Formula Used
1) Crosswind component
The sideways wind that produces drift is the component perpendicular to travel:
Vcross = Vwind · sin(θ)
2) Time of flight
A constant forward-speed estimate gives:
t = Range / Vforward
3) Drift models
Basic: assumes instantaneous coupling of wind to lateral motion:
y = Vcross · t
Aerodynamic lag: assumes lateral velocity approaches the wind with a first‑order response:
τ = 2m / (ρ · Cd · A · Vrel)
y = Vcross · ( t − τ(1 − e−t/τ) )
4) Angular correction
Convert lateral drift into small-angle aiming correction:
α = atan(y / Range) → mrad = 1000α, MOA = α·(180/π)·60
How to Use This Calculator
- Enter the range and an average forward speed over that distance.
- Add wind speed and wind angle. Use 90° for full crosswind.
- Pick Basic for quick checks, or Aerodynamic lag for a smoother response estimate.
- If using lag, provide mass, Cd, area, and air density.
- Press Estimate Drift. Then export results using the CSV or PDF buttons.
Example Data Table
| Range (m) | Forward (m/s) | Wind (m/s) | Angle (deg) | Model | Estimated Drift (cm) | Correction (MOA) |
|---|---|---|---|---|---|---|
| 100 | 300 | 4 | 90 | Basic | 133.33 | 45.84 |
| 300 | 800 | 5 | 90 | Lag | ~134.5 | ~15.4 |
| 500 | 250 | 8 | 60 | Basic | 1385.64 | 95.18 |
Professional Notes on Wind Drift Estimation
1) Understanding Wind Drift in Practical Terms
Wind drift is the sideways displacement caused by a crosswind acting over the object’s time of flight. Even modest winds can create meaningful offsets at long range. For example, a 5 m/s crosswind acting for 0.40 s produces about 2.0 m of basic drift if coupling is immediate.
2) Crosswind Components and Angle Effects
Only the perpendicular component matters for drift. The calculator uses Vcross = Vwind·sin(θ). A 10 m/s wind at 30° yields 5.0 m/s crosswind, while the same wind at 90° yields the full 10 m/s. Small angle mistakes can double the correction.
3) Time of Flight as the Key Driver
Drift grows with time of flight, which is why slower average speed or longer distance increases deflection. If range is 600 m and average forward speed is 750 m/s, time of flight is about 0.80 s. At 4 m/s crosswind, basic drift becomes roughly 3.2 m.
4) Choosing Between Basic and Lag Models
The basic model is a fast upper-bound style estimate: it assumes the lateral motion responds instantly. The aerodynamic lag model introduces a time constant τ to represent gradual lateral acceleration. Over short flights or large τ, lag reduces drift compared with the basic result.
5) Using Aerodynamic Inputs Responsibly
In the lag approach, τ depends on mass, drag coefficient, reference area, air density, and relative speed. Heavier objects and smaller areas tend to increase τ, delaying lateral response. Typical Cd values vary widely (about 0.05 to 1.2) depending on shape and regime, so treat Cd as a tuning parameter unless measured.
6) Interpreting Outputs in cm, inches, MOA, and mrad
The calculator converts drift to an angular correction: α = atan(y/Range). At small angles, α≈y/Range, so unit conversions are straightforward. Corrections are shown in milliradians and minutes of angle to support optics adjustments and targeting workflows.
7) Typical Ranges and Sensitivity Checks
Common surface wind speeds are often 2–10 m/s, while gusts can exceed that. Because drift is proportional to crosswind and time, a 20% wind-speed error roughly creates a 20% drift error. Re-run calculations for 0°, 45°, and 90° to bracket directional uncertainty.
8) Limitations and Field Validation
This tool intentionally simplifies reality: it assumes constant average forward speed, steady wind, and no vertical dynamics. For improved fidelity, estimate average speed over distance, update air density for altitude and temperature, and compare predictions against observed offsets to refine inputs and expectations.
FAQs
1) What wind angle should I use?
Use the angle between wind direction and forward travel. Enter 90° for full crosswind, 0° for tailwind/headwind, and 45° for quartering wind. Estimating angle correctly is as important as wind speed.
2) Why can the lag model predict less drift?
The lag model assumes lateral velocity builds gradually toward the crosswind component, governed by a time constant τ. When flight time is not much larger than τ, the object cannot fully “catch up” laterally.
3) Should forward speed be muzzle speed or average speed?
Use average speed over the distance whenever possible. If you only know the initial speed, the estimate may understate time of flight and drift, especially at longer ranges where speed typically decays.
4) How do I estimate Cd and reference area?
Area is the projected frontal area normal to flow (for a circle, πr²). Cd depends on shape and regime; if you lack measurements, start with a reasonable literature value and adjust using real-world observations.
5) Does air density significantly change drift?
Air density affects τ in the lag model, changing how quickly lateral response develops. Lower density (higher altitude or warmer air) increases τ and can reduce drift in the lag estimate, all else equal.
6) Can this handle wind that changes with distance?
Not directly. For variable winds, run multiple segments using different winds and approximate with a weighted average time of flight per segment. The result will still be an estimate, not a full trajectory simulation.
7) Which correction unit should I use, MOA or mrad?
Use the unit your sighting system supports. Milliradians are common on metric-based optics, while MOA is common on many imperial-based systems. Both represent the same small-angle correction.