What this calculator works out
This calculator takes a list of temperature and duration pairs and returns a weighted mean temperature across the whole period, along with the total hours covered.
Weighting by duration is what makes the mean meaningful. Twelve hours at 20°C and two hours at 30°C do not average to 25°C, and treating them as though they do overestimates the heat available for development. 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.
Weighted periods
Enter one period per line as temperature,hours — for example: 18,6
Before you enter data
- Enter one period per line as temperature, then hours, separated by a comma.
- Periods should reflect genuinely different conditions — day and night, sun and shade, before and after a body was moved.
- Temperatures should already be corrected to the scene rather than taken raw from a weather station.
- The total hours shown in the result is worth checking against the period you intended to cover.
A simple average treats a two-hour warm spell as equal to a twelve-hour cold night. Because development accumulates with time as well as temperature, the duration of each condition is exactly as important as its value. Weighting is not a refinement — an unweighted mean is simply the wrong quantity.
How this calculator works
A duration-weighted mean:
Weighted mean = Σ(temperature × hours) ÷ Σ(hours)The calculator rejects the whole input if any line is malformed or has a negative duration, rather than silently skipping it. A quietly dropped period would understate the total and produce a plausible but wrong mean.
Where hourly data is available, accumulating it directly with the Accumulated Degree Hours Calculator is preferable to condensing it into a mean. This tool is for cases where conditions are known in blocks rather than hour by hour.
Worked example: three periods
Using the default figures — 16°C for 8 hours, 20°C for 10 hours, 18°C for 6 hours:
- Weighted sum: (16 × 8) + (20 × 10) + (18 × 6) = 128 + 200 + 108 = 436
- Total hours: 24
- Weighted mean: 436 ÷ 24 = 18.17°C
The unweighted average of 16, 20 and 18 is 18.00°C — close, because the durations are similar. Change the periods to 16°C for 20 hours and 30°C for 4 hours and the difference becomes stark: the weighted mean is 18.33°C against an unweighted 23.00°C. Against a 10°C threshold that is 8.33 versus 13.00 degree hours per hour, a 56% overstatement of development.
Common mistakes
- Taking a simple average of the temperatures. Duration must weight the result.
- Using raw weather station data. Correct it to the scene first.
- Letting the periods not sum to the interval. Check the total hours in the result.
- Condensing highly variable data into a single mean. Accumulate hourly data where you have it.
- Including periods after the body was moved. Conditions changed; the reconstruction should too.
Frequently asked questions
When should I use a mean rather than hourly accumulation?
Only when hourly data is unavailable or the conditions genuinely fall into a small number of distinct blocks — a body indoors for a known period and then outdoors, for instance. Where hourly records exist, accumulating them directly preserves information that any mean discards.
How is weather station data corrected to a scene?
By placing a data logger at the scene position after the fact, recording for a period of days to weeks alongside the station, and building a regression between the two series. That relationship is then applied to the station's historical records. The correlation period should span comparable conditions to the period of interest.
How far can a scene differ from the nearest station?
Substantially. Sun-exposed positions can run many degrees above a shaded station reading, sheltered and enclosed spaces retain heat overnight, and indoor conditions may bear no relation to outdoor records at all. This is why correlation is done rather than assumed, and why the distance and terrain between scene and station are noted.
What if the body was moved?
Then the reconstruction has to be built in segments, with each period reflecting the conditions at that location. This tool handles that directly — enter one line per location and duration. Establishing when the move occurred is a separate investigative question that the temperature data cannot answer.
Is what I enter stored?
No. Temperature data is processed entirely in your browser and never transmitted or retained.
Related tools
References
- Met Office — historic and hourly UK temperature records used for scene correlation
- GOV.UK — Forensic Science Regulator guidance on method validation
Sources are checked at publication and can change — how I choose and check references.