Quick Answer
An arrow speed calculator estimates your real-world arrow velocity (FPS) by adjusting a manufacturer's rated IBO or AMO speed for your draw weight, draw length, arrow weight and string accessories. The standard adjustment is roughly ±2 FPS per pound of draw weight, ±10 FPS per inch of draw length, and −1 FPS per 3 extra grains of arrow weight.
How to Use the Arrow Speed Calculator
The calculator estimates how fast your finished arrow actually leaves your bow, then converts that speed into kinetic energy and momentum. It works from the speed your bow was rated at and adjusts for everything about your setup that differs from the rating conditions.
- Pick your bow type. Compound uses the IBO rating standard, Recurve / Trad uses the AMO standard, and Crossbow asks for the bolt weight your crossbow was tested with instead of a draw length.
- Enter the rated speed. This is the manufacturer’s advertised figure, usually printed on the limb sticker or the spec sheet — for example 320 fps. If your figure is in m/s, km/h or mph, change the unit beside the box rather than converting by hand.
- Enter your draw weight and draw length. Use your real numbers, not the bow’s maximum. Draw weight accepts pounds or kilograms; draw length accepts inches or centimetres.
- Enter your total finished arrow weight in grains. That means shaft, insert, point, nock and fletching all together — not the bare shaft weight printed on the box.
- Open Advanced and toggle anything attached to your string — peep sight, kisser button, D-loop, silencers, brass nock. Each one costs speed.
- Read the results panel. It shows estimated speed in fps with m/s and mph beneath it, then kinetic energy, momentum, total string add-on weight, and a game-suitability label.
The Formula Behind the Estimate
A published bow speed is measured under one fixed set of conditions. Your bow is almost never at those conditions, so the calculator starts from the rating and adds or subtracts for each difference:
Adjusted speed = rated speed + (your draw weight − baseline weight) × weight factor + (your draw length − baseline length) × length factor − (your arrow weight − baseline arrow) ÷ 3 − string add-on grains ÷ 3
The baselines are not arbitrary. The compound figures are the IBO/ATA rating conditions, which specify a 30-inch draw at 70 pounds shooting an arrow of 5 grains per pound of draw weight — exactly 350 grains at 70 pounds. The recurve figures are the AMO conditions of 60 pounds at 28 inches with a 9 grains-per-pound arrow, which comes to 540 grains.
| Bow type | Baseline | Per pound | Per inch | Baseline arrow |
|---|---|---|---|---|
| Compound (IBO/ATA) | 70 lb at 30 in | 2 fps | 10 fps | 5 gr per lb → 350 gr |
| Recurve / Trad (AMO) | 60 lb at 28 in | 2.5 fps | 8 fps | 9 gr per lb → 540 gr |
| Crossbow | Rated bolt weight | not used | not used | entered directly, default 400 gr |
A useful check on all of this: enter the rating conditions exactly and the calculator hands the rating straight back. Put in 320 fps, 70 pounds, 30 inches and a 350-grain arrow and it returns 320 fps. Do the same on a recurve with 320 fps, 60 pounds, 28 inches and a 540-grain arrow and it also returns 320 fps. Every number below the rating is the gap between the marketing conditions and your conditions.
Kinetic Energy and Momentum
Once the speed is known, the two energy figures follow directly from the arrow’s weight and speed:
Kinetic energy (ft-lb) = grains × fps² ÷ 450,240
Momentum (slug-ft/s) = grains × fps ÷ 225,218
Both divisors exist because grains are a unit of mass and the answers are in imperial force units, so each constant folds in the 7,000 grains in a pound along with gravitational acceleration. If unit conversions of that kind are unfamiliar, the Lbm to Lbf Calculator covers the same mass-versus-force distinction in isolation.
The two constants do not use the same gravity. 450,240 is 2 × 7,000 × 32.16, while 225,218 is 7,000 × 32.174. Both are the constants the archery industry has published for decades, so the calculator uses them as they stand — but the energy card and the momentum card are not derived from one internally consistent value of g, and the difference is small compared with the uncertainty in a rule-of-thumb speed estimate.
Worked Example
Take the setup the calculator opens with: a compound rated at 320 fps, drawn at 60 pounds and 28 inches, shooting a 350-grain arrow with nothing on the string.
- Draw weight is 10 pounds under the 70-pound baseline, so that is −20 fps.
- Draw length is 2 inches under the 30-inch baseline, so that is another −20 fps.
- The baseline arrow at 60 pounds is 5 × 60 = 300 grains, so a 350-grain arrow is 50 grains heavy, costing 50 ÷ 3 ≈ −16.7 fps.
