Arrow Spine Calculator
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Calculator
Use this arrow spine calculator to find a starting carbon spine (700, 600, 500, 400, 340, 300) from your bow type, draw weight, arrow length and point weight. It follows the same adjustments manufacturer charts use, so the result is a starting shaft to confirm with a bare-shaft or paper test, not a final answer.
Estimate a starting arrow spine
How it works: the calculator turns your setup into an effective draw weight, then reads the spine band the way a manufacturer chart does.
- Each inch of arrow over 28 inches counts as about 4 lb more draw weight.
- Each 25 grains of point over 100 grains counts as about 3 lb more.
- Finger shooters need a weaker spine than a release shooter at the same weight.
- Confirm the result on the chart for your exact shaft model before you cut a dozen.
How the arrow spine calculator works
Every spine chart is built around one reference setup: a 28-inch arrow with a 100-grain point shot from a compound bow with a release aid. Anything that makes the arrow act weaker (a longer shaft, a heavier point) is treated as extra draw weight, and anything that makes the bow push the arrow more gently (a finger release, a recurve without cams) is treated as less draw weight. The result is an effective draw weight that you read against the spine bands below.
| Effective draw weight | Starting carbon spine | Typical aluminum sizes |
|---|---|---|
| Under 22 lb | 1000 or weaker | 1816, 1913 |
| 22 to 30 lb | 700 | 1916, 2013 |
| 30 to 40 lb | 600 | 2016, 2114 |
| 40 to 50 lb | 500 | 2117, 2213 |
| 50 to 60 lb | 400 | 2216, 2314 |
| 60 to 70 lb | 340 | 2315, 2413 |
| 70 to 80 lb | 300 | 2512, 2514 |
| Over 80 lb | 250 | 2613 |
Bands follow typical Easton, Gold Tip and Victory charts at a 100-grain point. Your shaft's chart is the final word.
Worked example
A 55 lb compound with a release aid, a 29-inch arrow and a 125-grain point.
| Step | Adjustment | Running total |
|---|---|---|
| Peak draw weight | 55 lb | 55 lb |
| Arrow length 29 in (1 in over 28) | +4 lb | 59 lb |
| Point 125 gr (25 gr over 100) | +3 lb | 62 lb |
| Release aid | 0 lb | 62 lb |
| Result | 60 to 70 lb band | 340 spine |
The same bow with a 27-inch arrow and a 100-grain point would land at 51 lb and a 400 spine.
What to do with the result
Weak arrows usually show a nock-left tear for a right-handed shooter; stiff arrows show nock-right. Compounds are far less sensitive than recurves.
Recurve and traditional bows
A recurve or longbow releases the string with fingers, so the string leaves the fingers sideways and the arrow has to flex around the riser. That is why finger shooters need a weaker spine than a compound shooter at the same weight, and why the calculator subtracts a few pounds for those bow types. Measure the real weight at your draw length with a scale (see how to measure bow draw weight), because the number on the limb is the weight at 28 inches only. If you draw 26 inches, the bow is roughly 4 to 5 lb lighter than the label.
Traditional bows shot off the shelf, and bows that are not cut past center, are more forgiving of a slightly weak arrow than a stiff one. When in doubt on a longbow, test the weaker option first.
Signs your spine is wrong
| What you see | Likely spine problem | Fix |
|---|---|---|
| Bare shafts land left of fletched (right-handed) | Too weak | Stiffer shaft, lighter point, or shorter arrow |
| Bare shafts land right of fletched (right-handed) | Too stiff | Weaker shaft, heavier point, or longer arrow |
| Broadheads plane away from field points | Usually weak, sometimes stiff | Re-test spine, then check FOC and fletching |
| Groups open up as draw weight goes up | Too weak | Move one spine stiffer when you add limbs or turn limb bolts in |
| Loud, harsh shot with a light arrow | Arrow too light overall | Check grains per pound with the arrow weight calculator |
Left and right are mirrored for left-handed archers.
Related tools and guides
Spine is one of four numbers that make an arrow work. Set the others with these pages.
Frequently asked questions
Short answers to the questions people ask most about this tool.
What spine do I need for a 50 lb compound bow?
With a 28-inch arrow and a 100-grain point, a 50 lb compound with a release sits on the 500/400 line. Most shooters end up on a 400 spine at 29 inches or a 500 at 27 inches.
What spine do I need for a 40 lb recurve?
A 40 lb recurve measured at your draw length, with a 28-inch arrow and a 100-grain point, usually starts at a 600 spine. Longer arrows or heavier points move it to 500.
Is a 400 or 500 spine stiffer?
A 400 is stiffer. Spine is deflection in thousandths of an inch, so a lower number bends less.
Does arrow length change the spine I need?
Yes. A longer arrow flexes more, so it acts weaker. Every inch over 28 inches is worth about 4 lb of draw weight on the chart.
Does point weight change arrow spine?
Yes. A heavier point makes the shaft act weaker. Each 25 grains over 100 grains is worth about 3 lb of draw weight.
Can I use this calculator for crossbow bolts?
No. Crossbow bolts are very stiff and are chosen by length and total weight from the crossbow manual, not by spine. Use the crossbow bolt weight chart instead.
Why does the maker's chart give a different spine?
Charts differ because each shaft model has its own real deflection, and some makers build in more or less safety margin. The calculator gets you into the right band; the chart for your shaft picks the exact one.
References
These sources support the formulas and guidance on this page.