Draw length is an extremely important component to good form and repeatable shot mechanics.
However, you will typically find that draw length is discussed in regards to compound bows. This is because a compound bow has a set maximum draw length, once you hit the “back wall” of the draw cycle (the back wall is the place in the draw cycle where you can no longer pull the bow any farther.)
If you don’t want to use the N1 Outdoors draw length calculator above and instead want to figure your estimated draw length with good ‘ole mathematics, the first thing you need to do is measure your armspan (or wingspan).
First you need to determine your armspan. Simply spread out your arms (don’t over-stretch) with your palms facing forward.
Measure the distance from the tip of one middle finger to the tip of the other middle finger.
Do some old school calculating
Take the measurement from step 2 and divide it by 2.5. This will provide you with a fairly reliable estimate of your appropriate draw length. Check with your local bow shop to help you fine tune.
“Draw length is a specified distance, or the distance at the archer’s full draw, from the nocking point on the string to the pivot point of the bow grip (or the theoretical vertical projection of a tangency line to the pivot point parallel to the string) plus 1 3/4”. Draw length from pivot point shall be designed at DLPP and shall be called TRUE DRAW LENGTH. EXAMPLE: 26 1/4” DLPP plus 1 3/4” is the equivalent of 28” draw”
AMO Standards Committee FIELD PUBLICATION FP-3, 2000
So, by this definition, if you have a “28-inch draw length”, that means that at full draw the distance from the deepest point of the grip to the nocking point of the string would be 26.25 inches.
It’s important not to confuse draw length with arrow length. While they may be similar lengths, they are measured differently.
Arrow length is measured from the front end of the arrow shaft (not including the broadhead or field point) to the throat of the nock.
Your arrow length can vary depending on what you are trying to achieve regarding the FOC and spine of your arrow. (Spine is how much your arrow flexes at a set length with a set weight attached. More on arrow spine here.)
Arrow length is the measurement from the end of the arrow shaft to the throat, or groove, of the arrow’s nock.
When it comes to archery, IBO stands for International Bowhunters Organization.
What is IBO Speed and how is it calculated?
You might have seen where compound bows have an IBO rating.
But, what does that really mean?
Well, the IBO speed of a bow is calculated using a bow with a draw weight of 70 lbs, a draw length of 30 inches, and an arrow weight of 5 grains per pound of draw weight. So, that would mean a 70-lb draw weight would be shooting an arrow that weighs 350 grains (70 x 5 = 350).
That arrow is then shot through that bow through a chronograph, which measures the arrow speed. The average speed becomes known as the IBO rating of that particular bow.
The problem with IBO ratings, however, is that they are not typical of most hunters’ setups.
For example, a 30-inch draw length is rather long for the average archer. Also, an arrow weight of 350 grains is simply not typical of most hunting setups. And, with high FOC arrows gaining popularity, the arrow’s grains per inch is often significantly greater.
So, when using the arrow speed calculator above, you must understand that the IBO rating you enter is based on this premise. The calculator will give you an estimate of how changing the inputs that make up IBO could affect your arrow speed.
The arrow speed calculator makes the following assumptions:
Every inch of draw length under 30″ will subtract 10 ft per second from the IBO value.
Every inch of draw length above 30″ will add 10 feet per second to the IBO value.
Every 3 grains of total arrow weight above draw weight multiplied by 5, will subtract 1 foot per second from the IBO value.
Every 3 grains of additional weight on the bow string will subtract 1 foot per second from the IBO value.
When it comes to your bowhunting setup, knowing the “Kinetic Energy” of your arrow allows you to know how much energy that arrow possesses due to motion, from being shot by your bow.The “Momentum” tells youhow much force it will take to stop your arrow when it reaches its intended target.
Kinetic Energy and Momentum Arrow Calculator
Kinetic Energy and Momentum Calculator
Arrow weight Value must be between 250 and 1000 grains.
move slider or enter value
Arrow speed Value must be between 100 and 500 Feet Per Second.
move slider or enter value
If you know your arrow’s weight (in grains) and your arrow’s speed (Feet Per Second), then you can use our Kinetic Energy and Momentum calculator above to find out each! Simply move the sliders or enter the values in the blanks. And, if you really want to take a deep dive into the Kinetic Energy of arrows, check out what the Ranch Fairy is up to below…
Kinetic Energy And Bowhunting (How I Got Here)
As you may already know, the ‘ole Ranch Fairy (that’s me) is quite out of the norm in his measuring of arrow systems. (If you aren’t aware, I am definitely one of the strange ones in the bowhunting world.)
Anyway, just to set the record straight, the biggest overlap between Dr. Ed, the Ashby Bowhunting Foundation, and the Ranch Fairy is simple: We want to know the highest performing projectile for all impact points to pass through the animal you are hunting.
The first time Rocketman said, “well, Troy, a bow is just a spring with fixed Kinetic Energy,” I thought… BLASPHEMY!
But, from what I understand, he is right.
The bow can’t “make” more KE. It is what it is.
BUT, you can change the arrow and gain some…..so hang on. Let me set the table here…
A bow is just a spring with a fixed Kinetic Energy. It can’t make more kinetic energy than what it already possesses.
KE Arrow Testing
On a basic level, radar measures a projectile’s speed over distance.
The testing unit that we used measures 5 total distances. So, if you want to shoot 60 yards, the computer divides that distance into 5 increments.
[NOTE TO SELF – you need to put the target further than 60 yards to capture the flight speed. To address this, we placed the target at 70 yards. Because, if impact is at 60 yards, the data would be flawed for velocity testing because the target stops the arrow at a yardage that it should be being measured.]
The top line is the launch velocity. The change in velocity is super boring… Until you look at the 60 yard impact KE.
The gap in the data sets shows the significant reduction in KE over distance. However, you see that gap narrow as arrow mass increases.
As you can see, in all the above graphs, the launch KE is relatively constant, but alas, further away, at 60 yards, with higher mass projectiles, we see something worth pondering. (Well, only if you think math is correct!)
What are the results telling us? (Please pardon the steam coming out of my ears)
So, despite my heavy arrow bias, (I’m not much of a hair splitter), increasing launch KE 3-6 ft/pounds is really boring.
But the lower line, at 60 yards, is worth chewing on.
If you search around, many of the wide mechanical broadheads suggest KE’s of 45-60 ft-lb’s. Now, they don’t go out on a limb and say, “that will create a pass through, or break bones.” It’s just a recommended impact KE.
Formula for Kinetic Energy: K.E. = 1/2mv2 (where m=mass of object and v=velocity)
And be clear, just like the firearms world, this is launch KE, maximum velocity. This is because a projectile can’t go faster once it leaves the muzzle or the string… It’s always slowing down.
Silly aerodynamic drag.
Now in a vacuum… oh wow, throw in some zero gravity and guess what?
It still doesn’t go faster….. it would maintain launch velocity and you wouldn’t be able to breathe to test it.
Some adult field points and some, ahem, “super weenie points.”
There have been multiple companies and YouTube personalities showing fixed blade vs. mechanical pressure testing on deer thoraxes and other items simulating a critter. They use very complicated mechanical devices down to something as simple as a bathroom scale.
Let’s just say, the HUGE differences are eye popping.
It’s not half a pound or 3, it’s exponential. The “precision” of the device doesn’t matter when the difference is 40 pounds. Please search those tests up, because I know you’ll go do it anyway.
When it comes to arrow penetration, harder things push back harder… you can just blame Sir Isaac Newton for that and keep my hate mail down!