided by the average of the square), and the total radiation efficiency n * = Pout/Pin associated with various losses. These losses include spillover, ohmic heating, phase nonuniformity, blockage, surface roughness, and cross polarization. Thus n = na * n *. For a typical antenna, n = 0.55.