Kinetic Energy (KE for short) is described as the energy stored within a moving body. In the world of compound bows, KE is used to determine how much damage an arrow will will cause, or how deeply it will penetrate, once it impacts the target. To calculate KE, an archer will need to know the following:
- Arrow speed
- Arrow weight
Let’s take a look at each of those separately and see how they impact kinetic energy.
The Impact of Arrow Speed on Kinetic Energy
The faster an arrow flies, the more kinetic energy it will store regardless of its weight. When purchasing a compound bow, you’ll notice it has an IBO (International Bowhunting Organization) speed rating, which could be for example 320 FPS. What this number tells you is that an arrow was shot from this bow and its point-blank speed was measured to be roughly 320 feet per second.
What you might not know is that all IBO speed tests are conducted using strict bow settings, which are considered the industry standard. These settings are:
- Draw weight of 70 lbs.
- Draw length of 30″
- Arrow weight of 350 grain
It’s very important to understand that as the values above change, so does arrow speed. So if you were to purchase a bow marketed as having 320 FPS, but you bought:
- The 50 lbs. version of the bow
- Had it set to 28″
- Used 400 grain arrows
Your arrows’ actual FPS would not be 320; it would be closer to 240 FPS. So just because it says something on the label do not assume that this is the value you’ll experience. We have prepared a simple guide that will help you determine your bow’s REAL speed – based on your draw length, draw weight, and arrow weight.
The Impact Of Arrow Weight On Kinetic Energy
The heavier the arrow you shoot from your compound bow, the more Kinetic Energy it will hold. When calculating the weight of your arrow, make sure to include the weight of everything:
It’s important to keep in mind that the heavier your arrow is, the slower it will fly and hence it will be harder to properly aim. Personally, I like my arrows to weigh around 7-8 grain for every pound of draw weight. So if I’m shooting a 60 lbs. compound, I’d use 7 x 60 = 420 grain arrows at a minimum, and 480 grain maximum . I feel this strikes the perfect balance as far as my tastes in accuracy/KE go. If you are a beginner, you’ll probably need to experiment with various arrow weights until you find one that is heavy enough but doesn’t noticeably impacting your aim.
How To Calculate Kinetic Energy?
So let’s assume you have a compound bow rated at 320 IBO speed. You get the 50 lbs. version and set it to 28″ draw length. You decide to use 400 grain arrows, and using our FPS guide you calculate the point-blank speed of your arrow to be 240 FPS.
Now we have what we need to calculate KE:
- Arrow speed: 240 FPS
- Arrow weight: 400 grain
The formula for calculating Kinetic Energy is:
(Arrow weight) x (Arrow speed) x (Arrow speed) / 450,240
Let’s run our numbers through the formula (and yes, I’m using a calculator 🙂
Step one: (400) x (240) x (240) / 450240
The above results in: 23040000 / 450240
Which finally gives us: 51.17
This means that the Kinetic Energy of a 400 grain arrow, shot from your specific bow using our example’s specific settings (50 lbs. and 28″ draw) will result in 51.17 ft-lbs of kinetic energy.
Kinetic Energy Deterioration Rate
In our above example, we’ve calculated Kinetic Energy to be 51.7 ft-lbs. It’s important to remember, however, that this is only point-blank KE. Obviously in everyday conditions you are not shooting at targets located at point-blank range. So how do you measure the exact KE your arrow will carry after having flown a specific distance? It’s fairly simple:
Subtract around 1.7 ft-lbs of Kinetic Energy for every 10 yards traveled by the arrow.
So for example:
- If shooting at a ~20 yard target, subtract around 3.4 ft-lbs
- If shooting at a ~40 yard target, subtract around 6.8 ft-lbs
And so on. In our example above point-blank KE was 51.7 ft-lbs. So if you were shooting a target located 60 yards away, the actual KE your arrow will carry upon impacting the target would be 51.7 – 10.2 = 41.50 ft-lbs.
How Much KE Do You Need To Take Down Game Animals?
The best way to answer this is by presenting Easton’s famous field 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)|
Just make sure you hit the vital organs, otherwise KE won’t cut it.
Is More Kinetic Energy Always Better?
No, it’s not. The heavier the arrow you shoot from your compound, the more kinetic energy it will carry. At the same time however, the heaver the arrow, the slower it will fly and as a result- the faster it will succumb to gravity and friction. What does this mean in practice?
The heavier the arrows you go for, the harder it will be to hit your target (especially past the 30 yard range) as the trajectory of the projectile won’t be as close to a straight line as it would be when using a lighter arrow.
So here is my recommendation:
- If you are a target shooter, go with lighter arrows (350-375 grain for most modern compound bows). Kinetic Energy should not matter to you and your only concern should be accuracy, which should be optimal when using these lighter arrows.
- If you are a hunter, kinetic energy is more important and you should probably take a closer look at it. Read our compound bow reviews, where we list kinetic energy calculations for every bow – an excellent starting point.
I hope this guide has been helpful. Please keep in mind that all numbers calculated above should be treated as rough (but very close) estimates. Actual KE can and most likely will vary slightly, depending on a few factors, such as:
- Weather conditions (wind, air thickness, etc.)
- The length and diameter of your arrow shaft
- The length and height of your arrow vanes
- Your shooting technique
These variables however will not make a big difference in KE – I’m just mentioning them to give you a better understanding of the subject. If you have any questions or ideas, kindly leave a comment below or contact me via e-mail. Have fun!