What this calculator works out
This calculator treats a bloodstain as an ellipse, calculates its area from the length and width, and reports the diameter of a circle with the same area.
The equivalent circular diameter is a way of comparing stains of different shapes on a single scale. It is a descriptive measure rather than an inference about the droplet that made it. This is a teaching aid, not a casework tool.
Dimensions
Before you measure
- Use the same measurement convention as for angle of impact: length along the long axis excluding the tail, width perpendicular at the widest point.
- Both figures are in millimetres here, so the area comes out in square millimetres.
- The ellipse model fits a stain on a smooth non-absorbent surface. Textured surfaces produce irregular outlines it does not describe.
- Stain size is not droplet size. Blood spreads on impact, and how much depends on the surface and the impact velocity.
A droplet spreads when it hits a surface, so the stain is always wider than the drop that made it. The ratio depends on the surface, the impact speed and the angle, which is why stain size alone cannot be converted reliably back to droplet volume without controlled comparison on the same substrate.
How this calculator works
Ellipse area, then the circle of equal area:
Area = π × (length ÷ 2) × (width ÷ 2)
Equivalent circular diameter = 2 × √(area ÷ π)The equivalent diameter always falls between the width and the length — it is the geometric mean of the two axes. For a circular stain, where width equals length, it equals both.
Worked example: a 10 by 6 mm stain
Using the default figures — length 10 mm, width 6 mm:
- Area: π × 5 × 3 = 47.12 mm²
- Equivalent circular diameter: 7.75 mm
7.75 mm sits between the 6 mm width and the 10 mm length, as it must. The same stain would have an impact angle of arcsin(0.6), or 36.9° — the two calculations use the same two measurements to answer different questions, which is why recording both dimensions carefully matters more than either result individually.
Common mistakes
- Treating stain size as droplet size. Blood spreads on impact.
- Including the tail in the length. It inflates both area and diameter.
- Applying the ellipse model to irregular stains. It describes clean ellipses only.
- Comparing stains across different surfaces. Spread varies with substrate.
- Reading precision into the fourth significant figure. The measurement rarely supports it.
Frequently asked questions
What is the equivalent circular diameter for?
It provides a single size figure for stains of differing elongation, which makes them comparable. It is descriptive — useful for recording and for comparing within a pattern — rather than a step towards an inference about the source.
Can stain size indicate the mechanism?
Only loosely, and with considerable caution. Broad size categories have historically been associated with different mechanisms, but the relationship is not reliable enough to determine mechanism from size alone, and this has been an area of specific criticism. Size is recorded as one observation among many rather than treated as diagnostic.
Why does the surface matter so much?
Spread depends on the balance between the droplet's momentum and the surface's resistance and wettability. The same droplet produces a markedly different stain on glass, painted plaster, unfinished wood and fabric. Comparisons are only meaningful between stains on comparable substrates.
How precisely should stains be measured?
To the precision the stain edge actually supports, which for a typical stain photographed with a scale is a fraction of a millimetre at best. Reporting four decimal places from a measurement good to half a millimetre implies precision that does not exist.
Is what I enter stored?
No. Measurements are processed entirely in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — Forensic Science Regulator guidance on measurement and reporting
- National Institute for Health and Care Excellence — clinical guidance relevant to injury interpretation
Sources are checked at publication and can change — how I choose and check references.