What this calculator does — and what it cannot support
This calculator applies a straight-line elimination rate to a measured concentration, either projecting forward in time or extrapolating backwards.
Back-calculation is contentious and this tool cannot support an evidential conclusion. It assumes elimination had begun, that absorption was complete, and that a single rate applied — none of which can be established from the arithmetic. This is a teaching and planning aid, not a casework tool.
Elimination model
Before you use it
- Elimination rates vary between individuals and within the same individual. Published ranges span roughly 10 to 25 mg/100ml per hour, and the default of 15 is a mid-range figure rather than a constant.
- The model only holds after absorption is complete, which takes 30 to 90 minutes from the last drink and longer with food.
- During absorption the concentration is rising, so extrapolating backwards through that period gives a figure that is wrong in direction as well as magnitude.
- Units are whatever you enter. Keep the concentration and the rate in the same units.
Retrograde extrapolation requires assuming the elimination rate, that absorption was complete at the earlier time, and that no further alcohol was consumed in between. Each assumption is a point of challenge, and the last is the basis of the so-called hip flask defence. Courts treat back-calculated figures with considerable caution, and in some circumstances they are not admitted at all.
How this calculator works
Linear elimination in one direction or the other:
Forward: concentration = measured − (rate × hours), floored at zero
Backwards: concentration = measured + (rate × hours)Zero-order elimination — a constant amount removed per hour regardless of concentration — is a reasonable description once absorption is complete and at concentrations above a low threshold. It breaks down at very low concentrations, where elimination becomes concentration-dependent.
The floor at zero on forward projection prevents a negative result. It does not mean the model is reliable near zero.
Worked example: three hours
Using the default figures — a measured concentration of 120, an elimination rate of 15 per hour, over 3 hours:
- Projecting forward: 120 − 45 = 75
- Back-calculating: 120 + 45 = 165
Now vary the rate across its published range. At 10 per hour the back-calculated figure is 150; at 25 per hour it is 195. That is a 45-unit spread from a rate that cannot be measured retrospectively for the individual concerned — which is precisely why a single back-calculated number should not be presented as a fact.
Common mistakes
- Back-calculating through the absorption phase. Concentration was rising, not falling.
- Using a single elimination rate as though it were established. The range is wide.
- Mixing units between concentration and rate.
- Presenting a point estimate without a range. The uncertainty is the substance of the result.
- Treating this as an evidential calculation. It is arithmetic, not analysis.
Frequently asked questions
What are the UK drink-drive limits?
80 mg of alcohol per 100 ml of blood in England, Wales and Northern Ireland, with equivalents of 35 micrograms per 100 ml of breath and 107 mg per 100 ml of urine. Scotland's limits are lower at 50 mg blood, 22 micrograms breath and 67 mg urine. Evidential specimens are taken and analysed under a defined procedure; no calculation substitutes for one.
What is the hip flask defence?
The assertion that alcohol was consumed after driving and before the specimen was taken, so the specimen does not reflect the concentration while driving. It shifts the analysis to a back-calculation, and it is one of the main reasons retrograde extrapolation is scrutinised so closely in court. Establishing it requires evidence of the post-driving consumption, not just an assertion.
How much does elimination rate vary?
Published figures generally span about 10 to 25 mg/100ml per hour, with regular heavy drinkers typically at the higher end due to enzyme induction. It varies with liver function, sex, food, medication and time of day. It cannot be measured retrospectively for an individual, which is why any back-calculation is presented as a range.
Is elimination really linear?
Approximately, above a low concentration, because the enzyme pathway is saturated and works at capacity. Below roughly 10 to 20 mg/100ml elimination becomes concentration-dependent and slows, so the linear model overestimates how quickly the last of it clears.
Is what I enter stored?
No. Everything you enter is processed in your browser and is never transmitted or retained. Even so, treat any public website as an unsecured environment and do not enter identifiable case information.
Related tools
References
- GOV.UK — drink-drive limits, evidential specimen procedure and Forensic Science Regulator codes
- National Institute for Health and Care Excellence — clinical guidance on alcohol and its metabolism
Sources are checked at publication and can change — how I choose and check references.