Bloodstain Impact Angle

How the bloodstain impact angle works

A blood droplet in flight is close to spherical. When it meets a surface at an angle it leaves an ellipse, and the more acute the angle, the longer that ellipse becomes. The ratio of its width to its length therefore encodes the angle of impact.

A droplet striking at 90° leaves a circular stain, so width equals length and the arcsin of 1 is 90°. As the angle shallows, width shrinks relative to length. Measure across the stain at its widest point, and along it excluding any tail or spine.

Frequently asked questions

How do you calculate the angle of impact of a bloodstain?

Divide the stain's width by its length and take the arcsine of that ratio. A 6 mm by 12 mm stain gives 0.5, and arcsin(0.5) is 30°.

Why can the width never exceed the length?

Length is measured along the axis of travel, which is always the longer axis for an angled impact. If the measured width is larger, the axes have been swapped or the stain is not a clean elliptical impact.

How accurate is the arcsin method?

It is reliable for well-formed elliptical stains on smooth, non-porous surfaces. Accuracy falls on textured or absorbent surfaces, for very small droplets, and where the stain has satellite spatter or a pronounced tail.

What is the tail of a bloodstain used for?

The tail or spine points in the direction the droplet was travelling, which gives direction of travel. Combined with the angle, several stains can be used to estimate an area of origin.

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