Arrow Spine Calculator — How to Pick the Right Arrow for Your Bow

Spine is the one number on an arrow that nobody can see and everybody gets wrong. It is the stiffness of the shaft — how far the middle sags, in thousandths of an inch, when you hang a 1.94 lb weight off a 28 inch span. A 300 is stiffer than a 400. That is the whole definition, and it is also where the useful part of most explanations stops.

What follows is the method this calculator actually runs, written out: the six steps, the coefficients, the two places our answer deliberately differs from the tool ranked above us, and how closely it reproduces the manufacturer charts. If you would rather just get an answer, the builder at the top of this page does all of it live. If you want to check our work before you trust it, this is the page for that.

The six steps, in order

Every spine chart in the sport is really one idea: convert your setup into an effective draw weight, then look that number up. All the disagreement between calculators is in the conversion.

1. Start at measured peak draw weight

Measured. Not marked. A bow with 70 on the sticker is frequently not 70, and a scale costs less than a dozen shafts. If you have never put yours on one, do that before you read the rest of this.

2. Adjust for bow speed

Easton and Victory publish an identical speed table. We adopt it verbatim rather than inventing our own, because the entire point of chart parity is that our answer matches the manufacturer's when nothing exotic is going on.

3. Adjust for arrow length

Easton's chart diagonal moves one spine group per roughly three 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. Longer is weaker; a long arrow on a heavy bow is the most common way people end up two classes off without noticing.

eff_dw += (arrow_length_in − 28) × 3.5

4. Adjust for front-end mass

Easton moves ±3 lb per 25 gr of point weight off a 100 gr baseline — 0.12 lb per grain. Inserts up to 25 gr are free, because the published charts already assume a standard insert is in the shaft. Only hardware above that 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 every heavy-collar build — which is to say most serious elk arrows built in the last three years.

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. Easton and Victory do not use let-off in spine selection, so we leave it off and match the charts exactly. It is an opt-in refinement, not a default. Turn it on and you have left chart territory; the tool says so when you do.

eff_dw += (85 − let_off_pct) × 0.25  (opt-in only)

6. Map effective draw weight to a spine class

Yes, two of those rows are the same, and no, it is not a typo. The 45–55 and 55–65 bands both land on 340–400 because the published charts put a 45 lb trad rig and a 60 lb compound hunter on the same shaft, and they are right to. 340 is the most-used spine in bowhunting; it covers a genuinely wide slice of the sport. We could have smeared the boundary to make the table look tidier. Making a table look tidier is not a reason to move a recommendation.

We show 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.

A worked example: 70 lb, 29 inch arrow, 125 gr point

A common whitetail setup, run through all six steps.

79.5 lb lands in the 75–85 band → 250–300 spine. A 300 is the pick, sitting toward the weak edge of the band, which is exactly where a heavy-front build belongs. Move the point to 175 gr and you are at 85.5 and the answer changes class. That is not the calculator being twitchy; that is what 50 grains on the front of an arrow actually does.

Three things this calculator does that the others do not

This is the part worth reading twice, because it is where our answer and the answer from the tool ranked above us diverge — and one of us is wrong.

Rear weight stiffens. It does not weaken.

This is the single most commonly inverted fact in arrow-building software. Mass at the nock end — a heavier nock, more vane, a lighted nock, 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 same mass at the back.

Think about where the shaft bends. The arrow buckles around its own inertia at launch; mass behind the flex point resists that buckle, mass ahead of it drives it. Archer's Advantage, which ranks above this page, has the sign backwards. So did we, once. It is now a locked invariant with regression tests behind it, because a mistake we made and fixed is worth more to you than a mistake we never admitted.

Practical version: if you add a 20-grain lighted nock and three inches of extra vane to a build that was already at the stiff edge of its band, you have moved further stiff, not weak. Half the people who add lighted nocks and then chase a new tear are chasing this.

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. Our model runs linear up to a knee at +100 gr over baseline — 200 gr of total front mass — then continues at half slope. The 75–150 gr band is where the model is exact, and that is where most hunting builds live.

A fully linear model blows up. Ours once returned about 500 spine at 290 gr of front mass, which is not a real number and would have sent somebody shopping for a shaft that does not exist. If you are building an Ashby-style 30% FOC arrow, this is the correction that keeps the answer sane.

