Archery Arrow Spine Calculator
Adjusted Draw Weight (lbs)
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Recommended Spine (deflection)
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Min Arrow Weight (grains)
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Grains per Pound
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How Arrow Spine Selection Works
Arrow spine selection is the process of matching arrow stiffness to your specific bow setup for optimal accuracy and safety. Arrow spine refers to the stiffness of an arrow shaft, measured by how much the shaft deflects (bends) under a standardized test load. According to the World Archery Federation equipment regulations and the ASTM International standard F2031, static spine is measured by supporting a 29-inch arrow shaft at two points 28 inches apart and hanging a 1.94 lb (880 gram) weight from the center. The resulting deflection in thousandths of an inch becomes the spine number -- a 500-spine arrow deflects 0.500 inches, while a 300-spine arrow deflects only 0.300 inches and is therefore stiffer.
Correct spine matching is critical because an arrow bends significantly during the shot cycle -- a phenomenon called the archer's paradox. When released from a compound or recurve bow, the string pushes the nock forward while the arrow tip's inertia resists, causing the shaft to flex around the bow's riser. A properly spined arrow recovers from this flex quickly and flies straight. Too-weak arrows flex excessively and fishtail, while too-stiff arrows fail to flex enough to clear the riser cleanly. This calculator accounts for draw weight, draw length, arrow length, and point weight to recommend the correct spine group, giving you a solid starting point that you can fine-tune through paper tuning and bare-shaft testing. Use it alongside our force calculator to understand the physics of arrow acceleration.
How Arrow Spine Is Calculated
This calculator determines the recommended spine by computing an adjusted draw weight that accounts for all factors affecting dynamic spine:
Adjusted Draw Weight = Draw Weight + (Draw Length - 28) x 2 + Point Weight Adjustment - Arrow Length Adjustment
Min Arrow Weight (grains) = Draw Weight x 5
Grains Per Pound = Total Arrow Weight / Draw Weight
Adjustment factors:
- Draw length: +/- 2 lbs per inch from the standard 28-inch reference
- Point weight: +5 lbs per 25 grains over 125 grains (heavier points weaken effective spine)
- Arrow length: -5 lbs if arrow extends more than 1 inch past draw length
- Spine ranges: 300 (70+ lbs adj), 340 (60-69), 400 (50-59), 500 (40-49), 600 (30-39), 700 (20-29)
Worked example: A compound bow at 50 lbs draw weight, 29-inch draw length, 30-inch arrows, and 100-grain field points. Adjusted weight = 50 + (29-28) x 2 = 52 lbs. Point weight is under 125, so no point adjustment. Arrow is 1 inch longer than draw, which is within tolerance (no adjustment). Adjusted weight = 52 lbs, which falls in the 400-spine range. Minimum arrow weight = 50 x 5 = 250 grains. This is a typical setup for a compound target shooter.
Key Archery Terms You Should Know
- Static spine: The stiffness of an arrow measured by the ASTM deflection test with the shaft supported at 28 inches and loaded with 1.94 lbs at center. This is the number printed on the arrow (e.g., 400, 500).
- Dynamic spine: The effective stiffness of an arrow as it actually behaves during the shot, influenced by draw weight, point weight, arrow length, cam aggressiveness, and string material. Dynamic spine is what truly determines arrow flight.
- Archer's paradox: The phenomenon where an arrow bends around the bow's riser upon release due to the string pushing the nock while the point resists. Proper spine selection ensures the arrow recovers from this flex and flies straight.
- FOC (Front of Center): The percentage of the arrow's balance point forward of the shaft's physical center. Target arrows typically have 8-12% FOC, while hunting arrows benefit from 10-15% FOC for better broadhead flight and penetration.
- Grains per inch (GPI): The weight of the arrow shaft per inch of length. Combined with component weights (nock, insert, vanes, point), this determines total arrow weight.
Arrow Spine Chart by Draw Weight
The following table shows recommended spine ranges for compound bows at standard 28-inch draw length with 100-grain points. Data compiled from major manufacturers including Easton, Gold Tip, and Carbon Express:
| Draw Weight (lbs) | Recommended Spine | Min Arrow Weight | Typical Use | Approx Arrow Speed |
|---|---|---|---|---|
| 20-29 lbs | 700-800 | 100-145 gr | Youth / Beginner | 150-200 fps |
| 30-39 lbs | 600 | 150-195 gr | Target recurve | 200-240 fps |
| 40-49 lbs | 500 | 200-245 gr | Target compound / light hunting | 240-275 fps |
| 50-59 lbs | 400 | 250-295 gr | Hunting compound | 275-300 fps |
| 60-69 lbs | 340 | 300-345 gr | Hunting compound | 290-320 fps |
| 70-80 lbs | 300 | 350-400 gr | Max performance hunting | 300-340 fps |
Source: Compiled from Easton Arrow Shaft Selection Charts and Gold Tip spine recommendations. Actual speeds depend on bow efficiency (IBO rating), arrow weight, and draw length.
Practical Arrow Spine Examples
Example 1 -- Whitetail deer hunter: A hunter shoots a 60 lb compound bow with 29-inch draw length and wants to use 125-grain broadheads on 29.5-inch arrows. Adjusted draw weight = 60 + (29-28) x 2 = 62 lbs. No point adjustment needed (125 grain is the threshold). Arrow is 0.5 inches over draw length (no adjustment). At 62 lbs adjusted, the recommendation is 340 spine. Minimum arrow weight = 60 x 5 = 300 grains. A good choice would be a 340-spine carbon arrow at 8.5 GPI (8.5 x 29.5 = 250.75 gr shaft) plus 125 gr broadhead plus 50 gr in components = approximately 426 grains total (7.1 GPP).
