How this calculator works
This estimate uses the published Altra virtual-mass model: adjust the IBO baseline for draw length and force, then scale for arrow mass. It adds a separate approximate string-accessory penalty of one foot per second per three grains. The reference arrow is 350 grains at 30 inches and 70 pounds-force. This is an empirical compound-bow model, not a prediction from peak draw force alone. Bow design, tuning and accessory position change actual chronograph speed. Use the bow maker's minimum arrow mass guidance.
Formula
baseline_fps=IBO+10(draw_in-30)+2(draw_lbf-70); v_fps=baseline_fps sqrt(377/(arrow_gr+27))-string_gr/3; p=mv; E=mv^2/2; 1 grain=0.00006479891 kg.
Worked example
At the reference settings, a 300 ft/s IBO rating gives 300 ft/s, or 91.44 m/s. At 28 inches and 60 lbf with the same 350 grain arrow and no accessories, the model gives 260 ft/s.
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Frequently asked questions
Does this replace a chronograph?
No. It is a reference estimate; the actual setup should be measured when accurate speed matters.
Are grains the same as grams?
No. One grain is 0.06479891 g. Enter the arrow and string masses in grains.
Why does a heavier arrow move more slowly?
More mass shares the bow's available launch energy. The virtual-mass fit represents that trend, with an added effective bow mass.