Internal footing stiffens by length, not just by weight.

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, and almost every calculator ignores it — a footing goes in as plain added weight, so a footed build comes out wrong.

It is not worth the footing's full physical length either, and that distinction matters more than it sounds. We convert the footing to a stiffness-equivalent length first: 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, which is what it physically resembles. 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 and counts for correspondingly less.

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 shows up as 54. That was a real bug in this engine, found and fixed on 4 July 2026. Front footing, back footing, or both — all three are modelled.

How close is this to the manufacturer charts?

Close enough that we publish the number, which as far as we can tell nobody else does.

Those figures are locked by an automated harness that fails our build on drift, and the reason they stay stable is structural: every adjustment above is additive with a zero default. Turning on let-off, adding a collar, adding a footing — none of them can silently move the baseline chart answer. That is what makes a parity claim worth making. The full test — every cell, every miss, and the places we are still guessing — is on How the Forge is checked.

Where we disagree with a chart, we disagree on purpose and we say so. Gold Tip's 15% gap is not us being sloppy; it is Gold Tip sitting a class stiffer than Easton and Victory across a wide part of the grid. Pick a shaft, not a chart.

Static spine, dynamic spine, and the only test that settles it

The number printed on the shaft is its static spine, from a controlled lab deflection test. Dynamic spine is how stiff the arrow behaves at launch with everything you have bolted to it. The six steps above estimate dynamic spine from static, which is all any chart does.

None of it beats a bare shaft. If a bareshaft groups with your fletched arrows at 20 yards, your dynamic spine is right regardless of what the static number says or what this page told you. The calculator's job is to get you to the right shaft to buy before you have spent the money — not to replace the test.

Frequently asked

What spine do I need for a 70 lb bow?

At a 28 inch arrow and a 100 gr point on a normal-speed bow, 70 lb is a 300–340. Add a longer arrow, a heavier point or a heavy collar and it moves stiffer fast: the worked example above — same 70 lb bow, 29 inch arrow, 125 gr point, 50 gr of front hardware — lands on 250–300 instead. Draw weight alone answers this badly.

Does a heavier nock or a lighted nock weaken my spine?

No. Rear weight stiffens dynamic spine. A heavier nock, more vane or a back footing all move you stiffer, at roughly half the magnitude of the same mass on the front. Calculators that show rear weight softening the arrow have the sign backwards.

Do I count the insert as point weight?

Count insert, collar and any front internal footing together as front-end hardware — but only the mass above 25 gr, because the published charts already assume a standard insert is installed. A 50 gr steel insert plus a 100 gr point is 125 gr of chargeable front mass, not 150 gr and not 100 gr.

Should I turn let-off on?

Not unless you know why you want it. It is off by default because Easton and Victory do not use it, and leaving it off is what keeps our answer identical to theirs. If you shoot 65% let-off or you are past the edge of the printed chart, turn it on and treat the result as an opinion rather than a chart lookup.

What if I am between two spines?

Go stiffer and add point weight. A slightly stiff arrow that you tune weak with a heavier head recovers cleanly; a weak arrow with a heavy head does not, and a badly weak arrow can paradox into the riser, which is a safety problem and not a tuning one. If you are on a band edge, buy one dozen and bareshaft before you buy three.

Is this free?

The builder, this calculator, the recommender, the phone chronograph and saving a build are free with no card. The tuning wizard, true-to-scale sight tape printing, the shot solver and unlimited saved builds are the membership. The math on this page is public either way.

Where to go next

• The Forge — the whole workshop, and the full written methodology this page condenses
• Spine what-if ladder — change one variable at a time and watch the recommendation move
• Spine chart by draw weight — the plain chart, if that is all you came for
• Forge vs Stu Miller — where we agree with the 2011 spreadsheet and where we do not
• Tuning, in order — spine is step one for a reason
• Paper tune chart — what the tear is telling you once the arrow is built
• How the Forge is checked — the parity test in full, with the scores
• Shafts we stock and components — every one of them is in the builder's catalog

Found a number here you cannot trace, or one you think is wrong? Write sal@sparrowexpeditions.com. That is a standing offer, and it is how the footing bug got fixed.