Why Does ‘Oz.’ Stand for ‘Ounce’?

Some measurement abbreviations are straightforward — for example, “foot” is abbreviated as “ft.,” and “inch” is shortened as “in.” But others in the English language don’t seem to make sense. In particular, there is no “z” in “ounce,” so why is the abbreviation “oz.”? We need to go back to ancient Rome to find out the answer.

Ancient Romans used a base-12 numbering system, as opposed to our base-10 system. “Ounce” is from the Latin uncia, meaning “12th part, ounce,” because there were 12 units to a pound. In Old French, uncia evolved to unce, and in Medieval Italian, it became onze. It’s from the Italian influence that we get the abbreviation “oz.” 

Interestingly, when “onze” entered the English lexicon in the 14th century, it could refer to any small quantity. In Middle English, it was also used specifically as a measure of time (7.5 seconds) and length (about 3 inches).

If you know your measurements, you might wonder why we now have 16 ounces in a pound now and not 12 as in Roman times. In the Late Middle Ages, merchants in the international wool trade preferred dividing the pound into 16 ounces because it was more easily divisible by eight, four, and two. That preference stuck, and now we have 16 ounces in a pound.

Speaking of pounds, why do we use the abbreviation “lb.” when neither of those letters appears in the word? The answer again lies in ancient Rome, where the Latin libra pondo meant “a pound by weight,” based on the Latin pondus, meaning “weight.” Libra, meaning “balance,” referred to the scales used to weigh things. So while the English word “pound” derives from pondo, the abbreviation for it came from libra.