ou’ll find any number of Roman numeral converters for whole numbers online — but really, just about anyone who made it past the fourth grade can manage that on their own.
Roman fractions, though, are another matter entirely. They were based on twelfths, or multiples or divisions of twelfths. The 1/12 fraction itself was vncia, from which we derive words like “inch” (1/12 of a foot) and “ounce” (1/12 of a troy pound). The word “as” for unity refers to a bronze coin of the Republic and Empire periods which likewise divided into twelfths.
The chart below shows the symbols most often cited1, their Latin terms, how those terms typically would have appeared in manuscript at the time of Hadrian or thereabouts2, and finally their modern equivalents. Below that are both slashed and decimal inputs for you to calculate any fraction down to a siliqva3 (1/1728, or 1/144 of an vncia).
|𐆔||dimidia sextvla||1/144 (≈0.0069)|
|𐆓𐆓||binae sextvlae||1/36 (≈0.0278)|
|S :||bes||2/3 (≈0.67)|
|S ∴||nonvncivm||3/4 (0.75)|
|S ∷||decvnx||5/6 (0.83)|
|S ⁙||devnx||11/12 (≈0.9167)|
|I||as or vnvs||1|
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1. As with Roman numerals, Roman fractional notation varied over time and also according to the whims of those who employed it. Accordingly, you’ll see symbols and terms that differ from what I show.
2. Many thanks to epigrapher Johan Winge for his enterprising alphabet reconstruction based on a cache of personal letters unearthed in 1992 at Vindolanda fort near Hadrian’s Wall. The relative illegibility of Roman penmanship — with its B and D so similar, as well as its P, T, and R — was groused and joked about even at the time.
3. Siliqva means “pod” or “husk"; scripvlvm, "scruple"; sextvla, "small fraction.”