Bow FPS Calculator

How the Calculator Turns an IBO Rating Into Real Speed

An IBO speed rating is measured under one fixed setup: 70 pounds of draw weight, 30 inches of draw length, a 350 grain arrow and a bare string with no peep and no D-loop. Your bow rarely matches that setup, so the number printed on the spec sheet is a starting point, not your arrow speed.

The calculator starts at the rating and walks it toward your setup in four steps. Each step uses a rule of thumb that bow shops have relied on for years, and each step is shown separately so you can see where the speed went. Draw length and draw weight change how much energy the bow stores. Arrow weight and string accessories change how much of that energy leaves with the arrow.

The output is an estimate, not a chronograph reading. It is close enough to tell you whether a bow makes the speed you want without a trip to the shop, and it is honest enough to show a 55 fps spread between two setups that carry the same rating.

What IBO and ATA Speed Ratings Measure

The Archery Trade Association publishes a speed standard that holds draw weight to 70 pounds within 0.2 pound, draw length to 30 inches within a quarter inch, and the arrow to 350 grains. The test bow is fitted the way it ships, with a fletched arrow, a nock point and often a D-loop. That tight protocol is why an ATA number is the fairer figure when you compare two bows.

The International Bowhunting Organization allows more room. Its competition rules permit draw weight up to 80 pounds with a 5 grain per pound arrow and set no fixed draw length, so a manufacturer can post a high number by testing a long, heavy draw and a bare string. The same bow can read 5 to 10 fps higher under IBO rules than under ATA rules.

Both ratings share the same reference arrow weight, 350 grains at 70 pounds, which is 5 grains per pound. That shared anchor is what lets one calculator serve both labels. Enter whichever number your spec sheet prints and read the result as a comparison, not a promise.

The Four Adjustments, One at a Time

Each rule below is linear, which keeps the mental math simple and keeps the error small over the range most bows live in. The compound column is the model the calculator uses by default. The recurve column shifts the reference to 60 pounds and 28 inches, because recurve speed is usually quoted there.

The draw length rule is the one that surprises people. One inch of draw is 10 fps, so a bow rated at 30 inches and shot at 28.5 inches loses 15 fps before any other factor lands. The rule holds because a longer draw pulls the string further along the power stroke, and the limbs store more energy for the same peak weight.

The arrow weight rule is the one that changes the feel of a setup. Fifty extra grains costs about 16.7 fps, and that is roughly the difference between a flat 30 yard pin and a pin you have to hold high. String extras are cheap in speed but expensive in noise, which is why a 20 grain peep and D-loop combo costs only 6.7 fps.

Worked Example: 335 IBO at 65 Pounds, 28.5 Inches, 420 Grains

Take a bow rated 335 fps at 70 pounds and 30 inches. You shoot it at 65 pounds, 28.5 inches, with a 420 grain arrow and 20 grains of string extras. Here is the full walk through the model.

Draw length first: 28.5 inches is 1.5 inches under 30, and 1.5 x 10 = 15, so subtract 15 fps. Draw weight next: 65 pounds is 5 pounds under 70, and 5 x 2 = 10, so subtract another 10 fps. Arrow weight: 420 grains is 70 grains over 350, and 70 / 3 = 23.3, so subtract 23.3 fps. String extras: 20 / 3 = 6.7, so subtract 6.7 fps.

Add the steps: 335 - 15 - 10 - 23.3 - 6.7 = 280. The estimate is 280 fps, which is 55 fps below the rating and about 16 percent slower.

Now the energy. Kinetic energy is 420 x 280 x 280 / 450,240 = 73.1 ft-lb. Momentum is 420 x 280 / 225,400 = 0.522 slug-ft/s. Arrow weight per pound of draw is 420 / 65 = 6.46 grains per pound, comfortably above the 5 grain floor. That setup sits in the middle of the deer range and below the usual elk recommendation, which is a sensible place for a 65 pound bow.

