How The Forge calculates arrow spine, FOC, and sight marks
The tool above is interactive. This page is the same math, written out — the coefficients, the tables, and the sources they were validated against. If you want to check our work before you trust a number, check it here.
What spine does Sparrow recommend, and how is it calculated?
Spine is the stiffness rating of an arrow shaft — the deflection in thousandths of an inch when a 1.94 lb weight is hung from a 28 inch span. A lower number is a stiffer shaft. Sparrow picks a spine by converting your setup into an effective draw weight, then mapping that number onto a spine class. The conversion is six steps, run in order.
Step 1 — start at measured peak draw weight
Measured, not marked. A bow labelled 70 lb frequently is not 70 lb.
Step 2 — adjust for bow speed (ATA/IBO rating)
Easton and Victory publish an identical speed table. Sparrow adopts it verbatim rather than inventing its own, because the whole point of chart parity is that our answer matches the manufacturer's when nothing exotic is going on.
| Bow | Adjustment | Why |
|---|---|---|
| 300 fps or slower | −5 lb | Gentler launch, less dynamic bend |
| 301–340 fps | 0 | Chart baseline — most modern hunting bows |
| 341–350 fps | +5 lb | Harsher launch |
| Above 350 fps | +10 lb | Speed bows load the shaft hardest |
| Longbow / traditional | −5 lb | Longer power stroke, gentler acceleration |
Step 3 — adjust for arrow length
Easton's chart diagonal moves one spine group per roughly 3 inches, which works out to about 3.5 lb of effective draw weight per inch from a 28 inch baseline. Length is measured back-of-point (BOP).
eff_dw += (arrow_length_in − 28) × 3.5
Step 4 — adjust for front-end mass
Easton moves ±3 lb per 25 gr of point weight from a 100 gr baseline, i.e. 0.12 lb per grain. Inserts up to 25 gr are free — the published charts already assume a standard insert is installed — so only insert and collar mass above 25 gr counts, at the same rate.
eff_dw += (point_gr − 100) × 0.12
eff_dw += max(0, front_hardware_gr − 25) × 0.12
Front-end hardware means insert plus collar or outsert plus any front internal footing. Passing insert weight alone is a common error in other calculators and it under-weights heavy-collar builds.
Step 5 — let-off (off by default)
Lower let-off stores more energy in the limbs at full draw, so the arrow sees a harsher launch and wants a stiffer shaft. The physics is real, but Easton and Victory do not use let-off in spine selection, so Sparrow leaves it off by default and matches the charts exactly. It is an opt-in refinement, not a default.
eff_dw += (85 − let_off_pct) × 0.25 (opt-in only)
Step 6 — map effective draw weight to a spine class
| Effective draw weight | Spine range |
|---|---|
| Under 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 |
| 85–100 lb | 200–250 |
| 100 lb and up | 150–200 |
Sparrow shows a tight ±1 class range rather than the wide "anything works" band older charts use. If your setup lands near a band edge, that is worth knowing — the honest answer there is to paper-tune and confirm, not to pretend the chart resolved it.
Worked example: 70 lb, 29 inch arrow, 125 gr point
A common whitetail setup, run through the six steps:
- Peak draw weight: 70 lb
- Bow rated 330 fps IBO → baseline band, +0 → 70
- Arrow 29" → (29 − 28) × 3.5 = +3.5 → 73.5
- 125 gr point → (125 − 100) × 0.12 = +3.0 → 76.5
- 50 gr front hardware → (50 − 25) × 0.12 = +3.0 → 79.5
- Let-off: off by default → no change
Effective draw weight 79.5 lb lands in the 75–85 band → 250–300 spine. A 300-spine shaft is the pick, sitting toward the weak edge of the band — which is exactly where a heavy-front build should sit.
Does adding weight to the back of an arrow weaken its spine?
No — and this is the single most commonly inverted fact in arrow-building software.
Rear weight stiffens dynamic spine. Mass at the nock end (heavier nock, more vane, a back internal footing) makes the shaft behave stiffer, which shows up as a lower dynamic-spine number. Front weight weakens it, at roughly twice the magnitude of the equivalent rear mass. Sparrow once had this backwards — the same error Archer's Advantage makes — and it is now a locked invariant with regression tests behind it.
