# Arrow FOC Calculator by Spine

> See how arrow spine and shaft GPI affect FOC. Compare typical FOC results for common spine sizes so you can pick a shaft that balances well.

Canonical page: https://archeryprotools.com/calculators/arrow-foc-calculator-by-spine (the interactive calculator runs on that page)

Calculate FOC and see typical ranges organized by spine class. Enter your build, or start from a spine preset to see the FOC range that spine class usually produces.

## 🎯 Arrow FOC Calculator

Enter your build details. All fields influence total weight and FOC.

FOC & Build Summary

**ATA/AMO formula:**

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.

## 📊 FOC by Spine Class

Typical FOC ranges for each spine class at common component stacks. Values are starting points — your actual FOC depends on exact GPI, length, and components.

| Spine | Common GPI | Standard Insert + 100 gr Point | Brass Insert + 125 gr Point | Heavy Insert + 150 gr Point | Typical FOC Range |
| --- | --- | --- | --- | --- | --- |
| 200 | 11–13 gpi | 12–14% | 15–17% | 17–19% | 12–19% |
| 250 | 10–12 gpi | 11–13% | 14–16% | 16–18% | 11–18% |
| 300 | 9–11 gpi | 10–13% | 13–15% | 15–17% | 10–17% |
| 340 | 8.5–10 gpi | 10–12% | 12–15% | 14–17% | 10–17% |
| 400 | 7.5–9 gpi | 9–11% | 11–14% | 13–16% | 9–16% |
| 500 | 6.5–8 gpi | 8–10% | 10–12% | 11–14% | 8–14% |
| 600 | 5.5–7 gpi | 7–9% | 9–11% | 10–13% | 7–13% |

**Notice the pattern:** heavier-stiffness shafts (200, 250, 300) naturally produce higher FOC because their thicker walls add mass to the shaft — but that mass is distributed evenly, so FOC still depends mostly on how much you concentrate in the front. Brass inserts and heavy points push any spine class into heavy-FOC territory.

## ⚖️ How Spine Choice Affects FOC Strategy

### Stiff shafts (200–300 spine)

Common at 65–85 lb compound draw weights and heavy arrow builds. Because the shaft itself is heavy, adding brass inserts and 150–200 gr points pushes FOC into EFOC territory (19%+) quickly. These builds are ideal for Ashby-style penetration hunting.

### Mid shafts (340–400 spine)

The most common hunting spine across compound bows from 45–65 lb. Mid-weight shafts respond well to brass inserts (50–75 gr) and 125–150 gr points, landing in the 12–16% FOC range — the sweet spot for deer and elk.

### Light shafts (500–600 spine)

Standard for recurve archers at 28–42 lb and youth compound shooters. Light shafts sit at lower FOC by default because the shaft itself contributes little forward mass. Broadhead tuning focuses on shaft length and point weight rather than insert mass.

## ❓ Frequently Asked Questions

**What FOC does 340 spine produce?**

At typical build lengths and components, 340 spine arrows usually land between 10% and 17% FOC. Standard aluminum insert + 100 gr point: 10–12%. Brass insert + 125 gr point: 12–15%. Heavy insert + 150 gr point: 14–17%.

**Does spine affect FOC?**

Indirectly. Spine does not appear in the FOC formula, but heavier-stiffness shafts (lower spine numbers) have thicker walls and higher GPI. That extra shaft mass stays at the center, so FOC stays close to midpoint unless front components are heavy. The bigger effect is which components you can safely use — stiffer shafts tolerate heavier front loads.

**Can I calculate FOC and spine together?**

Yes, but they are separate calculations. Use our [spine calculator](https://archeryprotools.com/calculators/arrow-spine-calculator) to match shaft to bow, then use this page to plan FOC.

**What is the FOC range for Easton Axis 340?**

Easton Axis 340 (8.1 gpi) typically produces 11–16% FOC depending on insert and point. The exact number depends on arrow length and total rear component weight.

**How do I get 20% FOC without going to a stiffer spine?**

You can achieve 20% FOC on 340 or 400 spine shafts by using heavy brass inserts (100 gr+) and heavy points (150–200 gr). The catch is that spine effectively weakens with point weight — check our [spine chart](https://archeryprotools.com/reference/arrow-spine-chart-by-draw-weight) to ensure tune stays clean.

**What is the ideal FOC for a spine-matched build?**

For deer hunting, 10–15% FOC. For elk or large game, 15–19%. For penetration-focused builds (Ashby-style), 19–25%. The "best" FOC depends on your target and how much broadhead control you need at distance.

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