Use the arrow FOC calculator to estimate Front of Center (FOC).
FOC, What’s the Big Deal?
By Bob Robb
An often misunderstood and misused term in the world of arrows is FOC, an acronym for front-of-center balance. In a nutshell, FOC describes the percentage of the arrow’s total weight that is located in the front half of the arrow. The more weight that is located in the front half of the arrow, the more forward the arrow’s center of balance. The best method of measurement is an arrow FOC calculator.
Why is this important? Simply stated, the FOC position on the arrow shaft is a key factor in determining the shape of the arrow’s trajectory curve. The right FOC is especially important in long-range shooting, both at targets and at game. An arrow shaft with a FOC that is too high will fly like a laser beam but will tend to nose-dive at distance. Conversely, an arrow with a FOC that is too low will fly with a more stable trajectory better, but will often fly erratically. The accepted standard for the correct FOC for a hunting arrow is 10-15 percent FOC for optimum accuracy potential — especially at long distances.
Calculating FOC With an Arrow FOC Calculator
So, how do you measure your arrow’s FOC, and manipulate it to achieve the optimum balance point? The standard FOC formula is FOC % = 100 x (AL/2)/L where L is the correct arrow length, and A is the distance from the nock groove to the finished arrow balance position, including the arrow point/insert weight, nock, and fletching.
First, build your hunting shaft using the exact components – broadhead, vanes, nock, and cresting if desired. Then measure your arrow shaft, from the nock’s throat to the tip of the arrow shaft. Divide this number by two. Now, find the balance point. I balance the shaft on my index finger, since it’s simple, though obviously not as precise as using a narrow edge to balance the shaft on. Mark that spot with a Sharpie. Time to enter the data into the arrow FOC calculator above.
Now, measure from this mark to the throat of the nock. Next, simply subtract the center of the arrow measurement from the balance point, and multiply this number by 100. Lastly, divide the resulting number by the arrow’s overall length. Boom! This number is the FOC of your arrow.
You can change the balance point of an arrow shaft by simply adding or subtracting weight to either the front or the rear of the shaft. Heavier vanes and a lighted nock, for example, will add weight to the back end of the shaft, decreasing FOC.) Trading out a 100-grain broadhead for a 125-grain head is a simple way to increase the FOC. You can also use a heavier arrow point insert or add a metal weight insert. After making adjustments, reenter the appropriate information in the arrow FOC calculator to determine the new percentage.
One arrow maker, Carbon Express, has a patented Built-In Weight Forward design that gradually shifts weight to the front two-thirds of the arrow, which incorporates a lighter, stiffer back end that improves recovery out of the bow while putting additional weight in the front end.
Arrow FOC Calculator Example
One of my Easton 4mm FMJ 340 shafts, which weigh 11.0 grains/inch, is built with three 2-inch Bohning Blazer vanes, a standard Easton nock, 50-grain Easton aluminum Half-Out, and a 100 arrow point, and is cut to 28 ½-inches. Half of that is 14 ¼-inches. The balance point is at 17.75 inches. 17.75 minus 14.25 equals 3.5. Multiplied by 100, that equals 350. 350 divided by 28.5 equals 12.28, which is the FOC for this shaft configuration. By switching to a 125-grain arrow point, the balance point moves forward to 18.375 inches. 18.375 minus 14.25 equals 4.125. Multiplied by 100, this equals 412.5, which divided by 28.5 equals 14.47, which is the FOC for this shaft configuration. I can tell you from my shooting that both these shafts fly like absolute darts at both close and long distances.
More on FOC
Arrow FOC Calculator Example
The FOC balance position of the arrow is one of the more important elements affecting the shape of the arrow’s trajectory curve. Ideal FOC is especially critical for target shooters participating in long-range shooting competitions operating under the Olympic Style rules, such as FITA Archery events. While FOC is less relevant in some bowhunting situations (short-range shots), it can be critical in others (long-range shots, when shooting low-poundage bows and fixing trajectory issues at the range). It’s generally accepted among archery experts that a high FOC will fly with good stability, but will shed its trajectory quicker and nose-dive. An arrow with low FOC will hold its trajectory better, but can fly erratically. Easton recommends an arrow with 10-15% FOC for hunting setups and optimal accuracy – especially at long distances.
FOC is one of many variables to be considered when tuning arrow flight. If you need a complete bow-tuning guide, download your free copy of Easton Archery’s Arrow Tuning and Maintenance Guide. –Learn More at Easton.com This arrow FOC calculator will help to estimate Front of Center (FOC).Easton Archery