Bit geeky I know, but am currently doing this on a course.
Radar Horizon
Single Object Above Earth
d = sqrt(2kRh)
k = ~4/3 or -3/2 for super-refraction
R (earth) = 6370,000m
h = metres
or roughly 1.23sqrt(height(ft)) = nm as previously posted
Hilltop to Hilltop or Aircraft to Large Antenna
d = sqrt((2kRh1) + (2kRh2))
h1 / h2 = object heights respectively
Hope this helps