Adding an arrow up
Total arrow weight is a sum with no cleverness in it. The shaft is its grains per inch multiplied by the cut length, and the rest are catalogue figures you can also just weigh. A 29 inch shaft at 8.5 grains per inch is 246.5 grains before anything is fitted to it; add a 100 grain point, a 20 grain insert, a 10 grain nock, a 6 grain wrap and three 8 grain vanes, and the finished arrow is 406.5 grains.
The reason to do it on paper rather than on a scale is that you can do it before you buy, and you can do it for four combinations in the time it takes to weigh one. Once the arrows exist, weigh them. Grain scales are inexpensive, catalogue figures are nominal, and glue has mass too. If the scale and this page disagree by ten grains, the scale is right.
Grains per pound, and whose number the limit is
Grains per pound is total arrow weight divided by peak draw weight, and it is the ratio that says whether the bow is being asked to push something substantial enough to absorb what it is giving. A 406 grain arrow on a 50 pound bow is 8.13 grains per pound.
There is a widely used minimum of around 5 grains per pound, and it is worth being precise about where that number comes from: it is published by bow manufacturers in their own manuals, it is a warranty condition, and it is not a figure produced by this calculator or by any general rule of thumb. Some makers publish a higher floor than five. The one that governs your bow is the one printed in the manual for your bow. Going under it is how limbs and cams get damaged, because a bow that cannot put its stored energy into the arrow puts it into itself instead.
Front of centre, in one line
FOC is the balance point expressed as a share of the arrow length, measured from the middle. Balance the finished arrow across a straight edge until it sits level, mark where it balanced, and measure from the bottom of the nock groove forward to that mark. Then:
FOC percent = (balance distance from the nock groove − arrow length ÷ 2) ÷ arrow length × 100
A 29 inch arrow balancing at 16.5 inches gives (16.5 − 14.5) ÷ 29 × 100, which is 6.90 percent. The same arrow balancing at 17.65 inches gives 10.88 percent. The whole measurement is that subtraction and that division, and the two things that most often make it wrong are measuring from the end of the nock rather than the groove, and using the overall length including the point instead of the carbon-to-carbon length.
What actually moves it
| Change | Total weight | Front of centre |
|---|---|---|
| Heavier point | Up, grain for grain | Up, and this is the main lever |
| Heavier insert | Up | Up, but less than a point of the same weight, because it sits behind the point |
| Longer arrow | Up by the grains per inch | Down — the added shaft is behind the balance point and the length in the denominator grows |
| Heavier vanes or a wrap | Up a little | Down, since it is all mass at the back |
| Heavier shaft, same length | Up a lot | Down slightly, because the extra mass is spread evenly |
Only one row raises both, and it is the one that also weakens the dynamic spine. That is the trade at the centre of arrow building: front weight buys you balance and mass, and it spends spine to do it. Check any point weight change against the spine band before ordering, because a 100 grain jump in point weight can move the requirement a whole group.
What FOC is and is not worth
For target work, front of centre is a stability and consistency parameter. An arrow with more weight forward tends to recover from the shot sooner and is generally less bothered by a crosswind, at the cost of dropping faster over distance because it is heavier and slower. Target arrows commonly sit somewhere in the high single digits to low teens as a percentage, and archers who shoot the same distances every week converge on a figure and stop thinking about it.
What it is not is a score. There is no value that is correct, and a change of one percentage point is not something you will see on a target face at twenty yards. It is worth calculating because it tells you which way your last change moved the arrow, and worth measuring because the calculation assumes each component sits exactly where the model puts it. Once the arrows are built, the total weight goes straight into the kinetic energy and momentum calculator, and the length that produced it comes out of the arrow length calculator.
Questions people ask
How do I calculate FOC on an arrow?
Balance the finished arrow, complete with point, nock and fletching, across a narrow straight edge until it sits level. Measure from the bottom of the nock groove forward to the balance mark. Subtract half the arrow length, divide the result by the arrow length, and multiply by 100. For a 29 inch arrow balancing at 17.65 inches: 17.65 minus 14.5 is 3.15, divided by 29 is 0.1086, times 100 is 10.9 percent. Use the carbon-to-carbon length rather than the overall length with the point, and measure to the groove rather than the end of the nock, or the answer comes out high.
What is a good FOC for target arrows?
There is no single correct figure, and anyone who offers one without asking what you shoot is guessing. Target setups commonly land somewhere from the high single digits into the low teens as a percentage, and the archers shooting them mostly arrived there by settling on a point weight that tuned well rather than by aiming at an FOC target. More weight forward generally means an arrow that settles sooner and drifts less in wind, and one that drops faster because it is heavier. Which of those you want depends on your distances and your conditions.
What is the minimum grains per pound for a bow?
That number belongs to the bow manufacturer, not to a calculator. Around 5 grains of arrow weight per pound of peak draw weight is the figure most commonly published, and shooting below the limit stated in your own manual is a warranty condition as well as a way to damage limbs and cams. Some makers set a higher floor. Look it up for the exact bow, treat it as a hard limit rather than a guideline, and do not average it against something you read elsewhere. This page reports the margin against 5 gpp so you have a reference point, but the governing figure is the one in your manual.
Does a heavier arrow always mean a slower one?
Yes, for a given bow, and the trade is more interesting than it sounds. A common shop rule of thumb is that adding roughly three grains costs about one foot per second, though the real rate depends on the bow. What the loss buys you is that a heavier arrow takes a larger share of the energy the bow releases instead of leaving it in the limbs, so it carries more momentum even though it is slower. Kinetic energy barely moves, momentum rises, and the arrow is usually quieter. Whether that is a good trade depends on your distances and what you value.
Should I weigh my arrows or trust the component figures?
Calculate before buying, weigh once they are built. Catalogue weights are nominal, grains per inch varies a little between batches, glue and fletching tape have mass, and inserts vary more than you would expect. A grain scale settles all of it and is one of the cheaper items in the sport. Weighing also tells you something the calculation cannot: how much your arrows differ from each other. A set that spans fifteen grains from lightest to heaviest will not group as well at distance as one that spans three, and the only way to sort them is to weigh them and match them.