How the speed estimate is built from IBO
Start from the bow’s IBO rating, then correct for the two things you change: your draw weight and your arrow mass. Lighter arrows and more draw weight raise speed; heavier arrows drop it.
The mass term costs about 1 fps for every 3 grains over the 350-grain standard. Draw length also matters — add or subtract roughly 10 fps per inch away from the 30″ test length.
Kinetic energy from speed and arrow mass
Speed sells bows, but energy is what drops the target. Because mass counts once and speed counts squared, a light fast arrow and a heavy slow one can carry the same punch — and the heavier one usually penetrates deeper.
Typical speed and energy by bow type
| Bow type | Speed | Kinetic energy |
|---|---|---|
| Compound, 70 lb | 320–370 fps | 55–75 ft·lb |
| Recurve, 50 lb | 200–240 fps | 30–40 ft·lb |
| Longbow, 60 lb | 160–200 fps | 25–35 ft·lb |
These are ballpark ranges. Your real numbers depend on cam design, string accessories, and how cleanly the arrow leaves the rest.
What throws the estimate off
- IBO is optimistic. It is a bare-string lab number. Real setups run 10–15% slower once accessories are added.
- Arrow mass is easy to forget. Skip it and light-arrow speeds look too low while heavy-arrow speeds look too high.
- Draw length is baked in. The estimate assumes a 30″ draw; a shorter draw loses roughly 10 fps per inch.
- Chronograph beats math. For a real number, shoot through a chronograph — formulas only approximate the string’s efficiency.
Common questions
What is IBO speed?
IBO is the rated speed a bow shoots under a fixed standard: a 70 pound draw weight, a 30 inch draw length, and a 350 grain arrow. It is the number printed on a bow spec sheet, and it is your starting point before adjusting for your real setup.
How much does a heavier arrow slow the bow down?
Roughly 1 fps for every 3 grains added above the IBO standard of 350 grains, or about 5 fps per 5 grains removed below it. A 100 grain heavier arrow lands near 30 fps slower, but it carries more kinetic energy and penetrates better.
Is real arrow speed the same as the IBO rating?
Usually no. IBO is a best-case lab figure with no accessories on the string. Add a peep, a D-loop, and a stabilizer and your real speed typically runs 10 to 15 percent below the rated number. Use a chronograph if you need the true value.
How much kinetic energy do I need to hunt?
A common guideline is at least 25 ft-lbs for small game, 40 ft-lbs and up for deer-sized animals, and 65 ft-lbs or more for large game. Energy comes from arrow mass and speed together, so a slower heavy arrow can out-penetrate a fast light one.
