How is arrow speed estimated from the IBO rating?

The IBO speed is measured at a 30 inch draw, 70 pound draw weight and a 350 grain arrow. This calculator adjusts from that baseline: about plus or minus 10 fps for each inch of draw length, plus or minus 2 fps for each pound of draw weight, and minus 1 fps for every 3 grains of arrow weight above 350, plus a further deduction for weight added to the string. These are the widely used rules of thumb.

How accurate is the estimate?

It is a good ballpark, usually within a few fps of a real reading, but not exact. Cam efficiency, let-off, string and cable condition, arrow spine, fletching and nick fit all shift the true speed. The only way to know your real arrow speed is to shoot through a chronograph.

What is a safe minimum arrow weight?

Most manufacturers set a floor of about 5 grains of total arrow weight per pound of draw weight, so a 70 pound bow should shoot at least around 350 grains. Shooting lighter than the rated minimum can damage the bow and void the warranty, so keep the arrow at or above 5 grains per pound.

Arrow Speed Calculator

This arrow speed calculator estimates how fast your arrow leaves the bow, in feet per second, starting from the bow manufacturer's IBO speed rating and adjusting it for your actual setup. The IBO figure printed on a compound bow is measured under fixed conditions: a 30 inch draw length, 70 pounds of draw weight, and a light 350 grain arrow. Almost nobody shoots exactly that, so the raw number overstates real speed for most archers. The calculator applies the widely used rules of thumb: draw length changes speed by about 10 fps per inch, draw weight by about 2 fps per pound, and each 3 grains of arrow weight above 350 costs roughly 1 fps, with a further small deduction for the peep, nock loop and other weight added to the string. From the estimated speed it also works out the arrow's kinetic energy in foot pounds and its momentum, the two figures that describe hitting power, plus the speed in metres per second. This helps you compare arrow builds, judge whether a heavier hunting arrow still carries enough energy, or sanity check a dealer's quoted speed before you buy. Enter your bow and arrow details to see the estimate, with the adjustment rules and a worked example set out below. Treat the result as a close ballpark: a chronograph reading is the only exact measure.

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Bow IBO speed rating

fps

Draw weight

Draw length

Arrow weight (total)

Added string weight

289 fps

estimated arrow speed

Kinetic energy74.4 ft-lb

Momentum0.514 lb-s

Speed88.2 m/s

An IBO 320 bow at 70 lb and 29 inches, shooting a 400 grain arrow, gives an estimated 289 fps. Chronograph the setup for an exact figure.

Estimate only, based on standard IBO adjustment rules of thumb. Cam design, string condition, spine and fletching all affect real speed. Do not shoot below the rated minimum arrow weight for your bow.

How it works

The estimate starts at the bow's IBO speed and adjusts for the four things that move it. Draw length: add or subtract 10 fps for every inch away from the 30 inch IBO standard. Draw weight: add or subtract 2 fps for every pound away from 70. Arrow weight: subtract 1 fps for every 3 grains above the 350 grain IBO arrow, or add it back for a lighter arrow. String weight: subtract 1 fps for every 3 grains of peep, loop and silencer weight added to the string. Kinetic energy in foot pounds is arrow weight in grains times speed squared, divided by 450,240. Momentum in slug feet per second is arrow weight times speed divided by 225,218. Metres per second is the speed in fps times 0.3048.

Worked example

Take a 320 fps IBO bow set to 70 lb draw weight and a 29 inch draw length, shooting a 400 grain arrow with 12 grains added to the string. Draw weight is at the standard 70, so no change. Draw length is 1 inch short, costing 10 fps. The arrow is 50 grains over 350, costing 50 / 3 = 16.7 fps. The string weight costs 12 / 3 = 4 fps. That gives 320 - 10 - 16.7 - 4 = 289 fps. Kinetic energy is 400 x 289.3 squared / 450,240 = 74.4 ft-lb, and momentum is 400 x 289.3 / 225,218 = 0.514 slug ft/s, which is 88.2 m/s.

Related calculators

Arrow FOC Calculator: front of centre balance.
Kinetic Energy Calculator: energy of a moving mass.
Momentum Calculator: mass times velocity.
Projectile Motion Calculator: range and flight path.
Velocity Calculator: speed with direction.

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