How it works — the rule, stated plainly
Start from a baseline of 300 fps at 70 lb draw weight, 30 inches draw length, and a 350 grain arrow. Then adjust: 10 fps for every 5 lb of draw weight away from 70 lb (subtract if under 70, add if over 70); 10 fps for every inch of draw length away from 30 inches (subtract if under 30, add if over 30); and 1 fps for every 5 grains of arrow weight away from 350 grains (subtract if over 350, add if under 350).
In symbols: estimated speed = 300 + ((draw weight − 70) / 5 × 10) + ((draw length − 30) × 10) − ((arrow weight − 350) / 5 × 1).
Worked sample: 60 lb, 28 in, 400 grains. Draw-weight adjustment: (60 − 70) / 5 × 10 = −20 fps. Draw-length adjustment: (28 − 30) × 10 = −20 fps. Arrow-weight adjustment: −(400 − 350) / 5 = −10 fps. Total: 300 − 20 − 20 − 10 = 250 fps.
This is rough only. It does not model bow efficiency, cam design, string weight, peep sight, D-loop, arrow spine, or let-off, and different bows at the same draw weight and length will shoot at different speeds.
Frequently asked questions
Why is my chronograph reading different?
Because this is a generic rule of thumb, not your bow's specification. Use a chronograph reading for your actual setup whenever accuracy matters.
Does a heavier arrow always mean a slower arrow?
In this rule, yes — heavier arrows cost speed — but heavier arrows can carry more momentum, so speed alone does not settle which arrow to choose.
Can I use this to pick a draw weight?
No. Choose draw weight you can draw and hold safely and comfortably, and that suits your bow and local regulations. Do not chase a speed number beyond a safe draw weight.