This is the easiest way I explain to my self what a milliradian is, from the picture below, we see the following:
A] 1 Mil = 3.6 inches at 100 yards.
B] That means that in 27.778 yards there is exactly 1".
Now it's simple:
If using a Mildot reticle, a target that is 18" high gives me exactly 3 mildots then here is how I figure the distance and still being able to understand it and not just memorize it.
If for every inch of target dimension I get 27.778 yards for one milliradian, then 27.778 X 18" = 500 yars for one milliradian. BUT... IT WAS NOT one milliradian, it was 3; therefore 500/3 = 167 yards rounded.
All that boils down to what has been previously said:
Distance = [Higth of target in inches]x[27.778]/[Higth of target in Mils]
Distance = [18" X 27.778]/ = 167 yards.
Now that I understand where those numbers are coming from, I pay attention to what meichele said:
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Most game I hunt is 18" from back to brisket. 18*27.778 = 500. That is always easy to remember. 500 / mils.
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So... if your hunting let's say Mule deer and we mesure him to have 1.5 mils all we do is 500/1.5 = 333 yards away.
I hope this helps.