- 320 − 20 − 20 − 16.7 gives 263 fps, which is 80.3 m/s or 180 mph.
- Kinetic energy is 350 × 263² ÷ 450,240 = 53.9 ft-lb, and momentum is 350 × 263 ÷ 225,218 = 0.41 slug-ft/s.
That setup lands on the calculator’s “Medium game (deer)” rung. It is also 57 fps below the number on the sticker, which is the single most common surprise for anyone chronographing a bow for the first time.
The baseline arrow moves with your draw weight. Because the baseline is 5 grains per pound rather than a fixed 350 grains, dropping from 70 to 60 pounds also drops the reference arrow from 350 to 300 grains. That is why the same 350-grain arrow is “on baseline” at 70 pounds but 50 grains heavy at 60.
Why a Heavier Arrow Is Not Always More Energy
The usual shorthand is that heavy arrows carry more energy and light arrows fly faster. Only the second half is reliably true. Because every extra 3 grains costs a full 1 fps, and kinetic energy depends on the square of speed, adding weight eventually loses more to the speed term than it gains from the mass term.
Running the calculator across arrow weights on one bow — 320 fps rated, 70 pounds, 30 inches — shows energy rising, peaking, and then falling away:
| Arrow weight | Speed | Kinetic energy |
|---|---|---|
| 300 gr | 337 fps | 75.5 ft-lb |
| 350 gr | 320 fps | 79.6 ft-lb |
| 400 gr | 303 fps | 81.7 ft-lb |
| 437 gr | 291 fps | 82.2 ft-lb — maximum |
| 500 gr | 270 fps | 81.0 ft-lb |
| 600 gr | 237 fps | 74.6 ft-lb |
| 700 gr | 203 fps | 64.3 ft-lb |
| 900 gr | 137 fps | 37.3 ft-lb |
The peak sits at about 437 grains and 82.2 ft-lb. Going from 350 to 437 grains buys 2.6 ft-lb; going on to 900 grains gives back more than half the energy the bow started with. There is a clean way to recognise the peak without any of this arithmetic: at maximum kinetic energy the arrow weight in grains is exactly 1.5 times the speed in fps. At the peak above, 437 grains against 291 fps is a ratio of 1.50.
Momentum behaves completely differently. It depends on speed to the first power, not the square, so it keeps rising long after energy has turned over. On the same bow momentum peaks at roughly 655 grains and 218 fps — 218 grains heavier than the energy peak — and there the ratio is exactly 3.0 grains per fps.
This is the whole kinetic-energy-versus-momentum argument in two numbers. On one identical bow, the arrow that maximises energy and the arrow that maximises momentum are 218 grains apart. Neither camp is making an arithmetic mistake — they are optimising different quantities, and those quantities genuinely peak in different places.
Arrow Balance (FOC)
Speed and energy describe how hard the arrow arrives. FOC — front of centre — describes how the arrow’s weight is distributed along its own length, and it is the one number on this page that has nothing to do with your bow. Open the Arrow balance panel and enter two measurements.
FOC % = 100 × (A − L ÷ 2) ÷ L
Both are measured from the bottom of the nock groove. L is the correct arrow length, from the nock groove to the end of the shaft — not counting the point or insert protruding past it. A is the distance to the balance point of the finished arrow, so you must fit the point, insert, nock and fletching before you measure it. Balance the completed arrow across a straight edge and mark where it sits. Those definitions are Easton’s own, and so is the recommendation the calculator scores you against: 10% to 15% FOC for hunting setups.
| Target FOC | Balance point from nock groove | Distance ahead of centre |
|---|---|---|
| 0% | 15.0 in | 0 in — balanced dead centre |
| 5% | 16.5 in | 1.5 in |
| 10% — bottom of Easton’s range | 18.0 in | 3.0 in |
| 15% — top of Easton’s range | 19.5 in | 4.5 in |
| 20% | 21.0 in | 6.0 in |
The pattern is worth internalising because it makes FOC easy to estimate at the bench: on any arrow, each 5% of FOC moves the balance point forward by exactly one twentieth of the arrow’s length. On a 30-inch shaft that is 1.5 inches per 5%; on a 28-inch shaft it is 1.4 inches.
A worked case from the calculator: a 28.5-inch arrow balancing at 16.2 inches gives 100 × (16.2 − 14.25) ÷ 28.5 = 6.8%, which the verdict cell marks as below Easton’s range. Getting that arrow to 10% means moving the balance point to 17.1 inches — in practice, a heavier point or a brass insert.
