Bullet Energy, Momentum & Power Factor Calculator
Calculate muzzle energy in ft·lbf and joules, momentum, IPSC/USPSA power factor (grains × fps ÷ 1000), Taylor knock-out value and sectional density from bullet weight, velocity and calibre, with minor/major thresholds and typical hunting energy guidelines.
How it works
E (ft·lbf) = ½ × (grains ÷ 7,000) × v² ÷ 32.174 = grains × v² ÷ 450,437; power factor = grains × fps ÷ 1,000; TKO = grains × fps × calibre ÷ 7,000; SD = (grains ÷ 7,000) ÷ diameter²
Muzzle energy is kinetic energy with the bullet mass converted from grains (7,000 to the pound) to slugs, and it scales with velocity squared, which is why light fast bullets post big numbers; the IPSC/USPSA power factor is momentum in convenient units and scales linearly with velocity, which better matches felt recoil and how a bullet knocks steel over — hence minor and major scoring divisions at 125 and 165. Taylor's knock-out factor adds bullet diameter for a dangerous-game ranking that favours heavy, wide, slow bullets, and sectional density (weight over diameter squared) predicts penetration for a given construction. Hunting guidelines quoting minimum energy at impact are rough; shot placement and bullet construction matter more.
Worked example
A 150 gr .308 bullet at 2,800 fps carries 2,611 ft·lbf (3,540 J) of energy and a power factor of 420; its sectional density is 0.226 and TKO 18.5. A 9 mm 124 gr load at 1,050 fps has 304 ft·lbf and a power factor of 130 — Minor.
Frequently asked questions
Does more energy mean more "stopping power"?
Energy is only one input; what the bullet does with it (expansion, penetration depth, wound channel) depends on construction and velocity window. Momentum and sectional density describe penetration better for heavy bullets.
How do I get power factor for a match?
Chronograph your actual load — factory box velocities are from test barrels. Aim for a 5% margin (e.g. 132 for Minor) to cover temperature and chronograph differences.
Related calculators
Sport-shooting and hunting arithmetic from standard published formulas (SAAMI energy, Miller stability, free-recoil energy). Follow all range rules and local law; verify every zero and correction on paper at the range — calculators inform, they do not replace live confirmation.