Arrow weight is measured in Grains Per Inch which is where the GPI abbreviation comes from. To break that down a little further, one grain = 1/7000 of one pound.
The reason why it is measured this way is because arrows can be cut to different sizes to suit the needs of the archer. By quoting their weight on a per inch basis ensures it is measurable regardless of the full arrow length. A 32 inch arrow shaft can have the same GPI as a 28 inch arrow.
When the weight of an arrow is quoted by the arrow manufacturer, they are referring to the weight of the bare shaft. This is regardless of the fact that the arrow may be sold with a point, nock and fletchings attached.
The GPI of an arrow shaft can be affected by a number of different factors that may not be obvious if you’re simply casting your eye over it.
The diameter of the arrow, the wall thickness and the material used to make the shaft are all factors in the arrow’s GPI. It won’t necessarily be the case that the arrow’s spine changes the GPI.
Similarly, a small diameter arrow is not necessarily going to weigh less than a thicker shaft at the same spine.
As an example compare these 2 Easton Archery Arrows:
4mm Carbon Injexion
Another factor to remember when talking about the arrow’s GPI is the fact that it will be variable if you are looking at a tapered arrow. In this case the GPI will drop as the arrow becomes narrower.
GPI Differs To GPP
Another arrow weight measurement is its Grains Per Pound. This measurement is related directly to the draw weight of the bow you are shooting.
An arrow that would be considered light in weight would have a GPP of between 5 and 6.5 grains per pound of draw weight. Someone who want to shoot a heavy arrow would be looking at an arrow that weighs more than 8 grains per pound.
Naturally, when you are working out the GPP you will be counting an arrow that is equipped and ready to shoot. In other words, it will include the point, insert, fletchings and nock in the weight calculation.
Lighter or Heavier Arrow?
Whether you shoot a light or heavy arrow is an important consideration and can have a big effect on your accuracy and, consequently, your results.
A light arrow will start its flight faster and will maintain its flight trajectory for longer. But it won’t absorb as much of the force of a high energy bow as a heavy arrow and this will create more vibration within the bow.
A heavier arrow will be more effective when shot from a more powerful bow which leads to quieter shots. They will also be less affected by the wind while in flight and are used by hunters who are trying to achieve the greatest penetration possible. However, they can be far slower and drop more quickly if shot from a lower powered bow.
Generally speaking, a hunter is going to require a heavier arrow because it will provide more penetration through the retention of kinetic energy. It will hit harder and with greater knockdown power.
Deciding What GPI Is Best For You
Ultimately, determining the best arrow GPI is going to come down to some trial and error as you tune your bow to each arrow.
Playing around with a range of arrows complete with their various weights will help you become familiar with the bow you’re using and the different performance with each arrow.
The more arrows you shoot, the more comfortable you will become. Unfortunately there is no universal “best” arrow weight that will suit everyone.