This topic seems to come up a lot so here goes.

In the US a unit of alcohol (standard drink) is:
12 oz. of beer (5% abv)
5 oz of wine (12% abv)
3-4 oz of fortified wine (17% abv) such as Port, Sherry etc.
1.5 oz of 80 proof spirits (Scotch, Whiskey etc.)
a 750ml bottle of wine contains about 5 units
a 750ml bottle of Bourbon contains about 17 units (750 ml=25.5oz, 25.5oz/1.5oz=17)

1 standard drink in the US is the same as 1.77 standard drinks in the UK. So a 50 unit a week drinker in the US is equivalent to a 88.5 unit drinker in the UK. In Australia and New Zealand the factor is 1.4 so 70 Aussie or Kiwi units equals 50 US units. Here is the calculator so you can figure out the equivalent values of unit drinks between countries

http://www.cleavebooks.co.uk/scol/ccalcoh1.htm

Damn, that means I drink a hell of a lot more than our alchy brethren across the pond.