Time of Death Calculator

Estimate post-mortem interval using Henssge's double-exponential body cooling model.

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Estimated Post-Mortem Interval
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Approximate Time of Death
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Confidence
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How to Use This Time of Death Calculator

  1. Enter the rectal temperature of the deceased, measured at the scene by a forensic pathologist using a calibrated thermometer.
  2. Record the ambient temperature of the environment where the body was discovered, as close to the body's location as possible.
  3. Enter the estimated body mass in kilograms. Heavier bodies retain heat longer and cool more slowly.
  4. Select the correction factor that best describes the body's coverings and environment at the time of discovery.

Understanding Henssge's Nomogram Method

The Henssge Nomogram is the most widely used forensic tool for estimating the post-mortem interval (PMI) based on algor mortis — the predictable cooling of the body after death. Developed by Professor Claus Henssge in the 1980s, the method replaced simpler linear cooling models with a sophisticated double-exponential equation that more accurately captures the complex thermodynamics of human body cooling.

After death, the body does not cool at a constant rate. Instead, cooling follows a characteristic sigmoid curve: an initial plateau phase (the "temperature plateau" lasting 1-3 hours during which body temperature drops very slowly), followed by a period of rapid cooling, and finally a gradual asymptotic approach to ambient temperature. Henssge's double-exponential model captures all three phases mathematically.

The Double-Exponential Cooling Formula

This calculator solves the Henssge equation computationally using bisection root-finding:

(Tr - Ta) / (37.2 - Ta) = 1.25 · e-Zt - 0.25 · e-5Zt

Where Tr = rectal temperature, Ta = ambient temperature, Z = cooling constant derived from body mass and correction factor, and t = time since death in hours. The constant Z is computed as:

Z = 10 · exp(-0.0284 · M · Cf)

Correction Factors in Forensic Thermometry

The correction factor is critical to the accuracy of the PMI estimate. Henssge empirically determined correction factors for various environmental conditions through controlled experiments. A naked body exposed to still air serves as the baseline (factor 1.0). Clothing acts as insulation, slowing heat loss — light clothing yields a factor of 0.75, while heavy clothing or blankets reduce it to 0.5. Water immersion dramatically accelerates cooling, with a correction factor of 0.35 for still water and even lower values for flowing water.

Limitations and Environmental Factors

While Henssge's method is the forensic gold standard for temperature-based PMI estimation, several factors can compromise accuracy. Fever at time of death (the model assumes 37.2°C), environmental temperature changes between death and discovery, direct sunlight or heating sources, fire or burning, and decomposition generating metabolic heat can all introduce significant error. The method is most reliable within the first 24-48 hours post-mortem and when environmental conditions have remained relatively stable.

Real-World Forensic Application

In practice, forensic pathologists use rectal temperature as the standard measurement because it most closely reflects core body temperature. Measurements are taken as early as possible at the scene, ideally before the body is moved. The ambient temperature should be recorded at body level, not from a wall thermometer across the room. Multiple temperature readings taken at intervals can improve accuracy by allowing the pathologist to verify the cooling trajectory against the predicted curve.

Frequently Asked Questions

Algor mortis is the gradual cooling of the body after death. Once metabolic processes cease generating heat, body temperature drops toward ambient temperature following predictable thermodynamic principles. This cooling curve, along with rigor mortis and livor mortis, forms the triad of early post-mortem changes used by forensic pathologists to estimate time of death.
Under standard conditions, Henssge's nomogram method can estimate PMI within approximately ±2.8 hours for the first 10 hours post-mortem, and within ±4-5 hours for longer intervals. Accuracy depends heavily on correct ambient temperature measurement, appropriate correction factor selection, and stable environmental conditions between death and discovery.
Several factors influence cooling rate: body mass (larger bodies cool more slowly due to lower surface-area-to-volume ratio), clothing and coverings (insulation slows cooling), ambient temperature, air circulation and wind, water immersion (dramatically accelerates cooling), body fat percentage (adipose tissue insulates), and surface contact with cold or warm materials.
The correction factor adjusts the cooling equation for environmental conditions surrounding the body. A naked body on a dry surface uses 1.0 (baseline). Light clothing reduces the effective cooling and uses 0.75. Heavy clothing or 1-2 blankets uses 0.5. A body in still water, which conducts heat roughly 25 times faster than air, uses 0.35. These factors were empirically derived by Henssge from controlled cooling experiments.
Henssge used 37.2°C as the standard body temperature at death because it represents the average rectal temperature in living adults. The commonly cited 37°C (98.6°F) is an oral temperature measurement. Since rectal temperature is used for post-mortem readings (providing the most reliable core temperature), the slightly higher 37.2°C baseline produces more accurate PMI estimates.