It must be stressed that all of this is written specifically in relation to the UK. There is a separate calculator which deals with other countries.
The most obvious purpose of this calculator is to find how many units of alcohol exist in a given quantity of liquid when its AbV (alcohol by volume) is known. The AbV should always be printed on the label of the container of the liquid, or be prominently displayed when the liquid is being supplied from a bulk source (such as when drawing from a barrel in a pub).
It is only a matter of keying in the AbV value, and then the numerical value of the size of the quantity, adjusting the type of measure (if necessary) and clicking on [Calculate It].
Or, put in the AbV of the liquid and the total number of units allowed (or required) and see what quantity of that liquid will contain that number of units. In this case it can be very interesting to experiment by changing the type of measure being used.
The significant figures option should not be needed (the default value of 3 should be adequate for all this work) but is there *just in case*.
Whatever the setting, unvalued zeros are not shown.
See also the General Notes on Alcohol Content of Drinks.
It is worth knowing that (in the UK) the AbV value tells you immediately how many units of alcohol there are in a litre of the liquid. It is the same figure. So, provided you have some familiarity with measures you can make good approximations.
For example. Seeing a bottle (75 cL) of wine with an AbV of 12%, we know that a full litre of that wine contains 12 units of alcohol. So, three-quarters of a litre (=75 cL) must hold 9 units of alcohol. Allowing 6 glasses to the bottle, gives 1.5 units per glass.
Similarly, on seeing an AbV of 5.3% for a beer, and knowing that a pint is a bit more than half-a-litre, it easy to estimate that one pint of that beer contains about 3 units of alcohol.
Clearly an AbV of over 100% cannot exist. An attempt to produce such a value will generate a warning message. To see this, try putting in a quantity of 1 cL and 10 units. (That is an AbV of 1000%!)
There is another calculator dealing with Other Countries
and also one for finding the %AbV from other data.