Most compound bows nowadays have an advertised speed of between 300 and 340 FPS. There are a few things you need to understand about these numbers so that you can reasonably manage your expectations.
This Is Point-Blank Speed
A bow advertised as delivering 320 FPS will shoot an arrow with a velocity of 320 feet per second. This is only true for point-blank range though. In other words, this speed will be considerably lower once the arrow actually reaches the target. This might be obvious and it’s definitely not a bad thing, but it’s important to understand.
IBO Speed Tests
To keep speed ratings consistent across the board, compound bow manufacturers employ the IBO (International Bowhunting Organization) testing standards.
Since the speed of a bow depends on many different factors (draw length, draw weight, arrow weight are the most important), it made sense to develop a set of common testing conditions. As such, all IBO tests are performed with:
- A 70 lbs. draw weight version of the bow
- A 30″ draw length
- A 350 grain arrow
Given how even the smallest change in the above settings can impact arrow speed, we need to look into this further.
Elements That Impact Arrow Speed
Let’s take a look at some estimates. Once we’re done, I’ll provide you with a practical example on how to use these numbers to your advantage.
Draw Length Impact On Speed
For every 1″ of reduction in draw length, you can expect to lose around 10 FPS of arrow speed. IBO speed tests are conducted using 30″ draw length, however most people have a draw length of around 28″. This is already a 20 FPS reduction compared to the IBO speed rating.
Draw Weight Impact On Speed
For every 10 lbs. of reduction in draw weight, expect to lose around 15-20 FPS. For many beginners using a 70 lbs. draw weight compound (like the ones used during IBO tests) is not possible. A beginner will likely go for a 60 lbs. version. That’s another 15-20 FPS reduction.
Arrow Weight Impact On Speed
For every extra 5 grain of arrow weight, expect the speed of your bow to be reduced by around 1.5 FPS. IBO speed tests are conducted using 350 grain arrows, however most people will be hunting using arrows weighing a minimum of 425 grain. That’s 75 grain over the IBO arrow weight, which reduces FPS by an estimated 22 FPS.
Extra Accessories On String
When hunting in the real world, you will be likely using some accessories. a D-loop and peep hole are standard, which together weigh around 15 grain. This can rob you of another 5-6 FPS.
Release Method Impact On Speed
IBO tests are conducted using an automated shooting machine that releases the string with absolute perfection. A human isn’t capable of such release accuracy as a machine. For this reason, you’ll need to subtract another 2-3 FPS compared to the IBO rating.
Demonstrating The Numbers Above Using a Real Life Example
Ok, so suppose you bought yourself a bow with an IBO speed of 320 FPS. You are a beginner, so you get yourself a 60 lbs. draw weight version of the bow. You also have the average draw length of 28″, and decide to use 425 grain arrows for an optimal speed-to-kinetic energy ratio. You also install some accessories onto your bow string. Here’s what happens:
- You will lose around 17 FPS due to using a 60 lbs. rather than a 70 lbs. bow
- You will lose around 20 FPS due to using a 28″ draw length rather than 30″
- You will lose around 22 FPS due to using a 425 grain arrow rather than the 350 used in IBO testing
- You will lose around 5 FPS due to extra accessories on the string
- You will lose around 3 FPS due to imperfections in human release mechanics
17 + 20 + 22 + 5 + 3 = 67 FPS that have been lost. This means that the actual FPS of your bow will be 320 – 67 = 253 FPS. Of course, things don’t need to be that bad. If you are using a 70 lbs. draw weight, your compound bow would have a 270 FPS. And if your draw length is indeed 30″ (as it is during IBO testing), this number would be closer to 290 FPS. And so on.
What’s The Bottom Line?
The bottom line is that you should never expect to achieve the same FPS speed as advertised by the manufacturer. The actual difference can vary significantly depending on the bow setup, but you are guaranteed that there will be at least some difference (even if only due to the human release factor).
How Do The Numbers Above Impact Hunting Feasibility?
Understand that how good a compound bow is for hunting depends not only on FPS (speed), but also on KE (Kinetic Energy). An arrow can fly slower but still deliver more punch upon impact than a much faster arrow. In our “real life” example above, we determined that a beginner will likely only achieve ~255 FPS from a compound bow rated at 320 IBO speed. Would these ~255 FPS be enough to hunt with?
We need to figure out the kinetic energy (KE) of the arrow. So assuming we have a bow rated 320 FPS IBO speed, an if you have a 60 lbs. version of the bow set to 28″ draw length, with some accessories on the string, a 425 grain arrow would deliver approximately 59 ft-lbs of kinetic energy at point-blank range. What can you do with this much energy?
To answer that, let’s take a look at Easton’s Kinetic Energy Hunting Chart:
|Kinetic Energy:||You Can Hunt:|
|< 25 ft-lbs||Small Game (groundhog, rabbit, wild turkey)|
|25-41 ft-lbs||Medium Game (Antelope, Whitetail deer)|
|42-65 ft-lbs||Large Game (black bear, wild boar, elk)|
|> 65 ft-lbs||Largest Game (Grizzly bear, Cape buffalo, Musk Ox, African elephant)|
As you can see, even though the actual FPS in our example is considerably lower than the bow’s advertised IBO speed, the arrow would still deliver enough kinetic energy to take down large game (between 42-65 ft-lbs). This bow would therefore be enough to take down pretty much any game animal in the United States. Of course, these are just numbers; whether you can actually harvest an elk or black bear with this bow would depend on how accurately you shoot.
Also, keep in mind that kinetic energy deteriorates the further the arrow travels. Expect to lose an average of 1.5 ft-lbs for every 10 yards traveled. So if the arrow has 59 ft-lbs KE at point-blank range, expect this value to be closer to 55 ft-lb if shooting a target located 30 yards away. This is still more than enough to take down large game, assuming your shot is well placed.