Ranging Distances With Your Riflescope

A little know-how and some basic math will allow you to calculate the distance to your target with relative accuracy.

Ranging Distances With Your Riflescope

Laser rangefinders are becoming smaller, more durable and best of all, more affordable. They’re a great add-on sales opportunity for hunters, target shooters and long-range competitors. Even still, few shooters own one.

Until the day comes that range finding is built into smartphones (yes, I know, in some ways it already is) knowing how to determine the range of a target the old-fashioned way is a valuable skill that anyone can master. Think of it like knowing how to drive a manual transmission car.

Fortunately, this little-known skill is easy to understand once you know the basics and memorize a simple formula. Let’s walk through the process of using just any scope with some graduation marks in the reticle to determine how far away that target is.

Proper Proportions 

Simple proportions make the system work. Your eye is in a fixed place. The target downrange is a fixed size. If something in between your eye and the target is also a fixed and known size, you can establish and calculate relationships between those things to determine how far apart they are.

The first step to understand ranging with a scope is to understand the measurements inside of the scope itself. They’re marked right on the reticle. As long as we know their “size” we can do some easy math to figure out how they relate to the target size downrange. Scopes use either milliradians or minutes of angle as unit measurements for a graduated reticle. By graduated, I mean a reticle that has marks that are a known distance from each other as measured by either mils or MOA.

What Are Milliradians? 

A milliradian measures an amount of arc around a circle. If you use a cheesecake as an example, and cut a slice, the amount of arc that goes around the edge of that slice can be communicated in terms of some number of milliradians. So, if you really want to confuse the staff at your local Cheesecake Factory, you could say something like “I would like 1,047 milliradians of that Oreo Crust cheesecake please.”

Not coincidentally, there are 6.28 (and change) full radians in a circle. You might have noticed that number is twice the value of π. We all learned about that in geometry class, and as you might recall, the circumference of a circle is calculated by 2(π)r where “r” is the radius of the circle. So, there are 6,283 milliradians (one-thousandths of a radian) in a circle. That’s why, if we want 1/6 of a cheesecake, we would order 1,047 milliradians of cheesecake.

All this is fun and hopefully we all love cheesecake, but the real value of a milliradian is that since it represents an arc of a circle, we can also use it to represent an angle. That 1,570 mils of pie will have a 60-degree (1/6 of a circle) angle at the pointy end. A single mil represents a really narrow angle. Imagine a cheesecake cut into 6,283 equal slices and you get the idea.

Here’s what a mil means in real life. Imagine two lines extending down range from your eyeball. The angle between them is one mil, so they are very close together, especially at shorter distances. Of course, they spread apart the farther they extend. At 1,000 yards down range, those two lines are 36 inches apart. At 100 yards, the lines are just 3.6 inches apart. And herein lies the secret of using mils. Hold this thought for a minute and we’ll come back to it.

What Are Minutes of Angle?

A minute of angle, while different in terms of what it represents, serves a similar purpose as a mil. A minute of angle is 1/60 of a degree. As we know, just like on a compass, there are 360 degrees in a full circle. Since a minute of angle is 1/60 of a degree, there are 21,600 (60 x 360) minutes of angle in a full circle. An MOA represents an even narrower angle than a mil since there are more of them in the complete circle.

Again, using our line analogy, at 1,000 yards, the two lines at a single MOA angle would be only 10.4 inches apart. So, at 100 yards, they are just 1.04 inches from each other. So, the concept is exactly the same, it’s just the values that are different.

Ranging Concepts

Here’s how ranging works conceptually with either the mil or MOA method. Let’s suppose, for simplicity’s sake, that we’re shooting at a target that’s exactly 3.6 inches high. If we’re using a scope that has marks on the reticle that are exactly 1 mil apart from each other, then if we place that 3.6-inch target 100 yards downrange, it will appear to be exactly 1 mil tall in our reticle, right? If it’s 200 yards away, it will appear to be ½ mil tall, and so on. So, by using the scope marks to estimate how many mils tall the target appears, we can determine how far away it is.

