Model extreme FOC (EFOC) and heavy arrow builds inspired by the Ashby Foundation penetration studies. Input your components and instantly see FOC class, total grain weight, and a build-tuning note.
Enter your finished build components. This calculator reports FOC, total grains, GPP, and an EFOC classification.
FOC% = ((A − L/2) ÷ L) × 100
A = distance from nock throat to balance point; L = total shaft length. Center of mass is estimated from component masses distributed along the arrow.
Dr. Ed Ashby's 2005–2022 research on terminal arrow performance classifies FOC into tiers. The classification below is a practical shorthand used by heavy-arrow builders.
| FOC Range | Class | Typical Use | Tuning Notes |
|---|---|---|---|
| < 7% | Rear Heavy | Not recommended for hunting | Broadhead control is poor; expect planing at distance |
| 7–10% | Standard | Field / target archery | Fine for field points at moderate distance |
| 10–15% | Hunting Standard | Deer, hog, antelope | Most factory arrows fall here; balanced flight |
| 15–19% | Heavy FOC | Elk, black bear, mule deer | Improved penetration with matched spine |
| 19–25% | EFOC | High-penetration large game | Requires stiff spine and matched tune |
| > 25% | Ultra EFOC | Specialty builds | Very demanding on dynamic spine and arrow straightness |
Source: Dr. Ed Ashby, Bowhunting Foundation — Arrow Lethality research series (2005–2022).
If you are building toward a specific FOC class, these component combinations are proven starting points.
| Target FOC | Shaft GPI | Insert | Point | Typical Total Weight |
|---|---|---|---|---|
| 10–12% | 7–8 | 12–20 gr alum | 100–125 gr | 380–420 gr |
| 13–15% | 8–9 | 50 gr brass | 125 gr | 425–475 gr |
| 16–19% | 9–10 | 75–100 gr brass | 125–150 gr | 480–550 gr |
| 20–24% (EFOC) | 10–11 | 100–150 gr brass/steel | 150–200 gr | 550–650 gr |
| 25%+ (Ultra) | 11–12 | 150–200 gr steel | 200–300 gr | 650–800 gr |
Heavier builds require stiffer spine. Re-check dynamic spine after any tip-weight increase above 25 grains using our arrow spine calculator.
EFOC stands for Extreme Front of Center — typically arrows above 19% FOC. The term became popular through Dr. Ed Ashby's penetration studies, which documented improved terminal performance on large game when arrow mass is heavily concentrated in the front.
Ranch Fairy is a popular YouTube and podcast channel that advocates heavy-arrow, high-FOC hunting setups. Their FOC tool uses the same ATA/AMO formula as this calculator. For a fair comparison, use identical input values in both.
FOC itself does not reduce speed — total arrow mass does. Because high FOC is typically achieved by adding front weight, the total arrow weight usually rises, and speed drops. Use our arrow speed calculator to model the tradeoff.
Many EFOC hunters run 55–70 lb. Below 50 lb the launch is still doable but arrow mass must be reduced, which limits how extreme the FOC can go while maintaining ethical trajectory.
Generally, EFOC pairs best with fixed-blade cut-on-contact broadheads. Mechanicals lose some of the penetration advantage that EFOC builds were designed to capture.
Run your draw weight, draw length, and new point weight through our arrow spine calculator. If the result shifts to a stiffer spine class, your current shaft will likely tune poorly with the heavier point.