Blood Alcohol Calculators

Here's one from the State of Iowa.  As of the original posting of this link (7/1/2003 when the .08 law went into effect), it is only somewhat reliable because it could be construed to mean that if you are male and weigh 170 pounds and stayed uptown all evening and had no more than 3 beers an hour you'd be OK.

http://www.iowaabd.com/alcohol/bac_chart/pr_bac_chart.asp

NOT!

Here's a better one that takes into account that you might get to talking with someone and stay uptown more than an hour:

http://www.intox.com/wheel/drinkwheel.asp

The same scenario; 170# male, 3 beers an hour over 3 hours would put you at .138 according to their estimates.  You're shitfaced!

Many factors affect a person's sensitivity to alcohol.  Know your limits and err on the side of caution as none of these guides can be 100% accurate.

Here's a Google Search for more