Front mass saturates; it is not linear forever
Piling weight up front stops weakening the shaft at the same rate once you get deep into high-FOC territory. Sparrow's model runs linear up to a knee at +100 gr over baseline (200 gr total front mass), then continues at half slope beyond it. A fully linear model blows up — ours once returned roughly 500 spine at 290 gr of front mass, which is not a real number. The 75–150 gr band is where the model is exact.
Internal footing stiffens by length — but by stiffness-equivalent length
A carbon internal footing or CTI does two things: it adds mass, and it shortens the length of shaft that is free to flex. The second effect dominates. It is not worth the footing's full physical length, though, and that distinction matters more than it sounds.
Sparrow first converts the footing to a stiffness-equivalent length: physical length multiplied by the footing's share of the composite beam's stiffness. That equivalent length is then worth about 3.5 lb of effective draw weight per inch — the same rate as trimming the arrow. A wall-doubling footing carries most of the stiffness over its span and counts close to one-for-one. A thin internal CTI carries a small fraction of it, and counts for correspondingly less than its physical length.
Feed raw tube length in instead and a thin CTI is over-credited by something like five to ten times: a 6 inch CTI reads as −21 lb of effective draw weight, so a 70 lb bow is shown as 54. That was a real bug in this engine, found and fixed on 2026-07-04. Front footing, back footing, or both. Any calculator that treats a footing as pure added weight gets a footed build wrong — and any calculator that credits its full physical length gets it wrong the other way.
What is FOC, and what range does Sparrow recommend?
FOC (front of center) is the percentage by which an arrow's balance point sits forward of its physical centre.
FOC% = ((balance_point_in − (arrow_length_in / 2)) / arrow_length_in) × 100
Sparrow's recommended hunter range is 11–15%. That is not a "conservative" or beginner setting — it is the range where penetration gain and trajectory cost trade off best for a hunting arrow. Extreme-FOC builds above 19% buy penetration with drop, and Sparrow will build one if you ask, but it will tell you what it costs.
Why should you trust these numbers?
Because they were validated against primary sources, and the validation is re-run automatically on every change:
- Easton "Hunting Arrow Size Selection" chart 301055-A (2023) — Sparrow reproduces 98% of the published grid cell-for-cell.
- Victory's own spine-chart calculator grids, read from Victory's published logic rather than retyped from a printed chart. Parity is locked by an automated harness that fails the build on drift.
- Gold Tip's compound chart as a cross-check only. Gold Tip is the outlier of the three and Sparrow does not tune to it.
- Dynamic-spine behaviour re-derived from Easton and Victory published method, Stu Miller's dynamic-spine work, OnTarget2, and first-principles beam physics.
Every adjustment above is additive with a zero default. Turning on let-off, adding a collar, or adding a footing cannot silently move the baseline chart answer — which is what makes the parity claim stable.
What is free and what needs a membership?
| Tier | Price | What you get |
|---|---|---|
| Free | $0 | Arrow builder, spine and FOC calculator, live weight and speed, the arrow recommender, and saving or downloading a build. No card, no login to start. |
| Membership | $8/mo or $72/yr | Tuning Wizard, true-to-scale sight-tape printing, Shot Solver, wind drift, A/B build comparison, drift history, Compete, unlimited saved builds. |
| Founding Flock | $36/yr | Everything in membership at the founding rate, locked for life. Annual only, first 100 members. |
Annual membership is $72 against $96 paid monthly — three months free.
Where to go next
- Arrow spine calculator — the spine tool with a full written walkthrough
- How to make a sight tape — the chrono-calibrated method, no trial and error
- Archery cut chart — angle-compensation reference tables
- Arrow wind drift chart — drift by speed, weight, and crosswind
- Altitude and your sight marks — density-altitude correction
- Field Notes — the long-form archive
- Membership and pricing
Sparrow Expeditions is run by Sal Misseri out of Chicago, Illinois. Every catalog value is verified against manufacturer specification rather than estimated, and every calculation above is explainable. Questions, corrections, or a number you think is wrong: sal@sparrowexpeditions.com. We would rather be corrected than be confidently wrong.