What does arrow grain mean?
Arrow grain refers to weight measured in grains, the unit used for shafts and arrow components. One grain is 1/7000 of a pound. Shaft catalogs often list grains per inch, or GPI, while points and inserts list individual weights in grains. A finished arrow's weight includes every component you shoot.
Grains, GPI, and spine numbers
A grain is much smaller than a gram. Check the unit on your scale before recording a reading; a value in grams entered as grains will make your calculations wrong. An archery grain scale lets you weigh the complete arrow directly.
GPI means grains per inch of shaft. Multiply it by the cut shaft length in inches to estimate the shaft's weight. Use the length of the shaft material for this multiplication, excluding any projecting nock and point. This differs from length conventions that begin at the nock throat.
The large number printed on a carbon shaft may describe arrow spine. A 400 spine marking does not mean the arrow weighs 400 grains. Look for the separate GPI specification for the exact shaft size.
Adding the components
Total arrow weight = shaft GPI x shaft length + point + insert + nock + fletching x count + wrap. Include an outsert, collar, or other fitted component if your build uses one. Adhesive adds some weight too, so the final scale reading is the best check.
| Part | Calculation | Weight in grains |
|---|---|---|
| Shaft | 8 GPI x 28 inches | 224 |
| Point | One point | 100 |
| Insert | One insert | 20 |
| Nock | One nock | 10 |
| Vanes | 3 x 6 grains | 18 |
| Wrap | One wrap | 8 |
| Total before adhesive | 224 + 100 + 20 + 10 + 18 + 8 | 380 |
These are example values, not a recommended build. Enter your own parts in the arrow weight calculator. Check the finished arrows individually if you want a matched set; changing a nock or point can explain an unexpected outlier.
Grains per pound and safe arrow weight
GPP means grains per pound: total arrow weight divided by bow draw weight. For a compound, use peak draw weight. The example 380-grain arrow shot from a 50 lb compound gives 380 / 50 = 7.6 GPP.
Follow your bow manufacturer's minimum arrow-weight requirement. An arrow below the specified minimum can damage the bow or cause injury. Use the manual's limit when checking your calculated GPP. Check the minimum again after increasing draw weight.
If you replace a point with a heavier one, you change both total weight and FOC, the forward balance of the arrow. Record which component changed so you can interpret the next flight test.
Questions
Does a 100-grain point make a 100-grain arrow?
No. That label describes the point alone. The shaft, insert, nock, fletching, and any wrap contribute to the finished weight. Add all the parts to estimate the build, then weigh the assembled arrow on a grain scale before using that number in speed or energy calculations.
Are grains per inch and grains per pound the same?
GPI describes shaft weight per inch of shaft length. GPP compares a complete arrow's weight with bow draw weight. You use GPI while estimating an arrow build and GPP when checking the arrow-to-bow weight relationship. Neither number tells you the shaft's stiffness or replaces a spine chart.
Will heavier arrows always shoot lower?
With the same bow settings, a heavier arrow generally leaves the bow more slowly and needs a different sight setting as distance increases. The exact change depends on the setup. Confirm sight marks on the range after changing weight, and check tune if the added weight is at the point.