Arrow FOC Calculator (Front of Center)
FOC percent equals the balance point minus half the arrow length, divided by the arrow length, times 100. A 28 inch arrow balancing at 16.5 inches has an FOC of 8.9 percent.
Front of centre describes where the weight sits along an arrow, not how much of it there is. It is the difference between a dart and a broom handle: move mass forward and the arrow steers itself, move it back and it wanders. This calculator returns FOC percentage and tells you which use case your number belongs to, along with what you gave up to get there.
The formula, and the one place people get it wrong
FOC percent equals balance point minus half the arrow length, divided by the arrow length, times 100. A 28 inch arrow that balances 16.5 inches from the nock throat is 2.5 inches forward of its geometric centre, and 2.5 divided by 28 is 8.9 percent.
The trap is the length measurement. Arrow length is measured from the throat of the nock to the end of the shaft, which for a modern build is the back of the insert. The point is not counted, even though it stays screwed in while you find the balance point. Measuring to the tip of a broadhead instead adds an inch and a half to the denominator and shifts the assumed centre back, and it typically understates FOC by three to four points. If your number looks strangely low next to everyone else's, this is almost always why.
To find the balance point in practice, lay the finished arrow across a narrow straight edge, a ruler on edge or the back of a knife, and slide it until it sits level. Mark the contact point and measure back to the nock throat. Doing it three times and averaging removes most of the error, because a sixteenth of an inch of slop moves the result by about 0.2 percentage points.
FOC ranges and what they are for
| FOC range | Category | Typical use | Tradeoff |
|---|---|---|---|
| under 7% | Low | Rarely intentional, usually a light point on a long shaft | Slow to stabilise, poor broadhead control |
| 7 to 12% | Normal target | Indoor, outdoor target, 3D, field points | Flattest trajectory, least forgiving of a bad release |
| 10 to 15% | Hunting | Fixed or mechanical broadheads at normal ranges | The mainstream compromise, no real downside |
| 15 to 19% | High | Heavy fixed broadheads, larger game | Noticeably steeper drop past 40 yards |
| 19% and up | EFOC and beyond | Penetration-first traditional and heavy hunting builds | Slow, demands correct spine, short effective range |
The bands overlap on purpose. An archer shooting 3D targets with field points and an archer shooting fixed broadheads at 30 yards can both be right at 12 percent, and neither should feel the need to move. The useful question is not "is my FOC good" but "does my FOC match what this arrow is being asked to do".
The tradeoff, stated plainly
Everything you gain from FOC is paid for in trajectory. Weight at the front makes the arrow recover from the shot faster, resist steering input from a fixed blade broadhead, and hold its line better in a crosswind. All three are real. But the only practical way to raise FOC on a built arrow is adding mass at the point, and mass costs velocity at roughly one foot per second for every three grains.
Work it through: moving from a 100 grain point to a 175 grain point on a 28 inch arrow raises FOC by about six percentage points and adds 75 grains, costing roughly 25 fps. Inside 25 yards that is invisible. At 50 yards it is several inches of additional drop and a meaningfully larger penalty for misjudging distance. Whether that is a good trade depends entirely on how far you shoot and how accurately you know the range.
There is a second cost that is easier to forget. Adding point weight weakens dynamic spine. Every 25 grains up front behaves roughly like adding five pounds of bow weight, so a build that gains six FOC points may well have moved a full spine group. Recheck it with the arrow spine calculator before you conclude your broadheads are the problem.
Ways to raise FOC, ranked by how much they cost you
- Heavier point or broadhead. The largest and simplest lever. About 2 percentage points per 25 grains on a 28 inch arrow.
- Brass or steel insert instead of aluminium. Adds 30 to 75 grains right where you want it, and costs almost nothing in fitting time.
- Cut the arrow shorter. Removes mass from the rear and shortens the lever arm, so it raises FOC twice over. Only available if your draw length and rest allow it.
- Lighter nock and fletching. Small effect, roughly half a point, but free if you were choosing vanes anyway.
