Rangefinding in the Wild
"The Rule of Thumb"

By Henri Montandon


There you are and there he is, the King of all elks, standing in plain sight across from you, oh, say 500 to 1000 yards away, right across the canyon. But what a difference 500 yards can make. Even if you are fortunate enough and good enough to be shooting 1 MOA, it's the difference between a five inch circle and a ten inch circle.

Unfortunately, your mil-dot reticule is on your other scope, and the lithium battery in your laser range finder is out of electrons. But don't worry. You probably have two other measuring instruments that will bring you a range with an accuracy of plus or minus 30 yards. I'm talking about your thumbs. Study and learn this simple method and you will have a range finding backup that you are not likely to forget or find inoperable. I call it, "The Rule of Thumb."

How to calculate distances using your thumb.

  1. Measure the width of your thumb nail in inches.
  2. Note that you should measure the width of your nail at its widest part, which is usually where the sides of your thumb nail are approximately parallel.
  3. Hold your arm straight out in front of you, with your thumb pointed up.
  4. Measure the distance from your eye to your thumb in inches. This distance is labeled B.
  5. Divide the distance from your eye to your thumb (B) by the width of your thumb (A), and call this C. ( B / A = C )

You can find the distance something is from you by using the following relationship:

SIZE OF OBJECT IN FEET / NUMBER OF THUMB WIDTHS OF OBJECT x C = DISTANCE OF OBJECT IN FEET

Example:

  1. My thumb measures 20/32 inches at its widest part where both sides are parallel. 20/32 is the same as 0.625.
  2. The distance from my thumb to my eye, when I hold my arm out straight with my thumb pointing up, is 24 1/4 inches, or 24.25 inches.

So: 24.25 / .625 = 39 40
(For the sake of simplicity here, I am rounding off 39 to 40.)

Let's say my friend Fred is six feet tall. He is standing some distance from me. How far away is he? I hold my arm straight out, and sight along my thumb. Fred measures one thumb width high. Using the relationship that we worked out above:

6 / 1 X 40 = 240 feet

In this example, 6 is Fred's height in feet, and 1 is the number of thumb widths he is when I sight him against my thumb held at arm's length.

What if Fred measures 1/4 thumb width?

6 / .25 X 40 = 960 feet

5. Even if you think that Fred is five feet, six inches tall, when his real height is six feet, your possible error is much smaller then what you are likely to find by guessing.

( 5.5 / .25 X 40 = 880 feet )

If you are shooting 1 MOA, the difference in target diameters is less than one inch.

A happy fact: The fovea is the part of the eye where vision is sharpest. It covers a segment of your visual field about 1 degree in width. The width of your thumb at arm's length is about 1.5 to 2 degrees. This means that when you are range finding by the rule of thumb, you automatically use the best vision your eye can produce!




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Copyright 2005 by Henri Montandon. All rights reserved.



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