How this instrument works
Glaister's rule treats the body as a cooling object obeying one flat rate: about 1.5°F lost every hour after death, starting from a baseline living temperature of 98.6°F. Subtract a rectal reading from that baseline and divide by 1.5, and the arithmetic returns a figure for hours elapsed. It is named for John Glaister, the Scottish forensic pathologist whose early-twentieth-century textbooks carried the rate into generations of teaching, though no single paper of his actually introduced it as a formally tested formula — it is a heuristic passed down through the forensic pathology literature rather than a discrete published study with its own citation.
The figure this calculator returns is only ever a rough estimate, and it is worth saying so plainly rather than burying it in a footnote. Real cooling ignores flat rates: ambient temperature, body mass, clothing and insulation, airflow, and even the position a body is found in all change how fast heat actually leaves it, sometimes by a wide margin. A thin frame in a cold room cools far faster than 1.5°F an hour; a heavy frame under blankets in a warm room cools far slower. None of those variables appear anywhere in this simple division, which is exactly why professional postmortem interval estimation reaches for more sophisticated tools instead — the Henssge nomogram chief among them, which folds in ambient temperature and body weight rather than assuming they don't matter.
The formula also gets noticeably less trustworthy the longer the interval grows. Real postmortem cooling doesn't follow a straight line at all — it traces something closer to a flattened S-curve, staying near living temperature for the first hour or two, dropping fastest through the middle stretch, then leveling off as the body nears its surroundings and has less heat left to shed. A flat 1.5°F-per-hour line fits that middle stretch reasonably well but drifts further from reality on both ends, and is generally considered too unreliable to trust much beyond roughly the first twelve hours after death. This tool exists to illustrate the classic teaching rule, not to produce a forensically defensible estimate — real casework leans on nomograms, scene investigation, and considerably more besides.
- Enter Normal body temperature — 98.6°F is the standard living baseline and rarely needs changing.
- Enter Current rectal temperature, the measurement forensic teaching treats as the most stable postmortem reading.
- Read Estimated hours since death — the gap between the two figures, divided by 1.5°F per hour.
- Treat the result as an illustration of the classic rule, not a substitute for nomogram-based or scene-based estimates once the interval likely exceeds about twelve hours.
Worked example — a rectal reading of 90.0°F
Starting from the standard 98.6°F baseline, a rectal temperature of 90.0°F leaves a gap of 8.6°F. Divide by the assumed rate of 1.5°F per hour: 8.6 / 1.5 ≈ 5.73 hours since death — the arithmetic the classic rule performs in one step.
The same division scales in both directions. A smaller gap reads faster: 98.6°F down to 95.0°F is a 3.6°F difference, giving 3.6 / 1.5 = 2.4 hours. A larger gap reads slower and, not coincidentally, less trustworthy: 98.6°F down to 80.0°F is an 18.6°F difference, giving 18.6 / 1.5 ≈ 12.4 hours — right at the edge of where this flat-rate rule is generally considered too far into the cooling curve's nonlinear tail to trust.
Questions
How accurate is the 1.5°F-per-hour rule?
Only roughly, and it says so on its own terms. The rate is a teaching average, and real cooling depends heavily on ambient temperature, body mass, clothing, and position — none of which this simple formula considers. It works best as an illustration of the underlying idea, not as a figure to defend in an actual investigation.
Why is there no single published study behind Glaister's rule?
Because it isn't the output of one experiment — it comes from a lineage of forensic pathology teaching traced to John Glaister's early-twentieth-century Scottish textbooks, refined and repeated across generations of instruction rather than established in a discrete, citable paper. Modern forensic pathology references describe and contextualize the rate rather than treat it as a standalone finding.
What do forensic pathologists actually use instead of this formula?
The Henssge nomogram is the standard, more rigorous alternative, factoring in ambient temperature and body weight rather than assuming a flat rate applies regardless of surroundings. Investigators also weigh rigor mortis, lividity, decomposition, stomach contents, and scene evidence together — postmortem interval estimation leans on multiple, converging methods, not cooling alone.
Why does this formula get worse after about twelve hours?
Because real cooling isn't linear. A body sheds heat fastest in the hours after death, then that rate slows as it nears the surrounding temperature, tracing a curve closer to a flattened S than a straight line. A constant 1.5°F-per-hour line approximates the middle of that curve reasonably well but strays further from reality toward both ends, which is why longer estimates from this rule deserve real skepticism.
Does room temperature affect this calculation?
It affects real cooling enormously, but this particular formula doesn't ask for it — that is its core limitation, not an oversight to work around. A cold room speeds cooling well past 1.5°F an hour; a warm room slows it well below that rate. Tools built to account for surroundings, like the Henssge nomogram, exist specifically to correct for what this simpler formula leaves out.
References
- StatPearls — Algor Mortis (Eden RE, Das JM, Thomas B; NCBI Bookshelf)
- Knight's Forensic Pathology, 4th ed. (Saukko P, Knight B), Routledge
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.