Back calculations of blood alcohol for drink driving offences
Produced in partnership with Red Lion Chambers

The following Corporate Crime practice note produced in partnership with Red Lion Chambers provides comprehensive and up to date legal information covering:

  • Back calculations of blood alcohol for drink driving offences
  • Back calculations of alcohol levels in a driver’s system
  • The back calculation process
  • Information required by the expert to conduct back calculations
  • Evidence to be used in drink-driving cases

Back calculations of blood alcohol for drink driving offences

This Practice Note explains how the proportion of alcohol in a driver’s system at the time they were driving, rather than at the time that the breath or lab blood or urine specimen was provided, can be produced through a back calculation. For detailed guidance on the power to request specimens of breath, blood or urine in relation to motoring offences, see Practice Note: Evidential specimens in road traffic cases. For detailed guidance on the factors which can influence whether the evidential specimens will be deemed admissible evidence during a criminal prosecution for road traffic offences, see Practice Note: Admissibility of specimens of blood, urine and breath in road traffic cases. Finally, for further information on the offences of failing to provide a blood, urine or breath specimen or failing to co-operate with a preliminary test, see Practice Notes: Road traffic offences—failing to provide a specimen and Road traffic offences—failure to co-operate with a preliminary test.

Back calculations of alcohol levels in a driver’s system

The proportion of alcohol present in the driver’s system at the time of driving rather than at the time that the breath or laboratory specimen was provided can be produced through a back calculation.

The presumption is that the proportion of alcohol in the driver’s breath, blood or urine at the time of the

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