The Mil-Dot Reticle is a targeting system that was created for extremely long-range
snipers
(primarily military, but target shooters have begun to use them too). Shooters
can use the Mil-Dot Reticle to find out
the range to a target of a known size. Using the range, the snipers can then
accurately adjust for elevation and windage. The reticles used in the United States Marine
Corps (UMSC) have small, oval-shaped dots spaced every one milliradian along the
axes of the reticle. This spacing allows for the shooter to divide the reticle
up into easily identifiable eighths of a milliradian.

#### Using the Mil-Dot Reticle

In order to find out the range to a target, the shooter must
measure the height of the target in milliradians, using the sight, and then
convert that measurement through a series of calculations to a corresponding
distance to the target. For example, lets assume that a shooter estimates that a
target is 6 feet tall (72 inches) and needs to know the range to
the target in order to make a accurate shot. Then, the shooter would put the top
of the target in middle of a mil-dot and then measure how far the target extends
down the vertical axis of the scope. Let's say that it goes five-eighths of a
milliradian. Using the calculations for the Mil-Dot reticle the shooter would
know that the target was 3200 yards away (about 1.65 miles) and could then
adjust accurately for elevation and windage.

#### Reading the Mil-Dot Reticle

In the example above, I said that the shooter measured the
height of the target in milliradians. Well, to read the scope the shooter must
be able to discern the set eighths of the Mil-Dot Reticle. These measurements
are so that the shooter can quickly determine the height of a target. The distance between the middle of an oval and the top or the
bottom of the same oval is one-eighth of a milliradian. The distance from the
bottom of an oval to the top of an oval is therefore one-quarter of a
milliradian. The distance from the top or bottom of a milliradian to the middle
of the line above or below it is three-eighths of a milliradian. The distance
from the middle of an oval to the middle of the line above or below it is
one-half of a milliradian. The distance from the bottom of an oval to the middle
of the line above it or the distance from the top of an oval to the middle of
the line below it is equal to five-eighths of a milliradian. The distance
between the top and bottom of two ovals is equivalent to three-quarters of a
milliradian. The distance between the middle of an oval and the top of the oval
below it or the bottom of the oval above it is equal to seven-eighths of a
milliradian. Lastly, the distance between the middle of two oval is equivalent
to one milliradian.

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#### Mil-Dot Mathematics

The calculations used to find distances to and
heights of targets are as follows:

tan(milliradians / 1000) = Height of target / (Distance to
target * 36)

The number of milliradians is read off of the reticle. The
height of the target is the unknown and the distance to target is known (100
yards). The number of milliradians is divided by 1000 to convert the calculator
from radians mode to milliradian mode. The distance to target is multiplied by
36 to convert yards to inches. Now, for example, lets put in 1 milliradian and
100 yards into their variables. The equation now looks like this:

tan(1 / 1000) = Height of target / (100 *
36)

tan(.001) = Height of target / 360

.001 = Height of target / 360

Height of target = 3.600
inches

These calculations mean that if a target
occupied one milliradian at one hundred yards, then the target must be 3.6
inches in height. However, long range military snipers do not usually fire at a
known distance, so they use this equation:

(((Height of target in feet * 12) / Number of
milliradians) / 3.6) * 100 = Distance to target

To find the distance to a target of a known
height. For example, if a sniper is taking a shot on a target of six feet and
the target occupies two milliradians, then:

(((6 * 12) / 2) / 3.6) *
100) = Distance to target

((72 / 2) / 3.6) * 100) =
Distance to target

(36 / 3.6) * 100) =
Distance to target

10 * 100 = Distance to
target

Distance to target = 1000
yards

This means that the target is 1000 yards away and
now the sniper can adjust his scope for elevation and windage. The Mil-Dot
reticle can accurately determine the distance to a target and therefore make the
success rate of shot increase many times over.

Sources:

I got most of my information from two very useful
websites: