What the Bow FPS Calculator Does
Every compound bow carries a speed rating from the manufacturer, usually quoted as an IBO or ATA speed. That number is measured under laboratory conditions that almost no hunter or target archer actually shoots: a 70 pound draw weight, a 30 inch draw length, and a 350 grain arrow. Change any one of those three variables and the arrow leaves the bow at a different speed, sometimes much slower than the box promises. The Bow FPS Calculator closes that gap by adjusting the rated speed for your real setup, so the number you plan around is the number your arrow actually flies.
The adjustment matters because arrow speed drives everything downstream. It sets your trajectory and therefore your pin gaps, it determines kinetic energy and momentum on impact, and it tells you whether a given arrow weight is a sensible match for your draw weight. A bow rated at 340 feet per second that really shoots 293 feet per second is not a defective bow. It is simply a bow shot at 60 pounds with a 28 inch draw and a heavier arrow, and the calculator shows exactly where those 47 feet per second went.
The Adjustment Rules Behind the Math
The calculator uses three widely quoted rules of thumb that bow technicians have used for years. First, draw weight: every pound below the 70 pound rating costs about one foot per second, and every pound above it gains about one foot per second. Second, draw length: every inch below the 30 inch rating costs about ten feet per second, and every inch above it gains about ten feet per second. Third, arrow weight: every three grains above the 350 grain rating costs about one foot per second, and every three grains below it gains about one foot per second. A fifth input covers speed lost to accessories such as a peep sight, string silencers, and a loop, which together typically cost a few feet per second.
These rules are estimates, not laboratory physics. String material, cam design, brace height, and tuning all shift real speed by a few feet per second in either direction. Treat the adjusted speed as a planning figure accurate within a small margin, and confirm it with a chronograph when the exact number matters, for example when you are setting sight tapes or checking that a hunting setup clears a legal or ethical energy threshold.
Worked Example With Real Numbers
Take a bow rated at 340 feet per second under IBO conditions. Shoot it at 60 pounds of draw weight, a 28 inch draw length, and a 400 grain finished arrow, with no accessory loss. The draw weight adjustment is 10 pounds below rating times one foot per second, a loss of 10. The draw length adjustment is two inches below rating times ten feet per second, a loss of 20. The arrow weight adjustment is 50 grains above rating divided by three, a loss of about 16.7. Subtract all three from 340 and the adjusted speed is 293.3 feet per second.
From that speed the calculator derives two impact figures. Kinetic energy equals arrow weight times speed squared divided by 450240, which gives 400 times 293.3 squared divided by 450240, or 76.4 foot-pounds. Momentum equals arrow weight times speed divided by 225218, which gives 400 times 293.3 divided by 225218, or 0.521 slug-feet per second. For comparison, the same bow shot exactly at rating conditions, 70 pounds, 30 inches, and a 350 grain arrow, delivers 330 feet per second only if the rating itself is 330, with kinetic energy of 84.7 foot-pounds and momentum of 0.513 slug-feet per second. The heavier arrow in the first example is slower yet carries nearly as much energy and slightly more momentum, which is why many hunters prefer a heavier arrow despite the speed loss.
What the Three Outputs Mean
Adjusted speed is the headline number. It feeds your sight setup, your trajectory estimates, and your expectations at distance. A flatter trajectory from a faster arrow forgives small range estimation errors, which is why target archers chase speed. Kinetic energy measures the energy the arrow carries to the target and is the figure most often quoted in hunting regulations and broadhead discussions. Momentum measures how hard the arrow keeps pushing through resistance, and many experienced bowhunters weigh it more heavily than kinetic energy when judging penetration on larger game, because momentum falls off more slowly as the arrow passes through tissue.
Neither figure alone decides whether a setup is good. A very light arrow can post an impressive speed and a respectable kinetic energy number while giving up penetration, and a very heavy arrow can be so slow that trajectory becomes unforgiving past 30 yards. The useful question is balance: enough speed for a flat trajectory at your normal shooting distance, enough momentum for reliable penetration on the game you hunt, and an arrow spine that matches your bow at your draw weight.
Inputs Explained
Rated speed is the manufacturer IBO or ATA figure for the bow, usually found on the product page or in the manual. IBO ratings use a 70 pound draw, 30 inch draw, and 350 grain arrow, while ATA ratings use a 60 pound draw, 30 inch draw, and 540 grain arrow, so check which standard your number comes from before entering it. Draw weight is the peak weight you actually shoot, not the maximum the bow can reach. Draw length is your measured draw length, not the module setting printed on the cam, since the two can differ by half an inch.
Arrow weight must be the finished arrow weight: shaft plus insert, point, fletching, nock, and wrap if you use one. Weighing a bare shaft and entering that number is the most common mistake, and it inflates the speed estimate by several feet per second. Accessory speed loss is an optional deduction for a peep sight, string silencers, and a D-loop. Enter zero if you shoot a bare string, or a few feet per second if your string carries the usual accessories.
- Rated speed: the manufacturer IBO or ATA figure for your bow model.
- Draw weight: the peak weight you actually shoot, in pounds.
- Draw length: your measured draw length, in inches.
- Arrow weight: the finished arrow weight, in grains.
- Accessory loss: speed lost to peep, silencers, and loop, in feet per second.
Common Mistakes That Skew the Result
The most frequent error is mixing rating standards. Entering an ATA rating of 320 feet per second into a calculator built around IBO conditions, or the reverse, shifts the answer by a large margin because the two standards assume different draw weights and arrow weights. The second common error is entering the arrow weight from a catalog table for a bare shaft rather than the finished arrow you actually shoot. The third is guessing draw length instead of measuring it, since a one inch error moves the result by ten feet per second, more than any other single input except a large draw weight change.
