Arrow Speed Calculator

Think of a bow as a spring-loaded catapult. When you draw back the string, you store energy in the limbs. When you release, that stored energy transfers into the arrow. How fast the arrow travels depends on three things: how much energy was stored (draw weight), over what distance the force is applied (draw length), and how heavy the object being launched is (arrow weight). These three variables account for the vast majority of speed variation between setups.

The IBO formula takes this physics and expresses it as practical rules of thumb. Each additional pound of draw weight above 70 lbs adds roughly 10 FPS. Each additional inch of draw length above 30 inches adds roughly 3 FPS. Each additional 3 grains of arrow weight above 350 grains costs roughly 1.5 FPS. These rates are derived from aggregate testing across many bow designs and hold surprisingly well across modern compound bows, though they will be less accurate on recurves and longbows.

String accessories like peep sights and D-loops add mass to the string, which the bow must also accelerate on release. This dead weight never leaves the bow, so it costs energy without adding useful momentum to the arrow. The effect is the same as making the arrow heavier — a small but measurable speed reduction. Keeping string weight low is one of the cheapest performance gains in archery.

Use this calculator when you are configuring a new bow setup and want to predict speed before committing to arrow components. It is most useful for compound bow archers selecting arrow spine, comparing point weights, or deciding whether to adjust draw weight. It is also a good check before purchasing a new bow — if the manufacturer's IBO rating is 320 FPS and your setup variables are well below IBO conditions, you can estimate your real-world speed without needing a chronograph.

This tool is appropriate for planning and comparison but not for precision measurement. If you need to know your exact arrow speed — for ballistic calculations, long-range compensation, or equipment certification — use a chronograph. No formula substitutes for direct measurement when precision matters.

Do not use this tool to compare recurve or longbow setups to compound bows. The formula is specific to the energy delivery profile of compound bows with modern cam systems. Applying it to traditional archery gives misleading results that could cause you to select the wrong arrow spine or underestimate the force required for a clean ethical harvest.

The most common mistake is using listed bow draw weight instead of actual peak draw weight. Most compound bows have a 10-pound adjustment range, and many archers set them below maximum. A bow listed as a 70-pound bow often gets set to 60 or 65 pounds, which changes the estimated speed by 50-100 FPS. Always confirm the actual peak draw weight, ideally with a bow scale.

The second mistake is using shaft weight instead of total arrow weight. The arrow shaft alone might weigh 300 grains, but add a 100-grain broadhead, insert, nock, and three vanes and the finished arrow is closer to 460-480 grains. Plugging shaft weight alone into the calculator overestimates speed by 30-80 FPS depending on point weight — which creates a false sense of performance for hunters who then feel their bow underperforms at the range.

A third mistake is assuming this formula applies equally to recurve and traditional bows. It was developed from compound bow data and relies on consistent energy storage across the draw cycle, which is a property of modern cams. Traditional bows have a linear or regressive draw cycle, and the IBO formula will meaningfully overestimate their speed — sometimes by 30-50 FPS at equivalent settings.

The formula behind this calculator follows the IBO adjustment model. Starting from a baseline of 300 FPS (the IBO standard at 70 lbs, 30-inch draw, 350 grains), each variable is adjusted independently.

Draw weight adjustment: (draw_weight - 70) x 10 FPS. If you shoot 65 lbs, that is -50 FPS. If you shoot 80 lbs, that is +100 FPS.

Draw length adjustment: (draw_length - 30) x 3 FPS. A 28-inch draw subtracts 6 FPS. A 32-inch draw adds 6 FPS.

Arrow weight adjustment: -((arrow_weight - 350) / 3) x 1.5 FPS. A 500-grain arrow subtracts 75 FPS versus IBO baseline.

Kinetic energy uses the classical formula KE = 0.5 x mass x velocity squared, with arrow weight converted from grains to pounds (7,000 grains per pound) and velocity in feet per second, divided by 32.174 to account for gravity units in the imperial system.

Momentum uses p = mass x velocity, expressed in slug-feet-per-second — a less common but technically complete measure that some hunters prefer over kinetic energy because it better predicts penetration through resistance.

The 1.5 FPS per 3-grain assumption becomes less accurate at extreme arrow weights. Very heavy arrows (above 700 grains) engage the bow limbs differently because the slower acceleration profile changes the timing relationship between peak draw force and the arrow's departure. At very light arrow weights (below 300 grains), the arrow leaves the string before the limbs have fully returned, which means the bow retains residual vibration energy that a heavier arrow would have absorbed. This is why ultra-light arrow setups produce noticeably more hand shock — and why the formula's linearity breaks down outside the 300-600 grain sweet spot for most compound bows.