Roman coins came in many different denominations. The weight and metal type of each coin determined how much purchasing power each coin had. Eventually, coin designs would to a certain extent explicitly state the value of the coin but it is uncertain whether these official values were honored by merchants and the public in general.

The absolute base unit in Roman coinage is the Uncia, a small copper coin the size of a small button which was never struck in large quantities and is today very rare. 16 Unciae are equivalent to an As which is the first commercially functional coin. In turn, 16 Asses make up the famous Denarius, a silver, U.S.-dime sized coin which circulated for hundreds of years and influenced coinage in just about every successive Western culture up into modern times.

Although a fascinating subject, it 's disappointing to learn that we lack a good understanding of what the actual value of Roman coins were. Necessary food staples were often heavily subsidized by the government to ensure their accessibility. The emperor and his officials understood that a hungry citizenry was a grave liability. What records survive, therefore, tend to point out these set prices but the going rate for other luxuries is largely speculative.

For what it 's worth, a rough sketch of salaries would have a gold Aureus or Solidus as a month 's pay for an ordinary soldier. And a Denarius or two could be earned in a day by a skilled laborer. In turn, a family meal consisting of bread, olive oil, wine and perhaps some meat would cost a Denarius or one of its equivalents.

For early imperial coinage the relative values are as follows: 1 Aureus = 2 Quinarii = 25 Denarii = 100 Sestertii = 200 Dupondii = 400 Asses = 800 Quadrans = 6,400 Unciae

Considering the expense in labor and materials that was borne in the production of the small-change Quadrans and Uncia it 's not hard to see why these denominations existed more as a theoretical currency keystone than as real coins.

The first crack in the Roman economical machine appeared under the reign of Nero who cut back the purity of the Denarius from 98% fineness (essentially as pure as could possibly have been refined on a large scale basis back then) to 93%. The debasement did not link up with an official decrease in the nominal value of the coin itself so that the extra 5% silver was clear and free profit for the emperor.

However, it took virtually no time for the public at large to see that the old Denarius was intrinsically worth more than the new one. This created an immediate hoarding of the old silver coins which could now be melted and then sold as scrap. In fact, finding today a pre-reform Denarius is considerably more difficult and expensive than Nero 's new Denarii.

From then on each new emperor lowered the fineness of the Denarii a percent or two so that by the time of Gordian III, the last emperor to issue significant quantities of Denarii, a Denarius was actually no more than about 35% silver by weight.

Another unintended effect was that as the silver coins became cheaper the copper ones became more expensive. After all, each Denarius was now being made by more and more copper to fill in for the missing silver. What was happening was that a 3 gram silver coin was 2 grams copper but the whole coin was still valued at several multiples more than the Sestertii, Dupondii and Asses which weighed between 10-30 grams a piece. Therefore, rather than the government risk striking copper coins which would only wind up being melted it chose to not strike them much in the first place.

Gold on the other hand was considered sacred. As much as it may have pained each emperor to part with his dwindling supplies of its most precious metal no soldier would risk his life unless it was for real gold. Not until the situation had grown into a series of deep crises in the middle of the third century that emperors decided to tinker with the next best thing: their weight. The Aureus which had traditionally weighed between 7-8 grams each went as far down as just over 2 grams under the reign of Gallienus. How the paymasters kept a straight face on pay day is anyone 's guess and it 's quite possible that the scam was masqueraded as salary increases by paying two or three of these Aurei while, of course, the total outlay of metal was still below the traditional amount.
As the fineness in silver was steadily lowered, and the weight of the Aureus became erratic, new denominations were introduced to further blur the government 's cost-cutting schemes and attempts to curb rampant inflation. The silver Antoninianus was introduced around the year 215 under Caracalla at a nominal value of two Denarii and, for gold, the Binio was introduced a few years later as a double Aureus. Since gold coins were never a major part of everyday commerce the Binio was a nonstarter but the Antoninianus drove the Denarius into extinction within 30 years of its introduction. And it, too, would suffer severe debasement and reduction in weight.

By the mid-250 's the Ant reached the critical low point in the silver-copper alloy, about 18%, where it no longer resembled a silvery coin even when freshly minted. Debasing this coin further served no practical purpose because it was blatantly obvious it was no longer silver. A decision was therefore made to stop making silver coins altogether and simply apply a silver wash to the Ants as a last processing step of the coin blanks.

When new, these coins looked much better than the previous 18% silver Ants. However, shortly after entering circulation the silver coating wore off across the high points of the coin to reveal the copper beneath. Many such coins continued to circulate long after the silvering was fully gone and yet they were still officially considered silver coins!

By the early 290 's the Roman economy was in a state of near-collapse as the old currency value schedules were maintained relative to a silver coinage that existed only as a dim memory. The emperor Diocletian set into motion a series of monetary reforms meant to rectify the situation. The Antoninianus was suspended and new denominations introduced including a new Denarius of high silver content termed "Argenteus" (but officially worth 2.5 Denarii each) and the Follis which had a negligible amount of silver but was as hefty as an old As. The Aureus would be reborn under more predictable weights as well and the whole coinage system was overhauled from top to bottom in the hopes of stabilizing the economy.

Some of the denominations caught on and some, specifically the Argenteus, would see a quick demise due to the chronic lack of silver. What little silver was initially breathed into the Follis was pulled and the weight, too, dwindled swiftly from a high of about 10g until it was a small copper coin of about 2-3g each within a few years ' time. This reduced Follis enters the fourth century as the new de facto standard copper coin to serve the same general purpose as the Denarius of two centuries before (if not the actual buying power). Since it is unclear what the Romans of the time called it today 's numismatists give it the generic term of a class three bronze or AE3 for short (AE is the abbreviation for Aeratus, Latin for copper).

Even though during the fourth century the AE3 is king there are several other important denominations. After Diocletian 's reforms settle into a new swing over the following years, a new gold standard is introduced under Constantine I with the flagship Solidus, a successor to the old Aureus which is now made to unerring precision at 72 to a Roman pound of gold, or about 4.4g a piece. It is so successful that it was still being made 500 years later under Byzantine emperors easily outlasting the Denarius itself and, possibly, any other denomination to this age.

While the relationship between bronze coins and their silver and gold cousins are poorly understood the relative values between silver and gold are as follows:
1 Solidus = 2 Semisses = 3 Tremisses = 24 Siliquae

The Siliqua is the last major successor to the old Denarius. It is thinner and lighter at only 2-3g each and never approaches the popularity of the Denarius. Except for rare occasions it is the one denomination that is not survived by the fall of the Roman empire itself in 476.

While the gold and silver remain stable into the fifth century and beyond the last days of the Western half of the Roman empire see the bronze coinage shrink quickly into a morass of teeny coppers known as AE4 's. They survive in large quantities today but prove difficult to identify due to careless minting methods and heavily debased alloys which fared poorly in the soil upon their loss or burial.

The following table lists the most important denominations with rare fractions and multiples being omitted.