A balance point behind the middle gives a negative FOC, and the calculator reports it rather than refusing it. A 30-inch arrow balancing at 14 inches reads −3.3%. That is a real arrow and a real problem — it will plane badly in flight — so hiding the result behind an error message would be less useful than showing it. FOC is also completely independent of the bow: leave every speed field empty and the balance figures still work.
What a Pound and an Inch Are Actually Worth
The adjustment table lists 2 fps per pound for a compound, but that is only the direct term. Adding a pound of draw weight also raises the 5-grain-per-pound baseline arrow by 5 grains, which refunds a further 5 ÷ 3 fps. The real figure the calculator produces is 11 ÷ 3, or about 3.67 fps per pound. Three pounds buys exactly 11 fps.
Draw length has no such second term, so an inch is worth its full 10 fps. That makes one inch of draw length worth about 2.7 pounds of draw weight — which is why a tall archer with a short, heavy bow is usually leaving more speed on the table than they realise.
On a recurve both terms are larger: 2.5 fps per pound directly plus 9 ÷ 3 from the heavier baseline arrow gives 5.5 fps per pound, half again as much as a compound. Two recurve pounds buy exactly 11 fps.
| Change | Compound | Recurve |
|---|---|---|
| +1 lb draw weight | +3.67 fps | +5.5 fps |
| +1 in draw length | +10 fps | +8 fps |
| +3 gr arrow or string weight | −1 fps | −1 fps |
| All five string add-ons (25 gr) | −8.3 fps | −8.3 fps |
String add-ons are charged at the same 1 fps per 3 grains as the arrow, and the calculator treats them identically — 25 grains hung on the string costs precisely what 25 grains added to the arrow costs. Toggling all five accessories on gives up 8.3 fps, which is more than most archers expect from a peep, a D-loop and a set of silencers.
Speed, Energy and Game Suitability
The results panel labels the setup against a kinetic-energy ladder with rungs at 25, 40, 65 and 80 ft-lb, shown on the meter beneath the results.
| Kinetic energy | Label |
|---|---|
| Under 25 ft-lb | Target / practice only |
| 25 to 39 ft-lb | Small game |
| 40 to 64 ft-lb | Medium game (deer) |
| 65 to 79 ft-lb | Large game (elk, bear) |
| 80 ft-lb and above | Heavy game (moose, buffalo) |
These thresholds are this calculator’s own, and they are deliberately demanding. Figures commonly quoted in bowhunting guidance put deer-capable setups lower than the 40 ft-lb used here, so a setup this tool calls “Small game” may still be legal and widely considered adequate for deer where you hunt. The label is a conservative guide, never a substitute for your state or country’s rules. Kinetic energy also says nothing about broadhead choice, arrow spine or shot placement, which matter more than any single number.
Legal minimums are set by equipment, not by energy. New York, for example, sets a minimum peak draw weight of 100 pounds for crossbows and states there is no minimum limb width, no minimum length and no maximum draw weight. Requirements differ from state to state and change between seasons, so check the current regulations where you hunt. If you are working out what a deer is worth on paper rather than what your bow will do to one, the Whitetail Deer Score Calculator handles the measuring side.
Crossbow Mode
Crossbows are rated differently. There is no meaningful draw length to adjust for, so the calculator hides the draw weight and draw length fields and asks instead for the bolt weight your crossbow was rated with — usually 400 grains. Speed is then the rating minus the difference between your bolt and the rated bolt, at the same 1 fps per 3 grains.
A crossbow rated 400 fps with a 400-grain bolt, shooting a 500-grain bolt instead, gives 400 − 100 ÷ 3 = 367 fps and 149.3 ft-lb. Draw weight is genuinely ignored in this mode; changing it moves nothing, which is correct for the way crossbows are rated.
Limits Worth Knowing
The speed result is a rule-of-thumb estimate based on the entered rating and fixed adjustments for weight, length and grains. It does not measure cam efficiency, brace height, string condition, limb wear or tuning. The calculation does not establish a universal accuracy band. Use a chronograph reading from your own bow and finished arrow to check the estimate, especially before relying on the derived energy and momentum figures.
The linear model also has an obvious floor. Load an absurd arrow — 5,000 grains, say — and the arithmetic would go negative, so the calculator clamps the answer at 0 fps rather than printing a nonsense number. Treat anything approaching that region as outside the model rather than as a real prediction. In 2026 the only way to know your true arrow speed is still to put it through a chronograph; this calculator tells you what to expect and, more usefully, what each change to your setup is worth before you pay for it.
