Question

Why Relative Density has no UNITS?​

Answer 1

Answer:

because it is a ratio of similar quantities.

Answer 2

This does'nt even make sense for you itself if you truly did understand relative density. Say, You want to measure the density of water! Then you collect 1m³(1000L) of water in a container and put it in a weighing machine to note that it has a mass of aprox 1000kg/m³ = 1kg/L makes much sense. So that means that One litre of water weighs One Kg. (There is a simple way to measure density .... think about it)  

Now you measure the density of oil and figure that out as 840 kg/m³ alright how to express the density of oil relative to that of water? Or It is similar to asking How much times the density of water is the denstiy of oil? In our example to find that "how much times"  We divide 840/1000 = 0.84

Does "how much times" have any unit? You see that to get the actual density You can just Multiply relative density to the density of water. (0.84 * 1000 = 840)

Why does it make sense? most of the relative densities are expressed in relative to water because we exactly know how much a 1L water bottle would weigh....so if someone brings a 1L oil bottle you could easily say that by looking at 0.84 that the oil bottle should weigh less than water without even knowing the density of water (for water it is 1.00 ! )