Arrow Speed Calculator

What the arrow speed calculator does

The speed printed on a bow is measured under one fixed set of conditions that almost no one actually shoots. This calculator estimates the speed your arrow will really leave the string at, starting from that rated number and adjusting it for your own draw length, draw weight, arrow weight, and any gear hung on the string. It also reports the arrow's momentum and kinetic energy, the two numbers that decide how it hits.

You enter the bow's IBO rating and the details of your setup. Below is what that rating means, the formula behind the adjustment, and a worked example.

How to use it

1. Enter the IBO rating in feet per second, the speed printed by the bow's maker.
2. Enter your draw length and draw weight, each with its own unit.
3. Enter the arrow weight, the finished total in grains or grams, and any additional weight on the string such as a peep or a D-loop.
4. Press Calculate for the arrow speed, momentum, and kinetic energy, or Reset to clear it.

The IBO rating and why your speed differs

The IBO rating is a standard set by the International Bowhunting Organization so that bows can be compared on equal terms. It is the speed measured with a 70-pound draw weight, a 30-inch draw length, and a light 350-grain arrow, which works out to 5 grains of arrow for every pound of draw. With no accessories on the string, that combination gives the fastest number the bow will show.

Most real setups are slower than that, and usually for sensible reasons. A typical archer draws less than 30 inches, hunts with a heavier arrow than 350 grains for better penetration, and adds a peep sight and a string loop. Each of those differences from the test conditions shaves speed off the rating, which is why the figure on the bow is a ceiling rather than what you will chronograph.

The equation it uses

The calculator starts from the IBO rating and applies the standard adjustments. With L for draw length in inches, D for draw weight in pounds, A for arrow weight in grains, and W for added string weight in grains, the speed in feet per second is:

v = IBO + (L − 30) × 10 − W/3 + min( 0, −(A − 5D)/3 )

Each piece is a simple rule of thumb. Draw length adds or subtracts 10 ft/s for every inch away from the 30-inch baseline, so a shorter draw costs speed. String weight subtracts 1 ft/s for every 3 grains of accessories. And arrow weight subtracts 1 ft/s for every 3 grains the arrow runs over the baseline of 5 grains per pound of draw, which is where your draw weight enters. That last term only ever slows the arrow, never speeds it up, since a lighter arrow does not gain you speed beyond the rating in this model.

Momentum and kinetic energy

Speed alone does not tell you how an arrow performs on impact, so the calculator also gives momentum and kinetic energy, working from the arrow's mass and the speed it just found. Kinetic energy, one-half of mass times speed squared, is often quoted as a rough guide to penetration potential. Momentum, mass times speed, describes how well the arrow keeps driving through resistance.

The two reward different builds. Because kinetic energy leans on the square of the speed, light fast arrows tend to look good by that measure. Momentum leans more evenly on mass, so a heavier arrow often carries more of it even at a lower speed, which is why many hunters favour a heavier arrow for deep, reliable penetration despite giving up some speed.

Units and precision

You can enter draw length in inches or centimetres, draw weight in pounds or kilograms, and arrow and string weight in grains or grams, with the calculator converting to the units the formula expects. The speed is reported in both metres per second and feet per second, momentum in kilogram-metres per second, and kinetic energy in joules. Results are shown to a few decimal places, but the real accuracy is set by the formula itself, which is a set of rules of thumb rather than a measurement.

A worked example: a real hunting setup

Take a bow rated at 340 IBO, drawn to 28 inches at 65 pounds, shooting a 425-grain arrow, with 18 grains of accessories on the string.

The draw length is 2 inches short of 30, so that is −20 ft/s. The string accessories take 18/3 = −6 ft/s. The baseline arrow for a 65-pound draw is 5 × 65 = 325 grains, and the actual arrow is 100 grains over that, costing 100/3 ≈ −33 ft/s. Starting from 340, the result is 340 − 20 − 6 − 33 ≈ 281 ft/s, which is about 85.6 m/s. That is a realistic chronograph figure, well below the 340 on the sticker.

Reading the result as an estimate

This formula is the one nearly every arrow speed tool uses, and for a typical compound setup it usually lands within a handful of feet per second of what a chronograph shows. It is a sound way to compare setups and to see how a change, a heavier arrow or a longer draw, will shift your speed before you spend money on it.

It remains an estimate, because real speed also depends on things the formula cannot see: the efficiency of a particular bow and its cams, the string material, how the arrow is tuned, and even the temperature. The single biggest lever is the IBO rating you enter, since everything builds from it, and some bows are rated a touch optimistically. For a number you can stake a shot on, a chronograph is still the final word.

Questions people ask

What is an IBO speed rating?

It is a bow's speed measured under standard conditions set by the International Bowhunting Organization: 70 pounds of draw, a 30-inch draw length, and a 350-grain arrow, with no string accessories. It is the fastest figure the bow will produce.

Why is my real speed slower than the rating?

Because most setups differ from the test conditions. A shorter draw, a heavier arrow, and string accessories each subtract speed, so the rating is a ceiling rather than your actual figure.

How much does draw length affect speed?

About 10 ft/s for every inch away from 30 inches. A longer power stroke accelerates the arrow over more distance, so a 28-inch draw gives up roughly 20 ft/s against the rating.

Does a heavier arrow always shoot slower?

Yes, a heavier arrow leaves the string slower, since the same stored energy moves more mass. But it often carries more momentum and penetrates better, which is why many hunters accept the lower speed.

How accurate is this estimate?

For a typical compound bow it is usually within a few feet per second of a chronograph reading. Bow efficiency, string condition, tuning, and temperature can move it, so verify with a chronograph when an exact number matters.

References

A quick note on where this comes from. The baseline test conditions are the speed rating standard of the International Bowhunting Organization, and the adjustment formula is the long-established set of rules of thumb used across the archery industry to bring that rating toward a real setup. The momentum and kinetic energy follow the standard definitions from physics.

1. International Bowhunting Organization (IBO), bow speed rating standard (70 lb draw weight, 30-inch draw length, 350-grain arrow). https://www.ibo.net
2. OpenStax, University Physics Volume 1, definitions of momentum and kinetic energy. https://openstax.org/books/university-physics-volume-1/pages/9-1-linear-momentum