From IBO rating to real speed
Compound bows are sold with an IBO (International Bowhunting Organization) or ATA speed rating, measured under standard conditions: a 70-pound draw weight, a 30-inch draw length and a 350-grain arrow, 5 grains per pound of draw weight. Few hunters shoot that setup. Shorter draws, heavier arrows and accessories on the string all slow the arrow, which changes its trajectory and the energy it delivers.
The rule-of-thumb corrections
Starting from the IBO speed:
- Draw length: subtract about 10 fps for each inch below 30 inches.
- Arrow weight: subtract about 1 fps for every 3 grains above 5 grains per pound of your peak draw weight.
- String accessories: subtract about 1 fps for every 3 grains of peep sight, D-loop, silencers or nock points.
These corrections are averages; modern cam designs vary, and a chronograph gives the true number.
Kinetic energy and momentum
Kinetic energy, in foot-pounds, is arrow weight in grains × speed² ÷ 450,240. Momentum, which predicts penetration better for heavy arrows, is grains × speed ÷ 225,400 in slug·ft/s. Common hunting guidance:
| Kinetic energy | Suitable for |
|---|---|
| under 25 ft·lb | small game |
| 25 to 41 ft·lb | medium game such as deer and pronghorn |
| 42 to 65 ft·lb | large game such as elk and black bear |
| over 65 ft·lb | the largest and toughest game |
Worked example
A bow rated at 340 fps IBO is drawn to 29 inches at 65 pounds, with a 400-grain arrow and 10 grains of string accessories. The draw length costs 10 fps, the arrow is 75 grains above 325 grains (5 × 65), costing 25 fps, and the string weight costs about 3 fps. The estimated speed is about 302 fps. The kinetic energy is 400 × 302² ÷ 450,240 = about 81 ft·lb, plenty for elk.
Arrow weight and safety
Never shoot an arrow lighter than 5 grains per pound of draw weight from a compound bow. The bow stores more energy than such an arrow can absorb, and the excess, like a dry fire, can break limbs, cables or cams and void the warranty. Many bowhunters choose 6 to 8 grains per pound for quieter shooting and better penetration.
Speed is not everything
A faster arrow drops less over distance, which forgives range-estimation errors. A heavier arrow is slower but quieter, more stable in the wind and penetrates deeper. Accuracy, shot placement and a sharp broadhead matter more than a few feet per second.