Sestertius, a silver coin in use among the ancient Romans, called also simply nummus, and sometimes nummus sestertius. The sestertius was the fourth part of the denarius, and originally contained two asses and a half. It was at first denoted by LLS, the two Ls signifying two librae, and the S half. But the librarii, afterwards converting the two Ls into an H, expressed the sestertius by HS. The word sestertius was first introduced by way of abbreviation for semisterius, which signifies two and a half of a third, or, literally, only half a third; for in expressing half a third, it was understood that there were two before.

Some authors make two kinds of sesterces, the less, called sestertius, in the masculine gender, and the great one, called sestertium, in the neuter; the former we have already described, the latter containing a thousand of the other. Others maintain that any such distinction of great and little sesterces was unknown to the Romans. Sestertius, say they, was an adjective, and signified, as sestertius, or two asses and a half; and when used in the plural, as in quinquaginta sestertium, or sestertia, it was only by way of abbreviation, and there was also understood centena milia, &c.

This matter has been accurately stated by Mr Raper, in the following manner. The substantive to which sestertius referred is either as or pondus; and sestertius as is two asses and a half; sestertium pondus, two pondera and a half, or two hundred and fifty denarii. When the denarius passed for ten asses, the sestertius of two asses and a half was the quarter of it; and the Romans continued to keep their accounts in these sesterces long after the denarius passed for sixteen asses, till, growing rich, they found it more convenient to reckon by quarters of the denarius, which they called nummi, and used the words nummus and sestertius indifferently, as synonymous terms, and sometimes both together, as sestertius nummus. In this case the word sestertius, having lost its original signification, was used as a substantive; for sestertius nummus was not two nummi and a half, but a single nummus of four asses. They called any sum under two thousand sesterces so many sesterii in the masculine gender; two thousand sesterces they called duo or bina sestertia, in the neuter, so many quarters making five hundred denarii, which was twice the sestertium; and they said dena, vicena, &c. sestertia, till the sum amounted to a thousand sesterces, which was a million of sesterces. But, to avoid ambiguity, they did not use the neuter sestertium in the singular number when the whole sum amounted to no more than a thousand sesterces, or one sestertium. They called a million of sesterces decies nummum, or decies sestertium, for decies centena milia nummorum, or sestertiiorum, in the masculine gender, omitting centena milia for the sake of brevity. They likewise called the same sum decies sestertium, in the neuter gender, for decies centies sestertium, omitting centies for the same reason; or simply decies, omitting centena milia sestertium, or centies sestertium; and with the numeral adverbs decies, vicies, centies, millies, and the like, either centena milia or centies was always understood. These were their most usual forms of expression, though for bina, dena, vicena sestertia, they frequently said bina, dena, vicena milia nummum. If the consular denarius contained sixty Troy grains of fine silver, it was worth something more than eightpence farthing and a half sterling; and the as, of sixteen to the denarius, a little more than a halfpenny. To reduce the ancient sesterces of two asses and a half, when the denarius passed for sixteen, to pounds sterling, multiply the given number by 5454, and cut off six figures on the right hand for decimals. To reduce nummi sestertii, or quarters of the denarius, to pounds sterling; if the given sum be consular money, multiply it by 8727, and cut off six figures on the right hand for decimals; but for imperial money diminish the said product by one eighth of itself. To be qualified for a Roman knight, an estate of 400,000 sesterces was required; and for a senator, one of 800,000.