Calculate how much energy your bow transfers to the arrow versus how much is wasted as sound and vibration.
Enter your bow's peak draw weight, your draw length, the arrow's chronographed speed, and the arrow's total weight. If you shoot a compound, also enter the let-off percentage; leave it at 0 for a recurve or longbow, while compounds typically run 65–80%.
Stored energy is modeled as the trapezoidal area under a force-draw curve: peak force × draw length × (1 − let-off ÷ 200). At 75% let-off this stores 37.5% less energy than a no-let-off bow with the same peak weight, because the holding force at full draw is so much lower. Delivered energy is the arrow's kinetic energy, ½mv², from your measured speed and mass.
Efficiency is delivered energy divided by stored energy, expressed as a percentage and capped at 100. The rating bands are: 80% and above excellent, 70–79% good, 55–69% fair, and below 55% poor. A chronograph reading matters here — speed enters the calculation squared, so a guessed velocity can swing the rating by a full band.
The rating bands are: 80% or higher excellent, 70–79% good, 55–69% fair, and below 55% poor. Efficiency is the arrow's measured kinetic energy divided by the energy the model says your draw stored, capped at 100%.
Stored energy is modeled as a trapezoidal force-draw curve: peak force × draw length × (1 − let-off ÷ 200). A compound at 70% let-off therefore stores 35% less energy than a no-let-off bow of the same peak weight, which is why entering 0 let-off for a compound understates its efficiency.
Effectively yes. Delivered energy is ½mv² from your arrow's actual mass and speed, and because speed enters squared, a guess that is off by 10 fps can move the efficiency result by several points — enough to cross a rating band.