This calculator is based on average efficiency gains as arrow weight is increased from 5 grains per pound (IBO). The most critical factor is the IBO speed entered as this is the starting point of the calculations. Keep in mind that most IBO ratings are not real-world numbers. In the majority of cases once a peep is added the speed will only be a few fps off but the ratings of some bows are significantly off.
Every bow gains efficiency a little differently. Bows 5-10 years old will tend to gain efficiency faster than one that is brand new. As the older bows were less efficient out of the box, they naturally have more room for improvement. With that said, the average efficiency curve utilized for the calculator is based on various bows from APA, Elite, Bowtech, Hoyt, and PSE ranging from 40lbs up to 85lbs. This average will get most bows within a few percent of real-world speeds. For the most accurate results, it is best to use a known speed and known arrow weight under 6 grain per pound to determine your “corrected” IBO value.
A 70#/29″ bow that’s rated at 350 fps is has been chronographed at 315 fps with a 400 grain arrow. At the rated 350 fps IBO the speed should be 319 so we know that the IBO is slightly high. Reduce the IBO speed until the calculator shows the verified speed of 315; in this case, the correct IBO value would be 346 fps.
(feet per second)
Calculated Arrow Speed
*This Calculator Was Developed With The Help of The Ashby Bow Foundation*