What is BAC?
Tuesday, May 10, 2022
What is BAC?
BAC stands for Blood Alcohol Concentration or Blood Alcohol Content. It’s a measurement of the percentage of alcohol in your bloodstream after you've been drinking. Your BAC, as opposed to the exact amount of alcohol you've consumed, determines the effect that alcohol has on you. It is calculated as the amount of alcohol in milligrams (mg) per 100 millilitres (ml) of blood. The legal BAC limit for driving in England, Wales and Northern Ireland is 80mg of alcohol per 100ml of blood. This is expressed as 0.8 per mille (per thousand). On a personal breathalyzer it reads as 0.8‰ BAC. The symbol ‰ means per mille. The drink drive limit in Scotland is lower than the rest of the UK. It is 50mg of alcohol per 100ml of blood or 0.5‰ BAC.
How is BAC measured?
BAC levels can be measured using an accurate alcohol breath tester, or breathalyzer. Good quality personal breathalyzers feature smaller versions of the same sensors found in many of the devices used by Police for roadside breath testing. An online BAC calculator can be highly inaccurate as it does not take into account all the variable factors. If your reading is over the BAC limit whilst driving, you will be arrested and taken to a Police station where an ‘evidential’ breath test will be carried out. ‘Evidential’ means the results can be used as evidence against you in Court. In certain circumstances, Police may check your BAC level in a blood test.
How can BAC affect your body?
our BAC level impacts your brain and behaviour, with higher blood alcohol content resulting in a higher degree of impairment. There are many factors that influence BAC including your weight, biological sex, physical condition, how much you’ve eaten, any medication you’re taking and the alcoholic strength of what you’re drinking. It’s not simply a case of so much BAC per shot, nor should you rely on a BAC calculator formula.
BAC level effects
BAC alcohol levels can affect behaviour, judgement, physiology and driving ability. The Centre for Disease Control and Prevention in America has compiled the following chart to illustrate the meaning of BAC.
The number of drinks above represents the approximate amount of alcohol that a man weighing eleven and a half stone would need to drink in one hour to reach the listed BAC in each category. A standard alcoholic drink is a 330ml bottle of beer (ABV 5%) or 125ml glass of wine (ABV 12%). If you’d like to try and cut down your consumption, read our article on how to reduce alcohol.
| BAC level | Standard alcoholic drinks | Typical general effects | Likely effects on driving |
| 0.2‰ BAC | Two | Some loss of judgement, relaxation, slight body warmth, altered mood | Decline in visual functions (rapid tracking of moving target) and ability to perform two tasks at the same time (divided attention) |
| 0.5‰ BAC | Three | Exaggerated behaviour, loss of small-muscle control (e.g. focusing eyes), impaired judgement, lowered alertness, release of inhibition | Reduced coordination, reduced ability to track moving objects, difficulty steering, reduced response to emergency driving situations |
| 0.8‰ BAC | Four | Poor muscle coordination (e.g. balance, speech, vision, reaction time, hearing), difficulty detecting danger, impaired judgement, self-control, reasoning and memory | Loss of concentration, short-term memory loss, impaired speed control, reduced information processing capability (e.g. signal detection, visual search), impaired perception |
| 1.0‰ BAC | Five | Clear deterioration of reaction time and control, slurred speech, poor coordination, slowed thinking | All of the above plus reduced ability to maintain lane position and brake appropriately |
| 1.5‰ BAC | Seven | Far less muscle control than normal, potential for vomiting, major loss of balance | All of the above and substantial impairment in vehicle control, attention to driving task, and visual and auditory processing |
The number of drinks above represents the approximate amount of alcohol that a man weighing eleven and a half stone would need to drink in one hour to reach the listed BAC in each category. A standard alcoholic drink is a 330ml bottle of beer (ABV 5%) or 125ml glass of wine (ABV 12%). If you’d like to try and cut down your consumption, read our article on how to reduce alcohol.
