// Evidence Lab — Interactive Demos

Evidence Lab

Hands-on calculators and demonstrations that let you explore the math behind forensic methods. Each tool explains its formula and what the result means.

Educational demonstrations only. These calculators use simplified formulas to teach concepts. They are not professional investigative tools and must never be used to draw real-world conclusions about actual people or cases.

FIELD NOTE

Bloodstain Angle of Impact

Estimate the angle at which a blood drop struck a surface from the shape of the resulting stain. A drop hitting at 90° is round; lower angles produce more elongated stains.

Stain width (mm)

Stain length (mm)

Angle of impact

—

angle = arcsin(width ÷ length)

This simplified model assumes a single clean elliptical stain on a smooth surface. Real scenes involve surface texture, drop volume, and travel distance that change shape — this is an educational demonstration, not a reliable investigative tool.

Time of Death (Algor Mortis)

A rough estimate of how long ago death occurred based on body cooling. After death the body cools toward the ambient temperature at an approximate rate — once it reaches ambient, this method can no longer estimate time of death.

Body temperature

Ambient temperature

Approx. hours since death

—

hours ≈ (98.6°F − body temp) ÷ 1.5 (valid only while body temp > ambient)

The 1.5°F/hour figure is a classroom approximation. Real cooling depends on body mass, clothing, ambient temperature, humidity, and air movement. Never use this alone for an actual estimate — professionals combine it with rigor, livor, and scene indicators.

DNA Random Match Probability

Multiply the frequencies of several independent DNA markers to see how quickly a combined profile becomes rare. This illustrates why many loci create powerful (but not absolute) evidence.

Allele / genotype frequencies (comma-separated, 0–1)

Combined probability

1.00e-3

≈ 1 in

1,000

3 factors combined. More independent markers shrink the probability fast — but a small number never proves uniqueness.

combined probability = f₁ × f₂ × f₃ × …

Real STR analysis uses validated population statistics with corrections for substructure, not simple independent multiplication. This toy model shows the concept of combining rare events — not a real random-match probability.

Population Genetics (Hardy-Weinberg)

Given the frequency of one allele (p), the Hardy-Weinberg principle predicts the expected genotype frequencies in an ideal population — the basis for allele-frequency reasoning in DNA statistics.

Allele frequency p (0–1)

q (other allele)0.400

Homozygous p²0.360 (36.0%)

Heterozygous 2pq0.480 (48.0%)

Homozygous q²0.160 (16.0%)

These three genotype frequencies sum to 1 (100%) in an ideal population.

p² + 2pq + q² = 1, where q = 1 − p

Hardy-Weinberg assumes an ideal population (no selection, drift, migration, mutation, or non-random mating). Real populations deviate, which is why forensic DNA statistics apply correction factors.

Dilution (C₁V₁ = C₂V₂)

Find the final volume needed to dilute a solution from one concentration to another. Widely used in lab preparation, including toxicology and chemistry work.

Initial conc. C₁

Initial vol. V₁

Final conc. C₂

Final volume V₂

—

V₂ = (C₁ × V₁) ÷ C₂

A straightforward laboratory relationship. In real analytical work, units must match, volumes must be measurable, and solute behavior (e.g. non-ideal solutions) may require correction.

Unit Converter

Convert common forensic-relevant units: temperature, length, and mass. Useful for moving between laboratory and field measurements.

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