Kinetic Energy and Momentum: Two Different Numbers

Kinetic energy measures the work the arrow can do, and it rises with the square of speed. The archery form is KE = (grains x fps x fps) / 450,240, where the constant folds in 7,000 grains per pound and twice the acceleration of gravity. Double the speed and you quadruple the energy, which is why 20 fps matters more than 100 grains.

Momentum measures how hard the arrow is to stop, and it rises in a straight line with speed. The form is p = (grains x fps) / 225,400, which returns slug-ft/s. Momentum is the better predictor of penetration through hide, muscle and rib, because a heavy slow arrow carries more of it than a light fast one at the same energy.

The table shows the trade in one glance. The 420 grain arrow is slower than the 350 grain arrow but carries more momentum, 0.522 against 0.512, and only 11.6 ft-lb less energy. The recurve row shows the other end: a 50 pound traditional bow with a heavy arrow is a close range tool, and no amount of tuning turns it into an elk bow.

Matching Energy to Game

The thresholds below are rules of thumb from the archery industry, not legal requirements. States regulate bowhunting by draw weight, arrow length and broadhead width far more often than by energy. Washington, for example, requires at least 40 pounds of pull at 28 inches or at full draw for big game, a rule you can read in the Washington archery regulation.

Read the two columns together. A 45 ft-lb setup with 0.52 momentum penetrates better than a 55 ft-lb setup with 0.38 momentum, because the second one gets its energy from speed with a light arrow. Bowhunters who chase speed often end up with less penetration than the energy number suggests.

Every recommendation here assumes a sharp broadhead, a tuned arrow and a shot placed in the vitals. Energy does not fix a poor hit, and no state agency sets a foot pound minimum that overrides its own draw weight rule.

Arrow Weight, the 5 Grains per Pound Floor, and the Speed Trade

The 5 grains per pound rule sets the lightest safe arrow for a given draw weight. At 70 pounds the floor is 350 grains, at 65 pounds it is 325 grains, and at 50 pounds it is 250 grains. Shooting below that floor sends too little mass downrange for the energy stored in the limbs, and most bow makers void the warranty because the bow absorbs the excess as noise and vibration.

Above the floor, more weight buys penetration and quiet at the cost of speed and trajectory. Easton's arrow selector starts from your bow's speed rating and draw weight for exactly this reason: the shaft has to be stiff enough for the energy and heavy enough to protect the bow.

Notice the shape of the energy column. It peaks near 420 grains and falls away on both sides, even though speed keeps dropping. The 320 grain arrow is the fastest and the least efficient, and it sits below the 5 grain floor for a 65 pound bow, so it is not a legal hunting setup in this example. The 520 grain arrow gives up 33 fps for 2.8 ft-lb less energy but gains a lot of momentum, which is the trade a hunter makes for elk.

Why Your Chronograph Reads Lower Than the Calculator

A chronograph measures the arrow a few feet from the bow, and the arrow is already slowing. Drag costs roughly 1 to 2 fps per foot in the first yards, so a reading taken at 3 feet can sit 2 to 4 fps under the true muzzle speed. Move the unit back and the number drops further.

Four other factors push the real number below a clean linear model. A new string stretches and settles, which changes brace height and let-off. A bow that is out of tune loses a few fps to fletching contact and nock fit. Temperature and altitude change air density, which changes drag. And the model treats the four adjustments as independent, while a real bow couples them: a longer draw also changes the holding weight and the limb load.

Treat a 3 to 10 fps gap between the calculator and your chronograph as normal. A gap above 15 fps usually points to a setup problem worth fixing, such as a draw length measured to the wrong reference, a draw weight scale that reads high, or an arrow weight that includes the insert twice.

Measure the arrow on a grain scale, measure draw length to the nock throat plus 1.75 inches, and use a bow scale at full draw. Those three numbers drive the whole estimate, and each one is easy to get wrong by enough to move the answer by 10 fps.