Minutes of angle work exactly the same way. Suppose our target is 10.4 inches tall, and we place it 1,000 yards downrange. At that distance, when we look through a scope that uses MOA measurements on the reticle, the target will appear to be 1 MOA tall. If it appears to be 10 MOA tall, it has to be 10 times closer, or at 100 yards. Make sense?

So that’s the idea of how it works. The rest is just simple math to use the size of the target and figure out how many mils or MOA tall they appear to be through the scope. Then you’ll know the distance to target. Let’s do some examples.

Ranging with Mils

First, figure out the height of your target in yards. If we’re shooting at a 20-inch steel plate, it’s .55 yards tall because 20 inches divided by 36 inches equals .55. The other piece of data you need is how many mils tall that object appears to be when viewed through your scope.

To see how that works, imagine your target is 36 inches tall and appears to be 1 mil tall in your scope. Plugging those values into the formula gives you an answer of 1,000. We know that’s correct because 1 mil is 36 inches at 1,000 yards.

Ranging with MOA

The MOA model works exactly the same although with different numbers. Since it’s an “inch-friendly” system, we’ll want to determine our target size in inches. Using the previous steel plate example, we’ll go with a 20-inch-tall target.

For close-enough work, and if we assume that a minute of angle translates to 1 inch at 100 yards instead of the actual 1.04 inches, the formula looks like this: Target height in inches x 100 / target size in MOA as seen through the reticle. So, using our 20-inch target, if it appears to be exactly 2 MOA tall in the scope, then it must also be 1,000 yards away.

If you want to be precise, and use the exact 1.04 value of a minute of angle, we just have to tweak the formula and use 95.5 instead of an even hundred.

A Real Example

Let’s stick with our 20-inch steel plate for consistency and use both mils and MOA to determine how far away it is. Suppose that using a milliradian scope, that 20-inch-tall (.55 yard) target appears to be 3.5 mils tall. Plugging those values into the milliradian formula, we find that the target is 157 yards away. To get that multiply .55 x 1,000 yards and then divide by 3.5 mils.

As a second example, suppose that the same target when viewed through a MOA scope appears to be 4.75 MOA tall. That calculates to 402 yards downrange when you multiply 20 inches x 95.5 and divide by 4.75 MOA.  

Target Sizes

“But wait,” you say, “I don’t always know the exact size of my target!” That’s true. Fortunately, the world is chock full of things we can learn measurements from.

Targets are easy. Paper and steel have fixed dimensions you can learn. Fence posts in different areas are usually standard heights. A full-grown buck has a somewhat predictable size from the ground to shoulder or top of the back. Vehicles also have easy measurements to learn. The average car will have a predictable height from the ground to the top of the hood, as will an SUV. You get the idea.

By learning the sizes of a couple of common things in your shooting environment, you can use those objects as range estimation helpers as long as they’re near your targets.

Reticle Focal Planes

There’s always a gotcha, right? Here it is. All of this assumes that your reticle remains in proportion with the magnification level. With a fixed magnification scope, you have nothing to worry about. A mil is a mil, and an MOA is an MOA. Since you can’t change the magnification, your reticle view stays consistent. With a magnified scope, you might have to exercise some care depending on how it works.

When using a first focal plane scope, the reticle grows and shrinks along with the target as you zoom in and out. So, the proportion between reticle lines and target size remains constant at all magnification levels. You can use these ranging formulas at any power setting.

With a second focal plane scope, the reticle always appears to be the same size as you zoom in and out. The target changes size in your view but the reticle does not, so you can’t use the reticle as a measuring tool at different magnification levels.

For this reason, most second focal plane scope manufacturers set up their reticle so all of this ranging and hold-over math works at the highest magnification level. If you zoom out, your math is off and the calculations won’t work as you expect. You can apply factors to account for the magnification level you’re using, but that’s another topic.

When you take a minute to explore why this works, the concept of ranging using MOA or mils makes a lot of sense. Fortunately, there is little to memorize and the formulas are simple. With a little studying of common target sizes and some practice estimating size in mils or MOA through your scope, you’ll be determining range in no time.


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