Arrow Speed Calculator
Why Your Bow is Slower Than the Box Says
When you buy a compound bow, the manufacturer advertises a blazing speed, like "340 FPS!" However, when you shoot it through a chronograph, you might only see 280 FPS. Why the difference?
Manufacturers test bows using IBO Standards (International Bowhunting Organization). To get that high number, they test under ideal conditions:
- Draw Weight: 70 lbs (Maximum).
- Draw Length: 30 inches (Long).
- Arrow Weight: 350 grains (Very light - 5 grains per pound).
- String: Bare string (No peep sight, no d-loop, no silencers).
Most archers have a shorter draw length, shoot a heavier arrow for hunting, and have accessories on their string. All of these factors reduce speed.
The "Lost Speed" Formula
Our calculator estimates your true speed by subtracting the penalties from the IBO rating:
- Draw Length: For every inch under 30", you lose roughly 10 FPS. (A 27" draw loses 30 FPS immediately).
- Arrow Weight: For every 3 grains of weight added above the minimum (5 grains/lb), you lose 1 FPS. Heavier arrows fly slower.
- String Weight: Peep sights and d-loops add weight to the string, slowing it down. You lose roughly 1 FPS for every 3 grains on the string.
Kinetic Energy (KE) vs. Momentum
Speed isn't everything. If you are hunting, you need penetration. There are two physics metrics for this:
1. Kinetic Energy (ft-lbs)
This is the "Punch." It favors speed. A light, fast arrow has high KE. High KE causes massive tissue damage upon impact.
2. Momentum (slugs)
This is the "Pass-Through Power." It favors weight. A heavy, slower arrow has high momentum. High momentum helps the arrow push through heavy bone (like a shoulder blade) without stopping.
If you hunt deer (soft target), high KE is fine. If you hunt Elk, Bear, or Hogs (hard targets), experienced hunters sacrifice speed for a heavier arrow to increase Momentum.
Ethical Hunting Standards
How much energy do you actually need to ethically harvest game?
| Game Size | Minimum KE (ft-lbs) |
|---|---|
| Small Game (Rabbit, Turkey) | 25 ft-lbs |
| Medium Game (Whitetail Deer) | 40-45 ft-lbs |
| Large Game (Elk, Black Bear) | 50-65 ft-lbs |
| Tough Game (Cape Buffalo, Grizzly) | 65+ ft-lbs |
The 5 Grains Per Pound Rule
For safety, never shoot an arrow that weighs less than 5 grains per pound of draw weight (e.g., 350 grains for a 70lb bow).
Shooting an arrow that is too light is essentially "Dry Firing" your bow. The energy has nowhere to go but into the limbs and cams, which can cause the bow to explode or shatter.