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 L's signifying two librae, and the S half. But the librarii, afterwards converting the two L's into an H, expressed the sestertius by HS. The word sestertius was first introduced by way of abbreviation for semisestertius, 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 sestertii; the less called sestertius, in the masculine gender; and the greater, called sestertium, in the neuter: the first, that we have already described; the latter containing a thousand of the other. Others will have any such distinction of great and little sestertii unknown to the Romans: sestertius, say they, was an adjective, and signified as sestertius. 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 a quarter of it; and the Romans continued to keep their accounts in these sesterces long after the denarius passed for fifteen 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 which 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 sesterces in the masculine gender; two thousand sesterces they called duo or bina sesterces, in the neuter; so many quarters making five hundred denarii, which was twice the sestertius; and they said dena, vicena, &c., sesterces, 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 sestertius. They called a million of sesterces decies nummum, or decies sestertium, for decies centena millia nummorum, or sestertiiorum (in the masculine gender), omitting centena millia for the sake of brevity. They likewise called the same sum decies sestertium (in the neuter gender) for decies centes sestertium, omitting centes for the same reason; or simply decies, omitting centena millia sestertium, or centes sestertium; and with the numeral adverbs decies, vices, centes, milles, and the like, either centena millia or centes was always understood. These were their most usual forms of expression; though for bina, dena, vicena sesterces, they frequently said bina, dena, vicena millia nummum.

If the consular denarius contained 62 troy grains of fine silver, it was worth somewhat more than eight-pence farthing and a half sterling; and the as, of 16 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 16, to pounds sterling, multiply the given number by 54.54, and cut off six figures on the right hand for decimals. To reduce nummi sesterces, or quarters of the denarius, to pounds sterling; if the given sum be consular money, multiply it by 87.27, and cut off six figures on the right hand for decimals; but for imperial money diminish the said product by one eighth of itself. Phil. Trans. vol. lxi. part ii. art. 48.

To be qualified for a Roman knight, an estate of 400,000 sesterces was required; and for a senator, of 800,000.

Authors also mention a copper sesterc, worth about one third of a penny English.

Sesterc, or sestertius, was also used by the ancients for a thing containing two wholes and an half of another, as as was taken for any whole or integer.