What this estimator works out
This estimator divides a published accumulated degree hour requirement by the temperature available above the developmental threshold, giving the time that development would take.
It is thermal summation run in reverse: instead of accumulating heat from observed temperatures, it asks how long a constant temperature would need to supply a known requirement. These tools are teaching aids. They are not validated for casework, and the reference values that make any result meaningful must come from published species-specific literature.
Thermal requirement
Before you estimate
- The ADH requirement must come from published data for the identified species and the specific developmental stage. Requirements differ by an order of magnitude between egg hatch and pupariation.
- The threshold must be from the same source as the requirement. Mixing values from different studies produces an inconsistent result.
- The temperature is a mean over the relevant period. Where it varied, use the Forensic Weather Adjustment Calculator to produce a weighted figure.
- Species identification comes first. Everything downstream depends on it.
The divisor is the temperature above the threshold, not the temperature itself. At 20°C with a 10°C threshold the divisor is 10; at 15°C it is 5, doubling the development time; at 12°C it is 2, multiplying it by five. Small absolute temperature changes matter enormously in cool conditions and much less in warm ones.
How this calculator works
A division:
Development time in hours = required ADH ÷ (mean temperature − lower threshold)
Days = hours ÷ 24The calculator refuses to run if the temperature is at or below the threshold, because development would not occur and the division would be meaningless.
This assumes a constant temperature throughout. Where temperature varied, accumulating actual hourly data with the Accumulated Degree Hours Calculator is the better approach.
Worked example: 1,200 ADH at 20°C
Using the default figures — a 1,200 ADH requirement, mean temperature 20°C, threshold 10°C:
- Available above threshold: 10 degrees
- Development time: 1,200 ÷ 10 = 120 hours, or 5.00 days
Drop the mean temperature to 15°C and the same requirement takes 240 hours — ten days rather than five. A five degree difference in the mean, well within the variation between a sunny and a shaded position at the same scene, doubles the estimate. This is why microclimate reconstruction is a substantial part of forensic entomology rather than a detail.
Common mistakes
- Using the temperature rather than the temperature above the threshold. The divisor is the difference.
- Taking the requirement and the threshold from different studies. They must be consistent.
- Applying a constant-temperature model to a variable period. Accumulate real data instead.
- Estimating without species identification. Requirements vary enormously between species.
- Presenting the result as a time since death. It is a development time.
Frequently asked questions
Which species are relevant in the UK?
Calliphora vicina and Calliphora vomitoria are the common bluebottles and are typically the first colonisers in cooler conditions. Lucilia sericata, the greenbottle, is more common in warm sunny conditions. Their developmental thresholds and requirements differ, and identification usually requires rearing specimens to adulthood or molecular methods rather than field examination alone.
How is a larva's stage determined?
By morphology — the number of slits in the posterior spiracles distinguishes the three instars — and by length, which correlates with age within a stage. Length alone is unreliable because larvae shrink before pupariation. Specimens are killed and preserved in a standard way precisely so that measurements are comparable.
Does this account for the pre-appearance interval?
No. There is a delay between death and the arrival of the first flies, which can be minutes outdoors in summer and days indoors, at night, in cold weather or where the body is concealed. Entomological estimates address the period since colonisation, and the pre-appearance interval is assessed separately from the circumstances.
Why not use larval length directly?
Length-based estimation exists and is used alongside thermal summation, but it is complicated by shrinkage in the late third instar, by preservation method affecting measured length, and by variation between individuals in a mass. Combining stage, length and thermal summation gives a more defensible range than any one alone.
Is what I enter stored?
No. Values are processed entirely in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — Forensic Science Regulator guidance on expert evidence and method validation
- Met Office — temperature records used to reconstruct scene conditions
Sources are checked at publication and can change — how I choose and check references.