F.O.C. stands for Front of Center balance point. This measurement results from the relative weights of the components used in the arrow: shaft, insert, head, fletching, and nock. Let's talk about how to come up with the F.O.C. calculation:
Step 1. Overall length:
Measure the total length of the arrow from the nock groove to the tip of the arrow with the point you plan on using.
Step 2. Balance Point:
Install the tip you will be shooting. If you are testing stability for 3-D shooting put your field point into the arrow. Of course, for hunting install your broadhead. Find the arrow's balance point by sliding it back and forth along a fairly sharp edge. You'll find the spot where the arrow just balances. Next measure in inches from the bottom of the nock grove to balance point. Then measure in inches the length of arrow from the nock grove to the edge of the arrow shaft (not the insert-also known as the cut length). Mark it carefully. Now measure from the point of the arrow to the balance point and enter the distance above.
Step 3. Determine F.O.C.:
To find the F.O.C. (which is always expressed as a percentage) divide the overall length by two. This should produce the physical center of the shaft. Now subtract this number from the balance point and divide by the overall length. Multiply by 100 to express the fractional value as a percentage.
A projectile's flight is most stable when most of the projectile's mass is positioned Front of Center (F.O.C.). As such, an arrow should be heavier in the front than in the back. But how much? Where's the "perfect" balance point?
This is another hotly debated issue among archery enthusiasts. Some claim that F.O.C. makes little or no difference, others swear that F.O.C. has a profound effect on accuracy. Even the industry experts don't seem to agree, as the ballistic physics for F.O.C. include some rather elastic variables that make finding an "mathematically optimal" F.O.C. very difficult to declare and prove. To make matters worse, we even see a variation in how F.O.C. itself is calculated, depending upon which "expert" you ask. So while we have no interest in the fine points of the debate, we will agree that the tricky issue of F.O.C. is at least worth considering when purchasing a new set of arrows.
With all that said, it is generally believed that an arrow with a high F.O.C. will fly well, but with premature loss of trajectory (nose-diving). While an arrow with a low F.O.C. will hold it's trajectory better, but it will fly erratically. So again, another trade-off for you to consider.