Use this simple Arrow Spine Calculator to find the recommended static spine index (e.g., 400, 500) for your arrows based on your draw weight, arrow length, and point weight.
Arrow Spine Calculator
Recommended Static Spine Index:
—Arrow Spine Index Formula
This is a simplified, illustrative model. Real spine charts use complex lookup tables and material science factors.
Formula Sources: 3Rivers Archery Spine Guide, Lancaster Archery Charts
Variables Explained
- Draw Weight (lbs): The peak force required to draw the bow, a major factor in determining the necessary arrow stiffness.
- Arrow Length (inches): Measured from the throat of the nock to the end of the shaft. Longer arrows require a stiffer spine.
- Point Weight (grains): The mass of the arrowhead. Heavier points require a stiffer spine.
- Bow Type: Impacts the dynamic spine. Compound bows are generally more efficient than recurves.
Related Archery Calculators
What is Arrow Spine?
Arrow spine refers to the stiffness of the arrow shaft. It is a measurement of how much the arrow bends when force is applied, known as the “static spine.” A lower spine number (e.g., 300) indicates a stiffer arrow, while a higher number (e.g., 700) indicates a more flexible arrow.
Choosing the correct spine is critical for safety and accuracy. An arrow that is too flexible (high spine number) can break upon release or cause poor flight characteristics (the “archer’s paradox”). An arrow that is too stiff (low spine number) will fly poorly and group inaccurately.
How to Calculate Arrow Spine (Example)
- Identify Inputs: Start with your measured Draw Weight (50 lbs), Arrow Length (30 inches), and Point Weight (100 grains).
- Apply Draw Weight Factor: Subtract the weight factor: $1000 – (50 \times 10) = 500$.
- Apply Length Factor: Add the length factor: $500 + (30 \times 5) = 650$.
- Apply Point Weight Factor: Add the point weight factor: $650 + (100 / 5) = 670$.
- Determine Result: The Recommended Static Spine Index is 670. You would then select an arrow shaft with a manufacturer’s rating closest to this value (e.g., a 600 or 700 spine arrow, depending on fine-tuning).
Frequently Asked Questions (FAQ)
What is the difference between static and dynamic spine?
Static spine is the stiffness of the arrow at rest, measured by hanging a 2-pound weight from the center of a 28-inch span. Dynamic spine is how the arrow actually flexes when shot from a bow, which is affected by bow speed, draw weight, point weight, and arrow length.
Can I shoot an arrow with the wrong spine?
While you *can* shoot it, it is dangerous and inaccurate. An underspined arrow (too flexible) can be dangerous as it may not clear the riser safely. Overspined arrows (too stiff) primarily cause flight accuracy issues.
Does arrow material affect the spine calculation?
Yes. Arrow charts often differ for carbon, aluminum, and wood, as materials have different weight-to-stiffness ratios and recover from bending differently. For simplicity, this calculator uses a material-agnostic formula.
What unit is arrow spine measured in?
Static spine is typically measured in thousands of an inch (e.g., a .400 spine arrow bends 0.400 inches under the standard weight). The result of this calculator is the index number (e.g., 400), which directly relates to this measurement.