What are Minutes of Angle or MOA?

MOA

There are 360 degrees in a circle and each degree is divided into 60 minutes. Hence "minutes" of angle - it's a circle broken into increments of minutes (total: 21,600 minutes). If we round to the nearest 1⁄100 of an inch, at 100 yards, 1 degree measures 62.83 inches. One MOA, 1⁄60 of that, measures 1.047 inches. 

This is usually rounded down to 1.04 to simplify the math.A minute of angle, usually used as a measure of group size, target size or shot dispersion, spans 1.047 inch at 100 yards but we round down to 1”. To calculate MOA at any distance, multiply 1.047 by the distance in yards and divide by 100.

There's another measure known as Shooter's MOA (SMOA)--rounding down MOA's true measure of 1.047 inches at 100 yards, to a much-simpler-to-calculate 1 inch at 100 yards. This is fine for short distances, but at 1000 yards this basic measure is too inaccurate, as it's not 10 inches but 10.5 inches.

For simplicity, using SMOA here, a single minute roughly equates to 1 inch at 100 yards, so shooters get used to thinking of MOA in terms of inches.  2 MOA at 100 yards is 2 inches, 1 MOA at 400 yards is 4 inches, and so on. It's great to use angles because, as you can see, they increase proportionally with distance.As long as the shooter sticks with 100-yard increments, the math is pretty simple to do in your head. MOA scopes are often adjustable by 1/4 (0.25) MOA increments (approx. 1/4 inch at 100 yards). You will hear the term MOA used a lot in long range.

For example, the term is often used to describe the size of targets, so a target at 500 yards that is 2 MOA in size (usually width), means that the target is 10 inches wide. 1 MOA at 500 yards would be 5 inches, therefore 2 MOA would be 10 (2x5) inches. However, if you are using an MOA reticle in your scope, you will use MOA as a measurement and not inches, and you will use the markings (subtension) in the scope reticle that size up targets or make adjustments. The same goes for a MIL scope reticle.