What does GPI on an arrow mean?
What does GPI mean on an arrow?
When you look at arrow shafts, you often encounter the term GPI – perhaps 5.8 GPI, 7.2 GPI, or 10.0 GPI. But what does GPI actually mean, and why is it important when choosing arrows? GPI stands for Grains Per Inch and indicates the weight of the arrow shaft itself per inch. It sounds simple – but GPI actually has a significant impact on the arrow's total weight, speed, F.O.C., and not least how you put together your arrow setup.
What does GPI mean?
GPI = Grains Per Inch = grains per inch. For example, an arrow shaft with 7 GPI nominally weighs 7 grains per inch. If the shaft is 30" long, the nominal shaft weight will therefore be: 7 GPI × 30" = 210 grains. But here's an important detail: GPI usually only describes the shaft – not the finished arrow. Point, insert, nock, vanes/fletching, and any collars or other components must be added. Therefore, it's a mistake to think that a 30" arrow with 7 GPI weighs 210 grains when finished. The finished arrow will weigh more.
An important rule of thumb
What are grains?
Grain (gr) is the unit of weight traditionally used in archery for arrows and components. 1 grain corresponds to 1/7000 of a pound. It is therefore not the same as a gram. As a rule of thumb: 1 gram ≈ 15.43 grains. If you want to compare arrows and components, you should therefore be aware of which unit the manufacturer is using.
Why is GPI important?
GPI is interesting because the shaft's weight is part of the arrow's overall weight, and this affects, among other things:
Therefore, two arrows with the same spine can be very different in practice. For example, they can both be 400 spine, but have different GPIs. This means that one arrow can be significantly heavier than the other when built to the same length and with similar components.
Low GPI or high GPI – what to choose?
There is no single right answer as to whether you should choose low or high GPI. It depends on what you want from your setup.
Lower GPI
A shaft with a lower GPI will typically result in a lighter arrow if the other components and the arrow's length are comparable. A lighter arrow can, among other things, provide:
Higher GPI
A shaft with a higher GPI typically results in a heavier arrow. A heavier arrow can, among other things, provide:
For hunting, a higher arrow weight can be advantageous, while a 3D or target shooter might prioritize a different combination of speed, trajectory, and precision. It is therefore not about finding the lowest or highest GPI – but about finding the right combination for your purpose.
GPI is not the same as spine
This is an important difference. Spine describes the stiffness of the arrow. GPI describes the arrow shaft's weight per inch. However, the two values are often related to the shaft's construction. Diameter, wall thickness, and materials, among other things, affect the shaft's GPI. This also means that you cannot conclude: "400 spine = X GPI." Two different 400 spine shafts can easily have different GPIs. It is therefore important to view spine and GPI as two different properties.
How do you calculate arrow weight from GPI?
If you want a quick estimate of the shaft's weight, you can use: GPI × shaft length = shaft weight. Example: You have a shaft with 7.5 GPI, and it's 30" long. 7.5 × 30 = 225 grains The shaft nominally weighs approximately 225 grains. But the finished arrow weighs more.
If you, for example, assemble: shaft: 225 gr, point: 100 gr, insert: 10 gr, nock: 9 gr, and vanes: 18 gr, the total arrow becomes: 362 grains. It is the total weight of all elements and not GPI that tells you how heavy the finished arrow is.
GPI and the finished arrow
Here's an important detail that often causes confusion. When a manufacturer states: 7.5 GPI, it usually doesn't mean that every inch of the finished arrow weighs 7.5 grains. GPI typically refers to the shaft. You must therefore add all the components if you want to know the actual weight of the finished arrow. This is also why two arrows with the same GPI can end up with different total weights if they are built with different points, inserts, nocks, vanes, collars, and wraps.
GPI and arrow length
The length of the arrow directly affects the total weight of the shaft. If a shaft weighs 7 GPI, a 32" shaft will naturally weigh more than a 28" shaft of the same shaft type. Example: 7 GPI × 28" = 196 grains and 7 GPI × 32" = 224 grains. That's a 28 grain difference – solely due to the shaft's length. This is one reason why you shouldn't compare GPI without also looking at the arrow's length.
GPI, F.O.C., and point weight
If you start delving into arrow building, the interplay between GPI, overall weight, and F.O.C. becomes interesting. F.O.C. stands for Front of Center and describes how far forward the arrow's balance point is located. For example, if you increase the weight in the front with a heavier point, heavier insert, brass insert, outsert, and collar, you typically increase both the total arrow weight and F.O.C. But you also change the arrow's dynamic spine. This means that changing the point weight isn't just about making the arrow heavier. You change several properties in your setup at once. This is where arrow building becomes truly interesting.
Also read about what spine means here.
What makes an arrow dynamically softer or stiffer?
There are various factors that generally cause an arrow to react dynamically softer or stiffer.