- A weight tube or internal footing. Effective but fiddly, and easy to end up with an arrow you cannot repeat across a dozen.
Notice that four of the five also change total arrow mass, which is why FOC should never be tuned in isolation. Confirm the finished weight with the total arrow weight calculator, including the 5 grains per pound safety floor, before you order components in bulk.
This is the practical consequence of a heavy front end that rarely gets mentioned: the steeper your trajectory, the more a range error costs you. An arrow that drops sharply turns a ten yard misjudgement into a miss that a flatter setup would have absorbed. If you are deliberately building toward the high end of the FOC range, a rangefinder is not an accessory, it is the thing that makes the build work.
The Ashby research, in context
Most of the enthusiasm for extreme FOC traces to the penetration studies published by Dr. Ed Ashby, which concluded that arrows above roughly 19 percent FOC penetrated measurably better on large animals, with further gains above 25 percent. The work is genuinely interesting and it is the most systematic body of arrow penetration research anyone has published.
It also deserves the honest caveat. High FOC builds in those tests were also heavier overall, ran stronger shafts, and used single bevel broadheads, so isolating FOC as the cause is difficult. The reasonable position is that FOC clearly matters somewhat, that total mass and broadhead quality matter at least as much, and that an archer who cannot hit consistently at 30 yards will not be rescued by three more percentage points. Confirm what your setup actually delivers with the kinetic energy calculator, which also returns momentum, the figure heavy arrow advocates prefer.
Related tools
- Total arrow weight calculator: FOC and finished mass move together
- Arrow spine calculator: recheck spine after any point weight change
- Arrow speed calculator: what the extra front weight costs in fps
- Best archery rangefinders: the fix for the trajectory penalty
Frequently asked questions
How do you calculate FOC on an arrow?
Balance the finished arrow on a straight edge and mark the balance point. FOC percent equals the balance point measured from the throat of the nock, minus half the arrow length, divided by the arrow length, multiplied by 100. Arrow length here is throat of nock to the back of the insert, with the point excluded, even though the point is still screwed in while you balance it.
What is a good FOC percentage?
Target and 3D archers generally run 7 to 12 percent, which balances a flat trajectory against enough forward weight to stabilise quickly. Hunting setups usually sit at 10 to 15 percent for better broadhead control. Extreme FOC starts at 19 percent and is a deliberate penetration-first choice. There is no single correct number, only a number that matches what the arrow has to do.
Does higher FOC really improve penetration?
The strongest published case comes from Dr. Ed Ashby, whose arrow penetration studies concluded that FOC above 19 percent produced measurably deeper penetration on large animals. The finding is credible but not universally accepted, and its effects are hardest to separate from the heavier total mass and stronger shafts that usually accompany high FOC builds. Treat it as a real effect of uncertain size, not settled physics.
How much point weight do I need to raise FOC?
On a typical 28 inch hunting arrow, every 25 grains added to the point raises FOC by roughly 2 percentage points. Going from a 100 to a 150 grain point moves most builds about 4 points, which is often the whole difference between a target-range FOC and a hunting-range one. Adding weight forward also weakens dynamic spine, so recheck your spine after any point weight change.
Does high FOC make arrows drop more?
Indirectly, yes. FOC by itself is a balance measurement and does not change trajectory, but the usual way to raise it is by adding point weight, and that extra mass costs velocity at roughly one foot per second for every three grains. A build that gains six FOC points typically gives up 15 to 25 fps, which is noticeable past 40 yards and irrelevant inside 25.
Where exactly do I measure arrow length for FOC?
From the throat of the nock, meaning the deepest part of the groove where the string sits, to the end of the shaft at the back of the insert. The point is excluded from the length measurement but stays installed while you find the balance point. Measuring to the tip of the broadhead instead is the most common error and it understates FOC by several points.
How we choose: we compare published manufacturer specifications, verified owner reviews, and pro shop guidance. We do not test gear in person. Draw weight and arrow spine affect safety, so follow your bow manufacturer's specifications and have setup work checked by a qualified pro shop.