Example 2 -- Target archer: A competitive compound target archer shoots 50 lbs at 27-inch draw with 100-grain field points on 28-inch arrows. Adjusted weight = 50 + (27-28) x 2 = 48 lbs. At 48 lbs adjusted, the recommendation is 500 spine. The archer selects lightweight 500-spine arrows at 6.0 GPI for maximum speed and flat trajectory, totaling about 300 grains (6.0 GPP) for indoor and outdoor target competition.
Example 3 -- Elk hunter with heavy arrows: An elk hunter shoots 70 lbs with 30-inch draw and 200-grain single-bevel broadheads on 31-inch arrows. Adjusted weight = 70 + (30-28) x 2 = 74 lbs. Point weight adjustment: (200-125)/25 x 5 = +15 lbs. Arrow length penalty: -5 lbs (arrow is 1+ inch past draw). Final adjusted = 74 + 15 - 5 = 84 lbs, firmly in 300-spine territory. The archer uses heavy 300-spine shafts at 12 GPI for a total arrow weight of approximately 575 grains (8.2 GPP), prioritizing penetration over speed for large, tough-boned game.
Arrow Tuning Tips and Strategies
- Paper tune to verify spine: Shoot through paper at 6 feet. A tear showing the nock right of the point (for right-handed shooters) indicates too-weak spine. Nock left indicates too-stiff. A clean bullet hole confirms correct dynamic spine.
- Bare-shaft test for fine-tuning: Shoot one bare shaft (no vanes) alongside fletched arrows at 20 yards. If the bare shaft impacts left of the fletched group (right-handed shooter), the shaft is too stiff. Right indicates too weak. Adjust point weight in 25-grain increments to fine-tune.
- Start stiffer when in doubt: It is easier to weaken an arrow's effective spine (by adding point weight or using longer shafts) than to stiffen it. When between two spine groups, choose the stiffer option.
- Consider broadhead weight early: If you plan to hunt with 125-grain or heavier broadheads, factor that weight into your spine selection from the start. Switching from 100-grain field points to 150-grain broadheads significantly weakens dynamic spine.
- Cut arrows to the correct length: Arrows should extend 1-2 inches past the arrow rest at full draw. Cutting shorter than necessary wastes arrows and makes them too stiff; leaving them long makes them too weak and increases wind drift.
- Always meet the minimum GPP: Never shoot arrows below 5 grains per pound of draw weight. Underweight arrows cause excessive vibration, void bow warranties, and can result in catastrophic bow failure (dry-fire equivalent damage). Use our kinetic energy calculator to compute the arrow's energy at your expected velocity.
Frequently Asked Questions
What is arrow spine and how is it measured?
Arrow spine is the stiffness rating of an arrow shaft, measured by how much it deflects under a standard test. In the ASTM F2031 static spine test, a 1.94 lb (880g) weight is hung from the center of a 29-inch shaft supported at both ends. The deflection in thousandths of an inch becomes the spine number -- a 500-spine arrow deflects 0.500 inches. Lower numbers mean stiffer arrows. Dynamic spine, which accounts for actual shot behavior, is influenced by draw weight, point weight, and arrow length.
What happens if I use arrows with the wrong spine?
Using arrows with incorrect spine causes erratic flight and poor accuracy. Too-weak arrows (high spine number, too flexible) fishtail in flight, producing inconsistent grouping. Too-stiff arrows (low spine number) do not flex enough to clear the riser, causing them to fly off-center. Either mismatch reduces accuracy significantly and may pose safety concerns with broadhead-tipped hunting arrows.
How does draw weight affect arrow spine selection?
Higher draw weight requires stiffer arrows because more force is applied at release, causing greater flex. As a general rule, every 5 pounds of additional draw weight shifts the recommendation one spine group stiffer: 50 lbs typically needs 400 spine, 60 lbs needs 340 spine, and 70 lbs needs 300 spine. Use our force calculator to understand the physics of the forces involved during arrow acceleration.
What is grains per pound and how much should my arrows weigh?
Grains per pound (GPP) is total arrow weight in grains divided by draw weight in pounds. The absolute minimum for safety is 5 GPP, as required by most bow manufacturers to prevent dry-fire damage. A 6-8 GPP range is ideal for hunting, providing good kinetic energy. Target archers often shoot 5-6 GPP for flatter trajectory, while heavy-arrow advocates use 10+ GPP for maximum penetration and quieter shots.
How does arrow length affect spine?
Longer arrows are effectively weaker because there is more shaft to flex. Each inch longer than the standard 29-inch test length acts as if the spine number is higher (more flexible). As a rule of thumb, each additional inch weakens effective spine by about 5 pounds of equivalent draw weight. Cutting arrows shorter makes them stiffer. This is why arrow length must be factored into spine selection.
What arrow spine do I need for a recurve bow versus a compound bow?
Recurve bows require weaker (more flexible) arrows than compounds at the same draw weight because the arrow must flex around the riser during the archer's paradox. A 40 lb recurve might need 600-700 spine arrows, while a 40 lb compound with a center-shot riser needs 500 spine. Compound bows with aggressive cams also generate more acceleration, requiring slightly stiffer arrows. Always consult manufacturer charts specific to your bow type.