What this calculator works out
This calculator sums the daily temperature above a lower developmental threshold, giving a total in accumulated degree days. It shows the contribution of each day as well as the total.
Thermal summation is the basis of almost all insect development estimation. Insects are ectothermic: they develop faster when warmer, and the total heat energy accumulated above a threshold predicts how far development has progressed. 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.
Temperature data
Before you calculate
- The lower developmental threshold is species-specific and comes from published laboratory studies. It is not a general figure — common blowfly thresholds range from around 6°C to 11°C depending on species.
- Use daily mean temperatures, not maximums. The mean of the daily maximum and minimum is the usual approximation.
- Temperatures must apply to the body's actual location, not a distant weather station. Retrospective correlation to the scene is standard practice.
- Days contributing nothing — where the mean is at or below the threshold — are counted as zero rather than negative.
One accumulated degree day equals 24 accumulated degree hours. Published developmental data uses whichever unit suits the timescale: ADH for the rapid early stages measured in hours, ADD for longer periods. Mixing the two units is a factor-of-24 error and one of the more common mistakes in coursework.
How this calculator works
A sum of daily contributions above the threshold:
Contribution for each day = max(0, daily mean temperature − threshold)
Total ADD = sum of all daily contributionsClamping negative contributions to zero reflects the biology: development effectively stops below the threshold rather than reversing. Some models apply an upper threshold too, above which development also slows or ceases.
The linear model assumes development rate rises proportionally with temperature above the threshold. This holds reasonably across the middle of a species' viable range and breaks down at both extremes.
Worked example: five days
Using the default figures — a 10°C threshold with daily means of 16, 18, 20, 19 and 17°C:
- Day 1: 16 − 10 = 6 · Day 2: 8 · Day 3: 10 · Day 4: 9 · Day 5: 7
- Total: 40 accumulated degree days
Now raise the threshold to 11°C, which is within the range of published values for a different blowfly species. The total falls to 35 ADD — a 12.5% reduction from a one-degree change in a single reference value. Over a longer period the effect compounds, which is why species identification comes before any calculation.
Common mistakes
- Using a generic threshold. It is species-specific and materially changes the result.
- Mixing degree days and degree hours. One ADD is 24 ADH.
- Using maximum rather than mean temperatures. The mean is what the model expects.
- Applying weather station data without correction. The scene microclimate can differ substantially.
- Ignoring maggot mass heat. A large aggregation runs well above ambient.
Frequently asked questions
Where do developmental thresholds come from?
Published laboratory rearing studies, in which a species is raised at several constant temperatures and the development rate plotted against temperature. Extrapolating that line back to zero development gives the lower threshold. Values differ between species, between populations of the same species, and sometimes between studies, which is why the source matters as much as the number.
Does this give a time since death?
No, and the distinction is important. Entomological methods estimate the time since colonisation — the point at which insects first laid eggs on the body. That gives a minimum post-mortem interval. Death may have occurred earlier if the body was concealed, wrapped, indoors, refrigerated or otherwise inaccessible to insects.
How accurate is thermal summation?
It works well within its assumptions and degrades outside them. The main sources of error are the threshold value, the temperature data, whether a maggot mass was generating its own heat, and whether development occurred at constant or fluctuating temperatures — laboratory data comes from constant conditions and field development rarely is.
What about an upper threshold?
Development slows above an optimum and stops at a lethal upper limit, so a simple linear model overestimates development in very hot conditions. More sophisticated models incorporate this. For UK ambient temperatures it rarely binds, but inside a maggot mass, which can exceed 30°C, it can.
Is what I enter stored?
No. Temperatures are processed entirely in your browser and never transmitted or retained. Do not enter identifiable case information into any public website.
Related tools
References
- GOV.UK — Forensic Science Regulator codes of practice and method validation
- Met Office — historic UK temperature records used for retrospective scene correlation
Sources are checked at publication and can change — how I choose and check references.