If you enjoy cannabis, there is one reason you should be happy the United States never fully adopted the metric system. In many states, you can probably legally possess more marijuana because America kept using customary units.
In Colorado, Washington state, Alaska, and Oregon the amount of marijuana an adult can legally possess under state law is up to one ounce (equal to 28.35 grams). Even in states where marijuana isn’t yet legal, an ounce is normally an important legal threshold. For example, in California, Vermont, Rhode Island, Massachusetts, Nebraska, Nevada, and the Virgin Islands, getting caught with an ounce will result in only a fine but any larger quantity could result in jail time. Across much of the United States one ounce of marijuana is legally seen as the dividing line between a mere personal supply and potentially more criminal conduct.
There is, though, no practical reason why one ounce should be treated as the dividing line. One ounce isn’t some tangible metric for marijuana use or production. For example, it isn’t the amount the average smoker consumes in a week or the average yield from a single marijuana plant. This amount was mainly chosen for political reasons. The ounce just happens to be the smallest unit of weight most Americans are very familiar with, since it is how we measure cuts of meat or beverages. Voters and politicians like to make policy based off round numbers and things they easily can relate to — so policymakers rather arbitrarily chose an ounce as a nice, relatable small amount.
On the other hand, in countries that use the metric system the smallest unit of weight most people are familiar with is a gram, which is much smaller than an ounce. In these countries marijuana rules are often based on some round number of grams for similar political reasons. For example, in the Netherlands marijuana possession of up to five grams is officially tolerated. Chile, Belize, Ecuador and Paraguay all decriminalized personal possession, which they define as up to 10 grams. The Czech Republic and Norway use 15 grams as their threshold for personal use. Note they are all less than an ounce (28.35 grams).
If in some alternative universe the United States had fully adopted the metric system decades ago, it is reasonable to suspect the recent state legalization campaigns might have instead only tried to legalize possession of up to 10 grams for adults, since it is a nice round metric number. Americans’ resistance to adopting the metric system meant these campaigns set their limit at one ounce — and stateside, we can enjoy more weed as a result.