You use the steve to measure how much it costs to buy a certain quantity of alcohol. It's very useful when you're at the off-license and you're wondering, "What's the most economically efficient way of getting drunk?" (I should probably say here that neither I, nor the institution that this website is hosted on, would ever advocate getting drunk. Drink within your limits, drink safe, drink sensibly etc etc).
Example: which is better value for money (in terms of the amount of alcohol for the amount of money you're spending): a bottle of wine for £3.99, or a can of "premium" strength lager at £1.20, or a 70cl bottle of vodka for £8.49? Well, using the steve, you can work it out.
Because one of the quantities in the formula for working out the steve is currency, you need a baseline. The baseline for the steve is a can (500ml) of Stella from my local off-license ("Collin's"), which for a number of years has cost £1.00 (one pound sterling). This is one (1) steve.
The actual formula is as follows:
Note that the steve takes no account whatsoever of quality. You may feel, as I occasionally do, that you may be willing to consider a lower steve rating for a nicer drink.
My ultimate dream would be if off-licenses and supermarkets throughout the land started labelling the alcoholic drinks with steve factors, so you could make an informed choice about your purchases. I'll add dropdown menus etc here at a later date so you can calculate how many steves some popular drinks work out at, for the price you can purchase them for. And perhaps a few more examples. And conversion factors for different currencies.