Arrow speed calculator

What the IBO speed rating tells you

The IBO reference setup uses 70 lb peak draw weight, a 30 in draw length, and a 350 grain arrow. That arrow weighs 5 grains per pound of draw weight. A bow advertised at a particular IBO speed reaches that figure under a reference setup; your target or hunting configuration can differ in several ways.

Start with the IBO rating for your exact bow model. A rating from a different model, cam option, or testing standard gives the estimate the wrong starting point. If the manufacturer lists a speed range, calculate both ends and keep the result as a range rather than choosing the highest number.

This approach is intended for compound bows. A recurve or longbow has a different draw cycle and no equivalent compound IBO starting point. For those bows, measure speed directly with the arrows and draw length you actually shoot.

Enter measurements from the bow you shoot

1. Enter the advertised IBO speed in feet per second, abbreviated fps. Keep that separate from any speed you have already measured.
2. Use your current draw length in inches. A draw length measurement describes the bow setup, while arrow length is a separate measurement.
3. Enter peak draw weight in pounds. On a compound, this is the highest weight during the draw, before let-off reduces the holding weight.
4. Weigh the complete arrow in grains, including its point and every installed component. The arrow weight calculator can help estimate a build before assembly.
5. Account for added string weight, such as a peep, D-loop, or silencers. If that weight is unknown, treat this input as an additional source of uncertainty.

Do not enter the weight printed on a bare shaft as the finished arrow weight. Likewise, a limb label may describe a range rather than your current peak draw weight. A shop can measure the bow if you do not have suitable equipment.

How the speed adjustments work

A common rule of thumb subtracts about 10 fps for every inch of draw length under 30 in. A 28 in setup therefore loses about 20 fps from the reference rating before accounting for the arrow or string. This is an approximation: the change depends on how the bow stores and transfers energy at different draw lengths.

For arrow weight, subtract about 1 fps for every 3 grains above 5 grains per pound of peak draw weight. At 70 lb, that reference arrow weighs 350 grains. At 60 lb, it weighs 300 grains. Keep the baseline tied to the actual peak draw weight instead of automatically subtracting 350 grains from every arrow.

The string adjustment subtracts about 1 fps for every 3 grains of accessories on the string. A peep sight and other additions move during the shot and affect the estimate. Their position and the bow design also matter, so equal accessory weights do not guarantee identical speed losses.

Draw weight affects the reference arrow mass and the energy available to launch it. These simple corrections cannot fully describe every bow at a reduced limb setting. Results farther from the IBO reference deserve a chronograph check. Never reduce arrow weight below the bow manufacturer's minimum to chase the advertised speed.

Worked example: a 340 fps bow at 28 inches

Suppose a compound has a 340 fps IBO rating and is set to 70 lb with a 28 in draw length. You shoot a 425 grain finished arrow and have 15 grains of added accessories on the string. Using the rules above gives the following estimate.

The estimate is 290 fps. Save the inputs with that number, then compare it with your chronograph readings. If the readings differ, recheck the measured draw weight, draw length, and arrow mass before assuming the bow has a problem.

Why the chronograph can show a different speed

Actual draw length, measured peak weight, cam setup, and the finished arrow mass can all differ from the values entered. String accessories are another source of error. Even with accurate inputs, the rules of thumb can miss how a particular bow responds to those changes.

Follow the chronograph manufacturer's placement and lighting instructions, and shoot toward a suitable backstop. Keep the measurement arrangement consistent when comparing arrows. Record several valid readings and their average; investigate an isolated odd reading before treating it as the bow's normal speed.

Use that measured average with the kinetic energy and momentum calculator. If you use an estimated speed instead, label the energy and momentum results as estimates too. Recheck speed after changing arrow weight or bow settings.

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