This calculator starts from the IBO speed on your bow's spec sheet and applies the usual rules of thumb for everything your setup does differently from the IBO test: about 10 fps for each inch of draw length under 30 inches, 2 fps for each pound of draw weight under 70, and 1 fps for every 3 grains of arrow weight over 350 and of accessories on the string.
Worked example: a bow rated 335 fps IBO, shot at 70 lb and a 29-inch draw with a 425-grain arrow and 30 grains on the string. The missing inch of draw costs 10 fps, the 75 grains of arrow over 350 cost 25 fps, and the string accessories cost 10 fps: 335 − 10 − 25 − 10 = 290 fps. At that speed the arrow carries about 79 ft·lb of kinetic energy.
The IBO rating itself tends to be optimistic. In our field report, about half the ranked brands measured within 7 fps of their rating at IBO conditions, and the least accurate 15 fps or more under it. Pick your bow's brand to see that range. For the exact number, shoot your own arrow through a chronograph.
