The Arrow Spine Calculator determines the correct arrow spine for any bow setup from draw weight, arrow length, bow type, and point weight. Matching spine to your bow prevents fishtailing and inconsistent groups — the single most important arrow selection variable for accuracy.
470
363
gr
7.3
gpp
13.1
%
470
363
gr
7.3
gpp
13.1
%
Every archer who has ever blamed their bow for bad groups has probably been shooting incorrectly spined arrows. Arrow spine — the stiffness of the shaft — must match the energy delivered by the bow; too stiff or too flexible and the arrow bends excessively during the shot, departing the bow at an inconsistent angle that no amount of aiming can compensate for. The calculator for arrow spine takes your bow's specifications and gives you the correct spine rating for your shooting setup.
Arrow spine is measured by the deflection (in inches × 1000) of a 29-inch arrow shaft under a 1.94 lb load at its center — the AMO standard deflection test. A "340 spine" arrow deflects 0.340 inches; a "500 spine" deflects 0.500 inches. Counterintuitively, a higher spine number means a more flexible (weaker) arrow; a lower number means a stiffer shaft. Key reference points:
The bow draw length calculator determines draw length, which directly affects the arrow length input to the spine calculation.
Spine selection is based on an adjusted draw weight that accounts for all factors that affect the dynamic spine requirement:
The adjusted draw weight is then matched against standard spine charts (ATA chart, manufacturer charts) to select the correct spine. Use this online calculator to apply all adjustments simultaneously. The sports equipment calculators category covers archery and other precision sports equipment selection tools.
The static spine (AMO deflection test) is a standardized measurement, but what matters in flight is dynamic spine — how much the arrow bends when launched. During the shot, the bowstring applies force to the arrow's nock while the point is still in contact with the arrow rest, causing the arrow to buckle (archer's paradox). A correctly spined arrow oscillates through this flex and leaves the rest flying straight; an incorrectly spined arrow departs at an angle that no sight adjustment can fix. The compound bow's cams and let-off mechanism, release aid vs. fingers, and rest type all affect dynamic spine differently — which is why the adjustments in spine selection formulas are empirically derived rather than purely theoretical.
Spine selection from a chart is the starting point, not the endpoint. Paper tuning — shooting through paper at close range to observe the tear pattern — reveals whether the arrow is entering at a perfect "bullet hole" (correct spine, rest height, and centershot) or tearing left/right (horizontal nock travel, indicating spine issues) or up/down (vertical issues). Fine-tuning is achieved by adjusting arrow rest position, nocking point height, and arrow spine selection (including use of different inserts to change point weight). Walk-back tuning provides a systematic method to achieve perfect left-right alignment at all distances.
The recommended spine is calculated using draw weight as the primary factor with adjustments:
Length adjustment (per inch from 28" reference):
$$\text{Length Adj} = (\text{Arrow Length} - 28) \times 5$$
Point weight adjustment (per 25 grains from 100gr reference):
$$\text{Point Adj} = \frac{\text{Point Weight} - 100}{25} \times 15$$
Compound bow spine:
$$\text{Spine} = \frac{1200}{\text{Draw Weight} + \text{Length Adj} + \frac{\text{Point Adj}}{2}} \times 100$$
Recurve bow spine (15% reduction for softer force curve):
$$\text{Spine} = \frac{1200}{\text{Draw Weight} \times 0.85 + \text{Length Adj} + \frac{\text{Point Adj}}{2}} \times 100$$
FOC (Front of Center):
$$\text{FOC (\%)} = \left(\frac{\text{Balance Point from Nock}}{\text{Arrow Length}} - 0.5\right) \times 100$$
Arrow spines are sold in standard increments (typically 300, 340, 400, 500, 600, 700, 800). Choose the nearest available spine to your calculated value. If between two sizes, stiffer is generally safer than weaker. A GPP below 5 risks bow damage; aim for at least 5-6 GPP minimum. FOC between 7-15% is ideal for hunting; 8-12% for target archery. If your calculated spine seems far from what spine charts recommend, verify your draw weight with a bow scale and re-measure your draw length. Fine-tuning spine selection is ultimately done through paper tuning and bare shaft testing.
Inputs
Results
A 60-pound compound at 29-inch draw with 125-grain broadheads needs a stiff 350-spine arrow weighing about 413 grains (6.9 GPP) with good 11.2% FOC for hunting.
Inputs
Results
A 38-pound recurve with standard 100-grain points requires a 600-spine arrow, a common spine for intermediate recurve archers with balanced 9.5% FOC.
Arrow spine measures the static stiffness of an arrow shaft. It is determined by suspending a 28-inch section of the shaft and hanging a 1.94-pound weight from the center. The amount of deflection in thousandths of an inch becomes the spine rating. A 400-spine arrow deflects 0.400 inches. This standardized test, defined by the ATA, allows comparison across manufacturers. Dynamic spine (how the arrow behaves during launch) is related but influenced by additional factors like point weight and arrow length.
Compound bows store and release energy more efficiently than recurve bows at the same peak draw weight due to their cam systems and let-off. A compound bow generates a longer power stroke with more aggressive force application, putting more energy into the arrow. This means a 50-pound compound effectively acts like a 55-60 pound recurve in terms of arrow flex. Therefore, compound archers need stiffer spines than recurve archers at the same draw weight.
Front of Center (FOC) is the percentage of the arrow's balance point forward of its geometric center. Higher FOC means more weight is concentrated at the front, which improves arrow stability, forgiveness to bow tuning errors, and broadhead penetration on game. Target archers typically aim for 8-12% FOC, while hunters prefer 10-15% or even higher. FOC below 7% can result in erratic arrow flight, especially with broadheads.
Arrows that are too stiff will not flex enough to clear the riser properly, resulting in a right-miss pattern for right-handed archers (left for lefties). While too-stiff arrows are generally safer than too-weak arrows, they reduce accuracy and forgiveness. Slightly stiff arrows can often be tuned to shoot well by adjusting point weight (adding weight weakens effective spine) or adjusting the bow's center shot and rest position.
Cutting an arrow shorter makes it effectively stiffer because there is less material to flex. Each inch removed from the front of the arrow increases effective spine by approximately one spine group. For example, a 400-spine arrow at 30 inches that is cut to 28 inches will behave closer to a 350-spine arrow. Always determine your final cut length before selecting spine, and use that cut length in spine calculations.
Most bow manufacturers specify a minimum of 5 grains per pound of draw weight (GPP). For a 60-pound bow, that is 300 grains minimum. Shooting arrows below this threshold can cause damage similar to dry-firing the bow, potentially cracking limbs or cams. The IBO minimum for speed testing is also 5 GPP. For hunting, 6-8 GPP provides a good balance of speed and kinetic energy, while heavy arrows (10+ GPP) maximize penetration.
How helpful was this calculator?
5.0/5 (1 rating)
Power Analysis Calculator
Advanced & Specialized Statistical Tools
Effect Size Calculator (Cohen's d)
Advanced & Specialized Statistical Tools
Cronbach's Alpha Calculator
Advanced & Specialized Statistical Tools
Intraclass Correlation Coefficient Calculator
Advanced & Specialized Statistical Tools
Fleiss' Kappa Calculator
Advanced & Specialized Statistical Tools
Cohen's Kappa Calculator
Advanced & Specialized Statistical Tools
