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.
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.
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