Use the arrow efficiency calculator to estimate the most efficient arrow for your setup.
Arrow Efficiency Calculator – Instructions
- Select Preferred Arrow Efficiency
- Max Speed: IBO arrow (5 grains/per pound of draw weight).
- Max Efficiency: arrow ~10-15% over IBO (5.5 grains/per pound of draw weight).
- Max Energy: arrow ~20% over IBO (6 grains/per pound of draw weight).
- Chronograph Arrow Speed
- Shoot three arrows and use the average of the three shots.
- Enter Data Below for Results
- Enter speed and arrow weight data into the calculator.
- View Results*
- Red: Highlights specific speeds in fps and MAX energy (KE and M).
- Row with Black background: represents arrow weight with Max efficiency (mE).
*All data are approximate. Use the Arrow Efficiency Calculator to find the optimal arrow for your bowhunting setup.
by Bob Robb
The foundation for accurate bow shooting is to create a bow-and-arrow set-up that balances all the required elements for precise arrow placement downrange. If either is out of kilter, accuracy will suffer. The above arrow efficiency calculator is designed to assist bowhunters with their setup.
First and foremost, the draw length of your compound bow must be set exactly to your own body morph. If your draw length is 28 inches, your bow must be set at 28 inches – not 27 ½, or 28 ½, but 28 inches. Also, the bow’s draw weight should be one that you can pull with a modicum of strain, but not one that forces you to “cheat” when pulling the string back. For maximum arrow speed, flat arrow trajectory, and power downrange, you want to pull the most draw weight you can pull, but not too much.
Choosing the right arrow shaft comes next. After using the arrow efficiency calculator, ensure you select a correctly-spined arrow for your setup. You must use a shaft with the correct arrow spine for your draw length/draw weight combination. Arrow spine is a measure of stiffness, gauged by how much the arrow flexes when weight is applied to the center of the shaft. To determine static spine, a 28-inch arrow is supported at both ends, and a 1.94 lb. weight is hung from the middle.
The amount of flex that is induced on the arrow shaft by the force of the weight is then measured, giving the “static” spine rating. For example, if an arrow bends .340-inch at the center, then the shaft has a static spine deflection of 340. A shaft’s spine is indicated by a number on the shaft and charts created by arrow manufacturers. The higher the number, the easier the shaft to bend. For example, an arrow with spine number 340 is stiffer than a 400 spine arrow.
Arrow Weight and Flight
Arrow shafts come in varying weights, measured in grains/inch, in the same spine size. Thus, you can choose how heavy an arrow you want to shoot and still use the proper spine. All the elements of arrow flight are controlled by your bow’s draw weight/draw length combination, the weight of your finished arrow shaft, and what the forward of center (FOC) balance of the shaft is. This, in turn, will determine how much kinetic energy and momentum the shaft has when it reaches the target. (This data is provided in the arrow efficiency calculator).
You must also be able to tune your bow-and-arrow combination so the arrows fly like laser beams, without unnecessary wobble that will destroy accuracy. All arrows flex when they leave the bow, but a properly tuned set-up minimizes this wobble. You can check the tune by shooting the shaft through paper, checking the tear made by the fletches, and making adjustments to your arrow rest position. There are online charts that will walk you through the process.
For most bowhunting, balance means settling somewhere in the middle of the extremes between ultralight and super-heavy arrow shafts. An ultralight shaft will leave the bow faster and initially have a flatter trajectory than a heavy shaft but will shed its velocity faster at longer ranges and not have as much penetrating power. The heavier shaft will not fly nearly as fast, but it will produce more penetrating power downrange. A medium-weight shaft will create a balance between raw arrow speed, trajectory, and penetrating power. Use the arrow efficiency calculator to determine arrow speed and penetrating power.
An Arrow Efficiency Calculator Example
Here’s an example, using my own 340 spine, 28 ½-inch shafts with both 100- and 125-grain broadheads shot from a 70 lb. compound bow. With the 100-grain heads, a Victory Archery VAP has a total arrow weight of 379.1 grains, while an Easton 4mm FMJ weighs 480.1 grains. The VAP leaves the bow at 283 fps, while the FMJ leaves the bow at 263 fps. Switching to 125-grain heads, raw arrow speed is 273 fps for the VAP, and 253 fps for the FMJ.
While none of these shafts fall in the ultralight category (which I would never go bowhunting big game with), all these combinations produce enough K.E. to ethically bowhunt any big game in North America. The lightest VAP with a 100-grain head generates 67.42 ft./lbs. of K.E., while the FMJ with 100-grain head generates 73.74 ft./lbs. The heavier FMJs may be flying slower, but they will definitely out-penetrate the medium-weight VAPs. However, the FMJ’s trajectory arc will be larger, making precise knowledge of the distance to the target more critical.
Arrow Efficiency Calculator Use
So, you can select the right shaft for any specific bowhunting adventure. Shy away from the extremes, and you’ll be good to go! To get the most from your setup, make sure to use the Arrow Efficiency Calculator above.