28.3495231. The number of grams in an ounce. It's not exactly a pretty number. English measurement and metric measurement just don't go together, you always have to end up doing annoying equations (or usually in the case of drugs, guessing) and the number always has to be rounded for the the two to coexist.
In the case of marijuana, 1 gram is typically the smallest unit, followed by 1/8 of an ounce (8ths). There are 3.54369039 grams in an 8th. Why does there have to be the jump from metric to english? Wouldn't a 16th (1.77184519 grams) work out much better? Wouldn't it be a lot less confusing? Then of course you have yet another measurement, money. Dime bags and nickel bags aren't even a standardised weight, they're just a pricetag. It all works out so that unless you have a calculator with you, it's hard to tell if you just got ripped off or if you just got a deal (that is asssuming you got the actual weight that you paid for, which is unlikely).
Cocaine is a little bit more straightforward, with most smaller purchases made in grams. For larger quantities you can buy an "eight ball" or 1/8 of an ounce, once again making a shift from metric to english. Getting a conversion correct is perhaps more important however as fractions of a gram are more precious and larger amounts of money are at stake.
Maybe that was all planned out, now that I think about it as long as you're selling being able to confuse your customers is genius.
This write-up focuses on marijuana/cocaine, if you know anything about measurements of other drugs let me know so I can make it more complete