A subtler mistake is treating the adjusted speed as exact. Chronograph readings vary with lighting, distance from the chronograph, and arrow-to-arrow consistency, so a real bow may read a few feet per second either side of the estimate. Use the calculator to compare setups and to set expectations, then verify with a chronograph before finalizing sight tapes or making a hunting decision that depends on a hard energy threshold.
Getting the Most From Your Setup
If the adjusted speed disappoints, change the variable that costs you the least elsewhere. Dropping arrow weight buys speed at roughly one foot per second per three grains, but it also trims momentum, so a small reduction is usually wiser than a drastic one. Adding draw weight buys about one foot per second per pound, but only if you can hold and shoot that weight with good form. Lengthening draw length buys ten feet per second per inch, but draw length is set by your body, not by preference, and shooting an overlong draw wrecks accuracy far more than the extra speed helps.
For most hunters the sweet spot is a finished arrow around 400 to 450 grains shot from a bow they can draw smoothly, producing adjusted speeds in the high 200s with kinetic energy well above the level needed for deer-sized game. Target archers can go lighter, since penetration is not the goal and a flat trajectory pays off at longer distances. Whatever you choose, run the numbers first, then confirm on a chronograph, and let the verified speed set your sight marks.
Chronograph Checks and Sight Setup
A chronograph is the final authority on your real speed, and using one well is straightforward. Shoot through the sensors from a consistent distance, usually a few feet in front of the chronograph, and record three to five arrows rather than trusting a single shot. Discard obvious outliers caused by a poor release or a sensor misread, and average the rest. If the measured average sits within a few feet per second of the calculator estimate, your inputs were accurate and the estimate did its job. If the gap is larger, recheck finished arrow weight first, then draw length, since those two inputs explain most large gaps.
Once speed is verified, set your sight marks from the verified number rather than the estimate. Sight tapes and multi-pin gaps are built from trajectory tables keyed to speed, so a ten foot per second error shifts point of impact noticeably at longer distances. Record the verified speed, arrow weight, and draw weight together in your bow log, so that a future string change or arrow change can be evaluated against a known baseline instead of from memory.
Frequently Asked Questions
What is IBO speed and how is it different from ATA speed?
IBO speed is measured at 70 pounds of draw weight, 30 inches of draw length, and a 350 grain arrow. ATA speed is measured at 60 pounds, 30 inches, and a 540 grain arrow. The two numbers describe the same bow differently, so always check which standard a rating uses before comparing bows.
How much speed do I lose per pound of draw weight?
Roughly one foot per second per pound below the 70 pound rating, and about the same gained per pound above it. Dropping from 70 to 60 pounds therefore costs about 10 feet per second, before draw length and arrow weight are considered.
How much speed does draw length cost?
About ten feet per second per inch below the 30 inch rating, and about ten gained per inch above it. A 28 inch draw costs roughly 20 feet per second against the rating, which is why shorter draw archers often see lower speeds than advertised.
How does arrow weight change speed?
About one foot per second for every three grains above the 350 grain rating, and about one gained for every three grains below it. A 400 grain arrow costs about 16.7 feet per second against the rating, while a 320 grain arrow gains about 10.
Why is my chronograph reading different from the calculator?
The calculator uses rules of thumb, while real bows vary with string material, cam design, brace height, tuning, and chronograph setup. A difference of a few feet per second either way is normal, and larger gaps usually mean an input such as finished arrow weight was estimated rather than measured.
What is a good kinetic energy level for deer hunting?
Many guides suggest at least 40 to 50 foot-pounds for deer-sized game, with more preferred for larger animals. The example setup in this article produces 76.4 foot-pounds, comfortably above that range, even though its speed is well below the rated figure.
Is momentum more important than kinetic energy?
Momentum often predicts penetration better than kinetic energy, because it falls off more slowly as the arrow meets resistance. A heavier, slower arrow can carry slightly more momentum than a lighter, faster one, which is why momentum is worth checking alongside kinetic energy.
Should I use a lighter arrow to gain speed?
A lighter arrow does gain speed, but it also gives up momentum and can make the bow louder and harder on the limbs. Small reductions are usually fine, while very light arrows trade penetration for trajectory and are better suited to target shooting than hunting.
Does a peep sight really slow the arrow?
A peep sight, string silencers, and a D-loop add weight to the string and typically cost a few feet per second combined. The effect is small, but it is real, which is why the calculator includes an accessory loss input.
How do I measure my finished arrow weight?
Weigh a complete arrow on a grain scale, including shaft, insert, point, fletching, nock, and wrap. Do not use the bare shaft weight from a catalog, since the finished arrow is usually 40 to 80 grains heavier.
Can I use this calculator for a recurve or longbow?
The adjustment rules were developed for compound bows with IBO or ATA ratings, so they do not transfer cleanly to traditional bows. Traditional bows are better evaluated by chronographing the actual setup rather than adjusting a factory rating.
What draw length should I enter if I am between sizes?
Enter your measured draw length, ideally confirmed by a pro shop, rather than the module setting on the cam. A half inch error shifts the result by about five feet per second, so an accurate measurement matters more than a round number.
Does brace height affect the result?
Brace height affects real speed, since a shorter brace height gives a longer power stroke, but the simple adjustment rules used here do not include it. Treat it as one reason the estimate may differ slightly from a chronograph reading.
Why does speed matter for pin gaps?
A faster arrow flies flatter, so the distance between sight pins is smaller and small range errors cost less. A slower arrow drops more between distances, so accurate range estimation becomes more important, especially past 30 yards.
Is a faster bow always a better bow?
Speed helps trajectory, but accuracy, quietness, smooth draw, and reliable penetration matter at least as much. A bow you shoot well at 293 feet per second will outperform a faster bow you shoot poorly, so treat speed as one factor among several.