Shorter arrow
A shorter arrow is generally dynamically stiffer than a longer arrow of the same shaft type and static spine.
Lighter tip
A lighter tip reduces the load on the arrow's front end and generally causes the arrow to react dynamically stiffer.
Lower draw weight
Reducing the draw weight reduces the force the arrow is subjected to during acceleration, which generally makes the dynamic response stiffer.
The simple rule of thumb
These are general tendencies – not a complete tuning formula. The overall setup determines the actual response.
What happens if the arrow is too soft or too stiff?
One often talks about under-spined and over-spined arrows. If the arrow is dynamically too soft in relation to your setup, it can, among other things, cause challenges with tuning, arrow flight, and grouping. If the arrow is dynamically too stiff, it can also make tuning more difficult and affect the arrow's flight and grouping. But be careful with rigid rules. A poor result on the target does not automatically mean that the arrow has the wrong spine. Shooting technique, release, nock fit, tiller, centershot, cam timing, and other tuning parameters can also affect the arrow's reaction. Spine should therefore be evaluated as part of the overall system.
How to choose the right spine?
When choosing spine, you should at least know:
You can then use the manufacturer's spine chart or shaft selector as a starting point. But a spine chart is precisely a starting point, not a guarantee that a specific arrow will be perfectly tuned in your particular setup.
Spine for compound, recurve, and longbow
Spine is relevant across bow types, but how the arrow reacts and how it is tuned is different. For compound bows, the cam system, let-off, and release aid, among other things, play a role. For recurve and longbows, finger release and archer's paradox are central factors. Here, the bow and arrow move through the shot differently than with a compound bow. Therefore, spine recommendations should not be transferred from one bow type to another without further consideration.
Spine and point weight
For the experienced archer, point weight is an interesting tool. For example, if you change from a 100 grain to a 125 grain tip, you do not change the arrow's static spine. But you change its dynamic spine. At the same time, you change:
- the arrow's total weight
- F.O.C.
- balance point
- speed
- kinetic properties
It is therefore important to consider point weight as part of the entire arrow's construction – not just as "the tip I like best".
Spine and F.O.C.
Spine and F.O.C. are not the same. Spine describes the stiffness of the arrow. F.O.C. (Front of Center) describes how far forward the arrow's balance point is located relative to the arrow's total length. However, these two things are often related in a practical arrow setup. If you increase the weight in the front with a heavier tip or a heavier insert, you typically increase F.O.C. while also making the arrow's dynamic spine softer. Therefore, it is important to look at the entire arrow's construction rather than a single number.
Spine and hunting arrows
For bowhunting, spine becomes particularly interesting. A broadhead can be significantly heavier than the field point you use for practice. This changes both the arrow's total weight, balance, and dynamic reaction. At the same time, broadheads place high demands on stable and consistent arrow flight. Therefore, hunting arrows should be tested and tuned with the actual broadhead setup you intend to use. An arrow that flies perfectly with a field point is not necessarily perfectly tuned with a broadhead.
See also the guide "How to choose arrow points for bowhunting"
Is a lower spine better?
In short, no! A 300 spine is not automatically better than a 500 spine. Spine primarily indicates the stiffness of the shaft. A good arrow also depends on, among other things:
- straightness tolerance
- weight tolerance
- diameter
- weight
- components
- durability
- F.O.C.
- consistency from arrow to arrow
For serious archers, consistency from arrow to arrow is particularly important. The less variation there is between your arrows, the more consistent your setup will be.
In short
If you are completely new to archery, remember:
Lower spine number = stiffer arrow.
Higher spine number = softer arrow.
But if you want to choose the right spine, you need to look beyond the number on the shaft. The arrow's spine must match your entire setup. Bow, draw weight, draw length, arrow length, point weight, components, and your shooting style all work together to determine how the arrow reacts. And that is precisely why choosing arrows is not just about finding an arrow with the right spine number.
Also read our guide to choosing the right arrow
Get help choosing the right arrow
At Bowgear, we have many years of experience with archery and help both beginners and experienced archers find the right equipment. Book a free and non-binding consultation here
We will help you find the perfect arrow for your bow.
Frequently asked questions about spine
What does spine mean on an arrow?
Spine is a measure of the arrow's stiffness. The lower the number, the stiffer the arrow. Also read our article about spine here
Which spine should I choose?
It depends on your draw weight, draw length, and arrow length. Use a spine calculator or contact us.
Can I use the same arrows for recurve and compound?
Not necessarily – the two bow types often require different spine and arrow types.
What does trajectory mean?
Trajectory is the path the arrow follows through the air.
What is F.O.C.?
F.O.C. stands for Front of Center and describes how far forward on the arrow the arrow's balance point is located in relation to the arrow's total length.
See more questions and answers here.
