Blood Spatter Analysis Calculator

When a roughly spherical blood droplet strikes a surface at an angle, the resulting stain is an ellipse whose width-to-length ratio directly encodes the sine of the impact angle — a droplet hitting straight down (90°) leaves a nearly circular stain (width ≈ length, ratio near 1, arcsin near 90°), while a shallow glancing strike leaves a long, narrow stain (ratio near 0, arcsin near 0°). The calculator takes the minimum of width and length over the maximum to guarantee the ratio stays at or below 1, since width should always be the shorter dimension for a valid elliptical stain, then applies Math.asin to recover the angle.

The calculator multiplies the distance you enter (from the stain back to the estimated point of convergence) by the tangent of the computed impact angle, which is basic right-triangle trigonometry: if you know the horizontal distance and the angle of descent, the tangent gives you the vertical rise. This is explicitly a simplified two-dimensional version of the real technique, where analysts typically converge lines from many stains in three dimensions using strings or lasers rather than a single stain's 2D estimate.

The calculator first estimates droplet velocity from stain width alone using the empirical formula v ≈ 0.85 × width^1.14, then applies fixed velocity thresholds: below 1.5 m/s is classified as 'Low Velocity (drip)' consistent with passive dripping under gravity, 1.5 to 8 m/s is 'Medium Velocity (blunt force)', and above 8 m/s is 'High Velocity (gunshot)'. Because the estimate depends only on stain width, two stains with identical width but very different lengths would get the same velocity classification even though their impact angles differ.

This calculator processes one stain's dimensions in isolation, but real bloodstain pattern analysis converges measurements from many stains across a scene to triangulate a genuine three-dimensional area of origin, which is far more reliable than any single reading. Individual stain shape can also be distorted by the surface texture it lands on, subsequent drying, or secondary splashing — none of which this simplified single-stain model accounts for — so treat any one result here as investigative rather than conclusive.