Wind Drift Projectile Shot Calculator

Wind drift: drift = (TOF − d/v₀)·v_wind·sin θ

Didion's formula gives a solid approximation of how much a crosswind pushes a bullet sideways: drift = (TOF − d/v₀)·v_wind·sin θ. Here TOF is the real time-of-flight (s), d/v₀ is the vacuum time (range over muzzle velocity), v_wind is the wind speed and θ the angle between wind and trajectory. The piece that matters is (TOF − d/v₀), the so-called "lag time" caused by deceleration, since that's the window in which the wind gets to work on the bullet. The old rule of thumb says a 10 mph full-value wind at 1000 yd deflects a .308 Winchester by ~3 mil (≈ 108 in). Run the numbers: TOF = 1.50 s, d/v₀ = 0.914 km / 840 m/s = 1.088 s, lag = 0.412 s, v_wind = 4.47 m/s (10 mph), θ = 90° → drift ≈ 0.412·4.47 ≈ 1.84 m ≈ 72 in. A heavier bullet with a higher BC carries less lag, so it drifts less.

Applications

Long-range competitive shooting (NRA F-Class, PRS, ELR matches at 1000–2000 yd, where reading the wind is what separates the winners from everyone else), military and police sniping against point targets at 600–1500 m, long-range big-game hunting, building ballistic apps like Hornady 4DOF, Applied Ballistics or Strelok, wind-call practice on known-distance ranges, plus forensic reconstruction when a bullet ends up somewhere other than the aim point.

FAQ

Why is lag time the key variable, not raw TOF? In a vacuum the bullet would land in d/v₀ seconds with no drift at all. Drift only shows up because drag keeps the bullet in the air longer, and that extra time is exactly TOF − d/v₀.

What does "full-value wind" mean? It's wind blowing perpendicular to the line of sight (θ = 90°, sin θ = 1). Swing it round to θ = 30° (clock positions 1, 5, 7, 11) and only half of it pushes the bullet sideways (sin 30° = 0.5).

Why does a high-BC bullet drift less? A higher ballistic coefficient means the bullet decelerates more slowly, so TOF stays closer to d/v₀ and the lag time shrinks. That's why a .308 175 gr SMK drifts less than a 150 gr fired at the same muzzle velocity.