Rigor, Livor and Algor Mortis: How Experts Estimate Time of Death

Last updated: 13 October 2026. Written for law students, lawyers and readers interested in forensic science.

Short answer: Rigor mortis, livor mortis and algor mortis are three early changes that happen to the body after death. Rigor mortis is the stiffening of muscles, livor mortis is the pooling of blood that colours the lowest parts of the body, and algor mortis is the cooling of the body. Forensic pathologists read all three together to estimate the time since death, known as the post-mortem interval. Each gives a range of hours, not an exact time.

Key takeaways

• Rigor mortis usually appears within about 2 to 6 hours, is fully developed at roughly 6 to 12 hours and passes off over the following 1 to 2 days, faster in heat and slower in cold.
• Livor mortis begins within the first hour or two as patchy discolouration and becomes “fixed” (it no longer blanches when pressed) at around 8 to 12 hours or later.
• Algor mortis is commonly approximated at a loss of about 1.5°F (0.8°C) per hour, but modern practice uses methods such as the Henssge nomogram that correct for body weight, clothing and surroundings.
• No single sign is reliable on its own. Courts expect experts to give a range and explain the factors that widen it.

Why time of death matters in law

Time since death can confirm or destroy an alibi, narrow the list of suspects, test a witness account or decide questions of inheritance where two people died close together. Because it is so important, it is also one of the most frequently challenged areas of forensic evidence. Defence lawyers look closely at how the estimate was reached, whether the right temperatures were recorded at the scene and whether the expert gave an honest range. For the wider role of the discipline, see our overview of forensic pathology in modern investigations.

Rigor mortis: the stiffening of the body

After death, the muscles run out of adenosine triphosphate (ATP), the molecule that lets muscle fibres relax. Without it, the fibres lock together and the muscles stiffen. That locked state is rigor mortis.

Typical timeline

Rigor often appears to spread from the head downwards, an observation known as Nysten’s law, although all muscles are affected at about the same time and smaller muscles simply show it first.

What speeds it up or slows it down

• Heat speeds up both onset and resolution. Cold slows both.
• Exertion before death, such as a struggle, a seizure or high fever, depletes ATP and can bring rigor on faster.
• Muscle mass, age and body build change how strongly it develops.

Cadaveric spasm is a rare, instant stiffening at the moment of death, for example a hand still gripping an object. It is different from rigor mortis and can be important evidence because it cannot easily be staged after death.

Livor mortis: the settling of blood

When circulation stops, gravity pulls blood into the lowest parts of the body. The skin there turns pink to purple. This is livor mortis, also called post-mortem lividity or hypostasis.

• Appearance: small patches can appear within about 30 minutes to 2 hours and merge into broader areas over the next several hours.
• Blanching: early on, pressing the skin makes the colour fade. As blood leaks into the tissues, the colour becomes “fixed” and no longer blanches, commonly from around 8 to 12 hours, though reported times vary widely.
• Pressure areas: where the body rests against a surface, such as the shoulder blades or buttocks of a person lying on their back, the skin stays pale because blood cannot settle there.

Livor mortis is often more useful for position than for timing. If the pattern of lividity does not match the position in which a body was found, it suggests the body was moved after the colour became fixed. Unusual colours can also be a clue: a cherry-red colour, for example, is associated with carbon monoxide poisoning.

Algor mortis: the cooling of the body

After death the body stops producing heat and gradually cools until it matches its surroundings. Measuring core temperature, usually rectally or in the liver, gives a clue to how long the cooling has been going on.

The rule of thumb

A traditional approximation, sometimes linked to the Glaister equation, assumes a loss of about 1.5°F (around 0.8°C) per hour:

Hours since death ≈ (98.4°F − rectal temperature in °F) ÷ 1.5

It is easy to use and easy to misuse. It assumes the body started at a normal temperature and cooled in a straight line, and neither assumption is reliable. Fever, hypothermia, exertion and drugs all change the starting temperature, and real cooling follows a curve with an early plateau.

The Henssge nomogram

Many forensic services now use the nomogram developed by Claus Henssge. It combines rectal temperature, ambient temperature and body weight, with correction factors for clothing, covers, wind and water. It produces an estimate with a stated confidence range of several hours. That range is the honest answer, and experts who give a single exact time should expect to be challenged in cross-examination.

Factors that change cooling speed include:

• air temperature, wind and humidity;
• whether the body is in water, which cools much faster than air;
• clothing, bedding and body fat, which insulate;
• body size, since smaller bodies and children cool faster.

Reading the three signs together

This table is a teaching guide, not a diagnostic tool. In real cases each column can shift by hours because of the conditions described above.

Other methods beyond body changes

When the early signs have passed, or to cross-check them, investigators use other evidence: potassium levels in the fluid of the eye (vitreous humour), stomach contents, the stage of decomposition and insect activity. Our articles on post-mortem interval methods beyond body changes and on insects in forensic investigation cover these in detail. You can also read our earlier explainer on post-mortem changes and time since death, or explore more topics in the Legal Desire Forensics hub.

How courts treat time of death evidence

In the United States, expert testimony on time of death has to meet the reliability standards in Federal Rule of Evidence 702 or the state equivalent. Judges look at whether the method is accepted, whether it was applied properly and whether the expert’s conclusion stays within what the data supports. A well-supported estimate usually:

• records scene and body temperatures at the time of discovery;
• uses more than one method and explains where they agree or disagree;
• gives a range rather than a precise time; and
• acknowledges factors, such as heating or moving of the body, that could change the result.

The scientific background for these methods is summarised in the peer-reviewed StatPearls chapter Methods of Estimation of Time Since Death.

Frequently asked questions

How long after death does rigor mortis set in?

Usually within about 2 to 6 hours, starting in the small muscles of the face and jaw. It is typically fully developed at around 6 to 12 hours.

How long does rigor mortis last?

It generally passes off within about 24 to 48 hours, sooner in hot conditions and later in cold ones.

What is the difference between livor mortis and bruising?

Livor mortis is blood settling under gravity after death and sits in the lowest parts of the body. A bruise is bleeding into tissue caused by injury while the person was alive. A pathologist can tell them apart by cutting into the area: in livor the blood stays inside the vessels and can be wiped or washed away, while in a bruise blood has leaked into the surrounding tissue.

How fast does a body cool after death?

A common rule of thumb is about 1.5°F (0.8°C) per hour, but the real rate depends on the surroundings, clothing, body size and starting temperature.

Can time of death be determined exactly?

No. Every method gives a range. The longer the time since death, the wider that range becomes.

What does “algor” mean?

It comes from the Latin word for coldness. Rigor means stiffness and livor means a bluish colour.