Angle of Impact Calculator (bloodstain width ÷ length, educational)

Understanding the angle of impact calculator — concept and principle

When a drop strikes a surface at an angle, it leaves an elongated, roughly elliptical stain. This calculator uses the textbook bloodstain-pattern model to estimate the impact angle from the stain's width and length, and, with a convergence point, the approximate height of the source.

A stain twice as long as it is wide means sin α = 0.5, an impact angle of 30°; teaching model only.

How it is calculated

sin α = W ÷ L, so α = arcsin(W ÷ L), where W is the stain width and L its length without the tail.
A round stain (W = L) means 90°, a drop falling straight down.
Height of the source above the convergence point: h = d × tan α, where d is the distance from the stain to the convergence point on the surface.

The principle

A falling drop is roughly a sphere whose diameter equals the stain width. When it hits at an angle, it slides along the surface and stretches the stain. The width, the length and the drop's path form a right triangle, so the ratio W ÷ L equals the sine of the impact angle. Lines drawn back through several stains meet at a convergence point on the surface; tracing each impact angle upwards from there estimates where the source was.

Worked example

Default inputs: width 5 mm, length 10 mm, distance to convergence point 80 cm.

W ÷ L = 5 ÷ 10 = 0.5; L ÷ W = 2.
α = arcsin 0.5 = 30°.
Height ≈ 80 × tan 30° ≈ 80 × 0.577 ≈ 46 cm.

Each point is the calculator re-run with only “Stain width W (mm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Stain width W (mm)	Angle of impact α (°)
1.25	7.2
2.5	14.5
3.75	22
5	30
6.25	38.7
7.5	48.6
10	90

How to read the result

Narrow, elongated stains mean shallow angles; nearly round ones mean steep angles. The straight-line height is an upper estimate, because real drops follow curved paths under gravity and air resistance. Measurement errors in W and L matter most at steep angles.

Common mistakes

Including the tail or satellite spatter in the length.
Measuring stains on rough or absorbent surfaces, where shapes distort.
Treating the height as exact rather than an upper bound.
Using stains from different events in the same reconstruction.

Keep in mind: This is a teaching model. Real forensic casework follows validated procedures by trained analysts; do not rely on this tool for investigations.

Frequently asked questions

How is the angle of impact calculated?

α = arcsin(width ÷ length) of the elliptical stain.

What does a round stain mean?

The drop struck the surface at about 90°, falling straight down.

Why is the estimated height an upper limit?

Drops travel on curved paths, but the method assumes straight lines.

Sources: James, S. H., Kish, P. E. & Sutton, T. P., Principles of Bloodstain Pattern Analysis: Theory and Practice. CRC Press · Scientific Working Group on Bloodstain Pattern Analysis (SWGSTAIN), terminology and guidelines

Checked 9 Nov 2026. M. Gomez, “Solving crimes with maths: Bloodstain pattern analysis”, Plus Magazine (University of Cambridge, 22 Nov 2021): sin θ = b/a (width over length) and source height h = d·tan θ above the convergence point.

Your numbers