Ever since the introduction of the breathalyser in 1967, official sources have been extremely unwilling to publish any figures on the amount of alcohol you need to consume to take you over the 80 mg legal limit for driving. The reason for this is that they feel it will encourage drivers to "drink up to the limit". But, in reality, nobody can do that, because of the extremely unpredictable rate at which alcohol is absorbed by the body. Either you play safe, and stay well below it, or you try to drink up to the limit, and run a serious risk of exceeding it.
Of course, everyone knows that the law lays down a limit, not a prohibition, and that a certain amount of alcohol can be consumed without taking drivers outside the law. This is expressed in the popular wisdom that the limit equals two pints. This can be misleading, but contains some truth. Broadly speaking, if a man of average weight consumes two pints of ordinary strength beer of 4% ABV or less, it is extremely unlikely to take him above the 80 mg legal limit, and in reality will probably lead to a maximum BAC of no higher than 60 mg.
The following is an attempt to express this in rather more detail. It is drawn from various sources, including individuals' experiences of being breath tested, but the primary source is a booklet entitled The Facts about Drinking and Driving, published by the Transport and Road Research Laboratory in 1986, which presumably can be regarded as reasonably authoritative. However, these guidelines are not a statement of fact, and must not be taken as a guarantee of keeping below the legal limit in any particular circumstances.
Alcohol is normally measured in "units" of 10ml of alcohol. This is the amount of alcohol contained a a half-pint of beer of 3.5% ABV, a single 25ml pub measure of spirits, or a small 125ml glass of light table wine.
The rate of absorption of alcohol into the bloodstream is unpredictable and depends on a number of factors such as the level of hydration, the type of alcoholic drink consumed and whether food is eaten at the same time. As a broad rule of thumb, the alcohol in a drink is fully absorbed about an hour after the drink is finished.
The rate at which alcohol is metabolised and removed from the bloodstream is rather more predictable, and averages out at one unit per hour, starting one hour after the first drink is finished. However, the capacity of the body to metabolise alcohol is finite, and is limited to about 16-20 units per day. If you consistently drink around or above this level, you will probably never be below the limit - and you also need to consider seriously whether you have a drink problem!.
To ensure you run no risk of being "over the limit":
This applies to people of average weight (around 12-13 st for men, 9-10 st for women). If you are particularly small, these figures should be reduced accordingly. But if you are particularly big, it is no guarantee that they can be increased. The figures are lower for women not only because they are usually lighter than men, but also because their metabolism is different.
If you drink more than this, it will not guarantee that you will exceed 80 mg, as the rate of absorption of alcohol is so unpredictable. But even with one unit more you will be running a tangible risk. The above figures are the maximum you can consume without any significant risk of exceeding the legal limit, and also without resulting in any significant increase in accident risk.
It is also important to remember that a half pint of beer or a glass of wine can contain considerably more than one unit. Given that most pubs serve draught beer 5 to 10% under measure, it is fairly safe to assume a half of any beer up to around 4% represents one unit. But, any higher than that, and you have to make adjustments. A pint of 5% beer is almost three units, and even one and a half pints may not quite be safe.
In 1967, very little beer was available above 4% ABV. Now, it is widespread, and in some pubs it can be be difficult to find beer below that strength. In particular, the 33cl bottles of "designer lagers" that are so popular are usually between 5% and 5.5% ABV, meaning that each contains almost two units, and any more than two could take a driver above 80 mg.
The risks with wine are even greater. Only the very lightest German wines have an ABV as low as 8%, and many pubs and restaurants serve wine in 175ml or even 250ml glasses, rather than 125ml. A 175ml glass of wine of 12% ABV, which is fairly typical, represents two units. Two such glasses could put a smallish woman over the limit, whereas two pints of bitter would not do so for an average-sized man. An increasing number of pubs are now also serving spirits in 35ml measures, containing 1.4 units, rather than 25ml which is exactly one unit.
If you feel this is all a bit complicated, then don't drink anything. It's very simple, and an alcoholic drink will never, ever, make you a better driver. But bear in mind that, to be consistent, that also means you should drink little or nothing the night before driving. It's no good making a point of not touching a drop immediately before driving, and then having a skinful in the evening and getting in your car the next morning. However you want to play it, you must look at your overall pattern of alcohol consumption in relation to driving.