How To Determine Arrow Length And Weight. Units of energy are often measured in joules. So if your draw length is 28″, you should get arrows with a maximum length of 29″.
Be advised that some archery retailers may be unaware of. The number of inches the arrow bends multiplied by 1,000 is the arrow’s spine. Matching an arrow to a bow arrow spine, arrow weight, and arrow length are all important factors in matching an arrow to a bow.
20 inches x 13 grains/inch = 260 grains.
Spine is measured by the number of inches a shaft deflects (bends) x 1000. To calculate this, arrows must be 29” long and supported at two points 28” apart. They allow for proper arrow flight, and also serve to indicate that you have nocked your arrow correctly. The right sized arrow can make or break your shot.