Post Mortem Interval (PMI) Calculator: Algor Mortis Method
Estimate the time since death by calculating post mortem interval using algor mortis, the cooling of the body.
Temperature Comparison Chart
What is Post Mortem Interval (PMI) and Algor Mortis?
The Post Mortem Interval (PMI) is the estimated time that has elapsed since a person has died. In forensic science, establishing an accurate PMI is critical for investigations, helping to verify alibis, identify suspects, and reconstruct the timeline of events. One of the earliest methods used for this estimation is observing algor mortis.
Algor mortis, from Latin meaning “coldness of death,” is the process by which a body cools after death. When a person dies, their body’s internal thermoregulation ceases, and it no longer produces heat. As a result, the body begins to lose heat to its surroundings through processes like conduction, convection, and radiation, eventually matching the ambient temperature. This calculator focuses specifically on calculating post mortem interval using algor mortis, a foundational technique in forensic pathology. While it has limitations, it provides a crucial first estimate. For more details on other methods, see our guide on the Crime Scene Investigation Techniques.
The Algor Mortis Formula and Explanation
The most well-known formula for a basic PMI estimation is the Glaister equation. It provides a linear estimation of the time since death based on the difference between the normal body temperature and the measured rectal temperature.
The formula is generally expressed as:
PMI (in hours) = (Normal Body Temperature – Measured Rectal Temperature) / Cooling Rate
The “Normal Body Temperature” is typically assumed to be 37°C (98.6°F). The “Cooling Rate” is the most significant variable. The standard Glaister formula uses a rate of 1.5°F per hour. This calculator adjusts the cooling rate based on the ambient temperature, which provides a slightly more nuanced estimate.
| Variable | Meaning | Unit (auto-inferred) | Typical Range |
|---|---|---|---|
| Normal Body Temp | The assumed body temperature at the time of death. | °C or °F | 37°C / 98.6°F |
| Measured Body Temp | The core temperature of the body when discovered. | °C or °F | Ambient to 37°C / Ambient to 98.6°F |
| Cooling Rate | The rate at which the body loses heat per hour. | °/hour | 0.4-0.8 °C/hr or 0.75-1.5 °F/hr |
Practical Examples
Example 1: Indoor Discovery
- Inputs: Measured Body Temp = 29.4°C, Ambient Temp = 21°C, Units = Celsius
- Calculation: The temperature drop is 37°C – 29.4°C = 7.6°C. Using a standard cooling rate of ~0.78°C/hour, the estimated PMI would be approximately 9.7 hours.
- Results: The individual likely died around 9-10 hours before their body was discovered.
Example 2: Outdoor (Cold) Discovery
- Inputs: Measured Body Temp = 68°F, Ambient Temp = 30°F, Units = Fahrenheit
- Calculation: The ambient temperature is below freezing, so a faster cooling rate is applied (1.5°F/hour). The temperature drop is 98.6°F – 68°F = 30.6°F. The estimated PMI is 30.6 / 1.5 = 20.4 hours.
- Results: The increased cooling rate due to the cold environment suggests a much longer PMI of over 20 hours. Understanding these factors is key, as explored in our article on the Rigor Mortis Timeline.
How to Use This Post Mortem Interval Calculator
- Enter Measured Body Temperature: Input the core temperature taken from the decedent, typically rectally.
- Enter Ambient Temperature: Input the temperature of the immediate surroundings where the body was found.
- Select Units: Choose whether your temperature measurements are in Celsius (°C) or Fahrenheit (°F). The calculator will automatically adjust the formula.
- Calculate: Click the “Calculate PMI” button. The calculator will display the estimated time since death in hours, along with the temperature drop and the cooling rate factor used.
- Interpret Results: The primary result is a scientific estimation. Always consider other factors and methods for a complete analysis. Our Advanced Time of Death Models course offers more insight.
Key Factors That Affect Algor Mortis
The simple Glaister formula is a starting point. In reality, many factors can alter the rate of body cooling, making calculating post mortem interval using algor mortis a complex task. A forensic professional must consider:
- Clothing and Coverings: Layers of clothing or blankets act as insulation and significantly slow down the cooling process.
- Body Habitus (BMI): A higher body fat percentage provides more insulation, leading to a slower cooling rate compared to a leaner individual.
- Ambient Environment: A body in water will cool much faster than a body in air. Likewise, strong winds (convection) will accelerate heat loss.
- Initial Body Temperature: The assumption of a normal 37°C/98.6°F temperature can be wrong if the person had a high fever or was suffering from hypothermia at the time of death.
- Surface Contact: A body lying on a cold concrete floor will lose heat faster via conduction than one on a carpeted surface.
- The “Temperature Plateau”: For the first few hours after death, the internal body temperature may not drop, or may drop very slowly. This “plateau” phase is highly variable.
Frequently Asked Questions (FAQ)
- Is the algor mortis calculation 100% accurate?
- No. It is an estimation, and its accuracy decreases significantly after 12-18 hours and is heavily influenced by external factors. It should be used in conjunction with other methods like rigor mortis and livor mortis.
- Why are rectal or liver temperatures used?
- The core body temperature cools more slowly and predictably than the surface skin temperature. Rectal or liver measurements provide a more reliable reading for PMI calculations.
- What happens if the ambient temperature is higher than the body temperature?
- The body will actually gain heat until it reaches equilibrium with its environment. The standard cooling formulas do not apply in this scenario.
- How does body size affect the calculation?
- Larger individuals with more body mass and fat will cool slower than smaller individuals or children, who have a higher surface-area-to-volume ratio.
- Does this calculator use the Henssge Nomogram?
- No, this calculator uses a modified Glaister formula for simplicity. The Henssge nomogram is a more complex graphical method that accounts for body weight and other factors, providing a more accurate range.
- How long does it take for a body to reach ambient temperature?
- This varies widely but typically takes between 18 to 36 hours, depending on all the factors mentioned above. Once a body reaches ambient temperature, algor mortis can no longer be used to estimate PMI.
- What is livor mortis?
- Livor mortis is the settling of blood in the lower parts of the body due to gravity, causing a purplish-red discoloration. You can learn about it in our guide, Understanding Livor Mortis.
- What comes after algor mortis?
- Following the initial changes of algor, livor, and rigor mortis, the body enters stages of decomposition, starting with putrefaction. For more details, see our overview on Forensic Entomology Basics.
Related Tools and Internal Resources
Expand your knowledge with our other forensic and scientific calculators and resources:
- Understanding Livor Mortis: Learn about the pooling of blood after death.
- Rigor Mortis Timeline: A detailed look at the stiffening of muscles post-mortem.
- Forensic Entomology Basics: A guide to estimating PMI using insect evidence.
- Crime Scene Investigation Techniques: An overview of standard procedures at a crime scene.
- Advanced Time of Death Models: Explore more complex mathematical models for PMI estimation.
- Forensic Pathology Explained: A career guide for aspiring forensic pathologists.