Arrow weight and FOC, grains per pound, front of center.

Grains per pound

GPP = arrow weight in grains / draw weight in pounds. It says how much of the bow's energy the arrow carries away and how much stays in the limbs as vibration. Too light an arrow is a dry-fire in slow motion: the limbs take the shock, the bow is loud, and wooden and older fiberglass bows can crack. Most makers of traditional bows set a minimum of 8 to 10 grains per pound, and some void the warranty below it; compound makers usually allow 5. Check your bow's own figure before shooting anything light.

• Under 8 gpp: lighter than most traditional bows allow. Ask the maker.
• 8 to 10 gpp: the usual target and field range for a longbow or recurve. Flat enough, kind to the bow.
• 10 to 12 gpp: quiet, forgiving, a little slower; the usual hunting setup.
• Over 12 gpp: very quiet and heavy, with a noticeable arc; fine for close work.

Front of center

FOC % = (balance point − arrow length / 2) / arrow length × 100, both measured from the throat of the nock. A heavier point moves the balance forward and the arrow flies straighter and recovers faster, at the cost of a steeper arc. Seven to twelve percent is normal for target and field arrows; hunters often run twelve to fifteen or more. Below about five percent an arrow is tail-heavy and wanders.

Draw length

The standard estimate is wingspan in inches / 2.5, which is within an inch for most adults. The true number is measured at a shop with a draw-check arrow at full anchor. Beginners buy arrows one to two inches longer than their draw, so the point never comes inside the bow hand; the calculator adds two.

One grain is 1/7000 of a pound, or 0.0648 grams. A bare carbon shaft is typically 8 to 10 grains per inch, a field point 100 to 125 grains, a nock and three feathers about 30.