Arrow Spine Calculator — How to Pick the Right Arrow for Your Bow
Spine is the stiffness rating of an arrow shaft. Too stiff: arrow hits left of point of impact (RH shooter), flies slow. Too weak: hits right, flexes unsafely, can paradox into the riser. Spine match is the foundation of every accurate arrow build.
Sparrow's free Arrow Builder computes spine match live as you adjust components. Pick a shaft, set draw weight + draw length + point weight, and the recommender tells you whether you're in spec, stiff, or weak.
The spine selection methodology (Easton standard)
Effective draw weight =
- + measured peak draw weight (lbs)
- + (arrow length − 28") × 5 lb/in
- + (total front mass − 100 gr) × 0.2 lb/gr
- + 5 lb for fast cams (IBO ≥ 335)
- − 3 lb for soft cams (IBO ≤ 310)
- + (85 − let-off %) × 0.25 (compound only)
- − 5 lb for traditional bows
Then map effective DW to spine range per Easton's tune chart. The Sparrow Arrow Builder does this automatically and warns you if your selected shaft spine is outside the recommended range.
Spine ranges (Easton 2024 chart)
| Effective DW | Recommended spine |
|---|---|
| < 25 lb | 600-700 |
| 25-35 lb | 500-600 |
| 35-45 lb | 400-500 |
| 45-55 lb | 340-400 |
| 55-65 lb | 340-400 |
| 65-75 lb | 300-340 |
| 75-85 lb | 250-300 |
| 85+ lb | 200-250 |
The insert-weight trap
Most spine calculators only consider POINT weight. That's wrong — a 50gr brass insert + 100gr point launches like a 150gr tip mass. Sparrow's calculator counts BOTH point and insert as front-side mass for spine math (matches Easton's chart).
Stiff vs weak — what each tells you
Spine too STIFF
Arrow flexes less than the bow's launch energy demands. RH shooter: hits LEFT of POI. Tells you to: switch to a weaker spine (higher number) OR add point weight OR use a longer arrow.
Spine too WEAK
Arrow flexes too much. RH shooter: hits RIGHT of POI. Can also flex unsafely and paradox into the riser — dangerous. Switch to a stiffer spine (lower number) OR reduce point weight OR shorten the arrow.
What about dynamic spine vs static spine?
The spine number printed on a shaft is its STATIC spine — measured under a controlled lab test. DYNAMIC spine is how stiff the arrow effectively is during launch given everything attached to it. The Easton formula above estimates dynamic spine reasonably well. Bareshaft tune is the empirical test — if a bareshaft groups with a fletched arrow, your dynamic spine is right regardless of what the static number says.
Free arrow spine calculator
The Sparrow Arrow Builder includes a live spine match indicator at every adjustment. The recommender mode lets you set a goal (penetration / speed / balanced), pick your bow + brand, and returns the top 5 fully-spec'd arrow builds with verified spine match.