Where I live new drivers can't have any alcohol in their system while driving. So if you blow anything above 0 this is a fail. I have never taken a breathalyzer and have no clue how long alcohol takes to go through system. For example, if I had 1 beer at lunch and am driving somewhere at 8:00 PM that does seem a bit excessive. Is there anyway to know what your alcohol content is? Obviously I'm not trying to bypass drinking and driving laws, I'm trying to comply with them.
I would say short of buying something which can check your breath you would have to work it out manually.
If you waited 6-8 hours after drinking a pint of ale or lager i would say it would be safe, but how quickly it leaves your system is different for everyone.
The NHS published this below in regards to how long it takes to break down alcohol. You can read the full article here.
On average, it takes about one hour for your body to break down one unit of alcohol. However, this can vary, depending on: your weight whether you're male or female your age how quickly or slowly your body turns food into energy (your metabolism) how much food you have eaten the type and strength of the alcohol whether you're taking medication and, if so, what type It can also take longer if your liver isn't working normally
At Respect Yourself they roughly claim
- under 8 units: number of units + 1 hour (2 pints lager = 6 units, so 7 hours)
- 8-11 units: number of units + 1.5 hours (3 large glasses of wine = 9 units, so 10.5 hours)
- 12 units plus: number of units x 1.5 (5 pints lager/glasses wine = 15 units, so 22.5 hours)
Remeber, though, nothing is certain and the person testing you won't care that you waited x hours if you still have it in your system.
You can read more on units of alcohol here but heres an image from the page.
You can work out the units manually following the information from the NHS
Using units is a simpler way of representing a drink's alcohol content – usually expressed by the standard measure ABV, which stands for alcohol by volume. ABV is a measure of the amount of pure alcohol as a percentage of the total volume of liquid in a drink. You can find the ABV on the labels of cans and bottles, sometimes written as "vol" or "alcohol volume" or you can ask bar staff about particular drinks. For example, wine that says "12% ABV" or "alcohol volume 12%" means that 12% of the volume of that drink is pure alcohol. You can work out how many units there are in any drink by multiplying the total volume of a drink (in ml) by its ABV (which is measured as a percentage) and dividing the result by 1,000. Strength (ABV) x volume (ml) ÷ 1,000 = units. For example, to work out the number of units in a pint (568ml) of strong lager (ABV 5.2%): 5.2 (%) x 568 (ml) ÷ 1,000 = 2.95 units
In my personal experience i have had 1 beer, been breathalysed with in an hour, and shown up as no alcohol in my system.