Your Bow
Bows are marked at a 28" draw. Your real draw weight — and everything downstream — depends on how far off 28" you actually draw.
Not sure how to measure your draw length? Quick tutorial
Quick estimate — no bow needed
- Stand with your arms stretched straight out to your sides, like a "T."
- Have someone measure fingertip to fingertip, in inches — that's your wingspan.
- Divide that number by 2.5.
Example: 70" wingspan ÷ 2.5 = 28" draw length. A fast starting point, not an exact number.
Accurate — AMO standard
- Nock a marked "measuring arrow" (or a dowel marked in inches) on the string.
- Come to full draw at your normal anchor point, with proper form — don't overdraw to hit a number.
- Have a helper mark where the arrow crosses the pivot point of the bow (the far edge of the grip).
- Measure nock groove to that mark, then add 1.75". That's your AMO draw length.
No helper or measuring arrow handy? Most archery shops can measure this for you in under a minute.
Advanced: bow setup
Your Arrow & Spine
A recommended spine range, built on the universal wood-shaft-equivalent pound bands every trad shaft is cross-referenced against — then converted to your material.
Advanced: adjustment rules
Balance Point (FOC)
Estimated from component weights before you've even built the arrow — arrow length and point weight are carried over from Step 2.
Diagram is proportional, not to exact physical scale. The balance point (orange) sits forward of the geometric center by an amount that defines your FOC%.
How this is calculated (and where it can be wrong)
This tool uses transparent, widely-used traditional-archery rules of thumb — not a hidden formula, and not a physics simulation of your exact bow. Every number above is one click away from being changed and re-run. Tap any line below to expand it.
Draw weight at your draw length
actual weight = marked weight + (your draw length − marked draw length) × lb-per-inch. The lb-per-inch figure is a community approximation (commonly 2–3) that varies by limb design — treat the result as close, not exact.
Spine range
center = actual draw weight + baseline spine offset + (arrow length − 28) × length-adjust + ((point weight − 125) / 25) × point-adjust + longbow/selfbow adjustment + Fast Flight adjustment, shown as a ± band around that center. Longer arrows and heavier points both flex an arrow more in flight, so both push the recommendation toward a stiffer (higher-pound) shaft. This is a starting bracket for shopping — cross-reference it against your specific shaft brand's chart (wood or carbon), and confirm with bareshaft or paper tuning before committing to a full dozen. Release style, form, and individual bow behavior all shift real-world dynamic spine in ways no calculator fully captures.
The baseline spine offset (default −9.5#) exists because recommended spine isn't a 1:1 match to draw weight — published trad charts consistently recommend a spine several pounds below actual draw weight, not equal to it, and early beta testing confirmed the gap on real, bareshaft-tuned setups. Without it, the tool ran noticeably stiffer than what beta testers were actually finding worked. It's applied before every other adjustment and is editable like the rest, since it's inferred from a limited number of real tuning results and will likely get more precise as more beta feedback comes in.
Two more adjustments live in Step 1's "Advanced: bow setup" and are optional, off by default. A non-center-shot bow (longbow or selfbow) flexes the arrow around the riser more than a center-shot recurve does, so published trad spine charts call for a stiffer spine — commonly around 5# — at the same actual draw weight; select "Longbow / selfbow" to add it. A Fast Flight (or other low-stretch) string transfers more energy to the arrow, which those same charts treat as roughly 5# of extra effective draw weight for spine purposes only — it doesn't change what your bow actually pulls on a scale. Both defaults are editable, since the exact figure varies by chart and bow.
Carbon conversion
The underlying calculation is always done in wood-shaft pounds — that's the physical, brand-agnostic unit the rest of the math is built on. For carbon, that range is converted to a static-spine deflection number (the 400/500/600-style rating, where a lower number is stiffer) using a smoothed version of the wood-to-carbon bands commonly published across trad shaft sellers, then rounded to the nearest hundred — matching how carbon shafts are actually sold. It's an approximation — carbon spine also shifts with shaft diameter and wall construction, so treat it as your starting search term, not gospel.
Front of Center (FOC)
FOC is estimated as a weighted balance point: the shaft's weight is treated as evenly spread along its length (center at the midpoint), point and insert weight are treated as concentrated at the front, and nock and fletching weight are treated as concentrated near the back. FOC% = ((balance point − arrow length / 2) / arrow length) × 100. Once you've actually built the arrow, balance it on your fingertip or a dowel to check the real figure — this estimate gets you very close but glue, wrap, and crest weight aren't accounted for.
What this tool doesn't do
- It doesn't know your release style, anchor, or form — all of which shift real-world dynamic spine.
- It doesn't replace bareshaft or paper tuning — treat every number here as your starting point at the shop, not your final answer at full draw.
- It isn't tied to any shaft brand's proprietary chart or catalog — cross-reference the spine range against whatever shafts you're actually buying.