Roman Imperial Coin Denominations
This page is an overview of the common denominations of silver and bronze Roman Imperial coins available to collectors. It is posted in response to a request from a visitor of this site and could be useful to the new collector or student. Unfortunately, denomination systems changed several times and numismatic knowledge of the details of each of these changes is not perfect. The listing is not complete but should serve the needs of its intended audience. Each of the photos has been sized roughly in proportions to the others. To assist further in showing size, each photo contains a 2 cm bar and, underneath a coin, the same U.S. Lincoln cent. The size images you see will depend on your monitor and software settings.
The foremost and most popular denomination of Roman coins is the silver denarius (represented here, left, by a coin of Nerva 2.8g, 96-98 AD). The denarius was first issued in the late third century BC as a 'modern' alternative to the unwieldy Aes Grave bronze coinage. First valued at 10 asses, the denarius was soon retariffed at 16 asses. Under the Republic, denarii contained nearly 4g of high grade silver. By the early Empire this had slipped a bit but most coins are found (except for the fourrees) to weigh around 3.5 g . Beginning with the reign of Nero (54-68 AD) and continuing for the rest of the life of the denomination, denarii were issued in metal alloyed with copper. This debasement became progressively worse until the coins of the early third century were struck in approximately 50% silver. Even at this level, the appearance of a good silver coin is maintained. Weights also fluctuated through this period with the average falling to under 3g. The actual amount of fine silver contained in any coin varied from issue to issue within reigns. Some coins are found heavier with more debased metal; others lighter in better silver. Runaway inflation in the middle of the third century brought an end to massive issues of denarii. The denomination was replaced as coin of choice by a new double denarius, named by collectors the antoninianus. Occasionally, the half denarius or quinarius was also issued. For most reigns they are rare and almost never seen by beginning collectors. Our example here (right) is of Hadrian (1.4g, 118-138 AD).
Left to Right: Caracalla antoninianus 5.6g 215 AD; Gordian III antoninianus 3.3g 243 AD; Gallienus antoninianus 2.7g 250's AD; Salonina (on U.S. cent to show size) 2.5g c.266 AD; Aurelian antoninianus 3.4g 270-275 AD; Diocletian antoninianus 3.3g before 295 AD; Diocletian post reform radiate 3.8g 295-298 AD
The antoninianus ('coin of Antoninus') was first issued under Caracalla (properly: Marcus Aurelius Antoninus) in 214 AD. The name was first given to the denomination after it was no longer current; we have no idea what the coin was called at the time of issue. While a good looking large coin of the then current 40% silver alloy, most fell short of weighing as much as two denarii so the immediate profit to the state was obvious. The Caracalla example (far left) is heavy at 5.6g; most are lighter. The first three decades of the issue saw weights vary quite a bit with some coins even overstruck on earlier denarii. Lighter coins (like our Gordian III, second from left) were usually made thinner so the large diameter appearance was maintained. To distinguish the double denomination from the denarii, antoninianus portraits show the emperor (and male Caesars!) wearing a radiate crown. Female portraits rest on a crescent (note the Salonina example above center). Inflation forced the debasement to a point that the appearance of silver was lost after the first issues (third from left above) of Gallienus (253-268 AD). From this point on, silver appearance was restored by a thin silver wash applied to the copper looking flans. Diameter fell to the point that out example of Salonina (wife of Gallienus) barely covers the Lincoln cent on which it rests in our photo.
A reform under Aurelian (270-275 AD; third from right above) restored the diameter of the denomination and stabilized the alloy at 1 part silver and 20 parts copper. Coins of this alloy still required the silver wash for sake of appearance and often (not always!) bore a mark 'XXI' testifying to the silver content. Some students use the term 'Aureliani' for these issues but most continue with the old name. The denomination came to an end with the reform (c.295 AD) of Diocletian (284-305 AD). Early coins of Diocletian (second from right above) bear the XXI mark and were issued on the old standard of Aurelian. After his reform, radiate coins were issued with no silver content and without silver wash. These 'post reform radiates' (far right above) are often confused by collectors and incorrectly called 'antoniniani'. None of them bear the XXI mark. This distinction is made difficult by the fact that many of the silvered antoniniani in collections have lost all traces of the silver wash.
Left to Right: Nero sestertius 22.6g c.65 AD; Domitian dupondius 11.9g 85AD; Nero as 11.1g c.65 AD; Domitian semis 4.6g 85 AD; Trajan quadrans 3.5g 98-117 AD
The silver coinage of the early Empire was supplemented by a series of bronze denominations. Foremost was the sestertius valued at 1/4 denarius. In the Republic, the sestertius was occasionally issued as a tiny silver coin but by the early empire it was an impressive yellow brass ('orichalcum') coin. Diameter and weight fluctuated over the following two centuries until the last ones were less than half the size of the originals. Half a sestertius was the dupondius. Smaller in diameter and weight, the dupondius is distinguished by the radiate crown on the emperor. There was no such distinction used for coins issued for women and Caesars; only the Augustus wore the crown on dupondii. Half a dupondius (1/4 sestertius, 1/16 denarius) was the as. Unlike the two larger coins, the as was struck in red copper to distinguish it from the yellow brass used for the other denominations. When new, this color served well to separate the as from the only slightly larger dupondius. It can be difficult to tell if a patinated coin of an empress was a dupondius or an as. As time progressed, this was made worse with the brass alloy and pure copper replaced by a more generic bronze used for everything. The two smaller denominations were used mostly in the first century AD. The semis was 1/2 as and the quadrans 1/4 as. Not all denominations were produced every year. Some periods saw huge productions of sestertii and rare asses; other years reversed the situation. The coins were probably produced to balance the need for circulating coinage. These coin did circulate. Many are found well worn from decades of use. People hoarded silver coins but spent the bronzes. Extremely high grade sestertii are the most beautiful and popular of Roman coins.
The denomination problem only gets worse as the end of the Empire approached. Circa 350 AD Constantius II and his co-rulers released a new series of coins. There appear to be three denominations with the largest struck in slightly finer metal than the others. Comparing actual silver content and weight, it seems reasonable that the three are half/double each other's value. The largest (the popular fallen horseman type) showed the obverse bust facing right. The middle showed the ruler facing left and holding a globe. The smallest returned to the bust right. Shortly afterward the smaller two were discontinued and the fallen horseman series continued down the familiar path of progressive decline in size. Surviving texts mention the denomination 'centenionalis' which must be one of these. Which is not absolutely certain. Finding more texts from that period that mention the matter would be very fortunate. Many listings (certainly incorrectly) use 'centenionalis' for either of the larger two and 'half centenionalis' for both the smallest of the early series and the reduced 'fallen horseman' coins that followed. Our examples show (Left to Right): Constantius II 4.1g (light for these); Constans 4.7g; Constans 2.3g. Individual specimens show considerable variation from published norms. Average weights declined over several coin issues which documented that fact with a series of letters in the filelds on one or both sides of the coin. Study is needed here. Meanwhile, what can collectors call their coins?
An answer is provided by a scale of coin sizes used by many people to describe late Roman bronze (including silvered bronze) coins when the correct denomination name is not certain. This can be used for any coin after the reform of Diocletian (follis, centenionalis or whatever). The system uses the abbreviation for bronze followed by a number 1 through 4: AE1 = over 25mm (Valentinian I); AE2 = 21-25mm (Honorius); AE3 = 17-21mm (Arcadius); AE4 = under 17mm (Theodosius I). Of course, there are still coin issues that straddle the lines with various specimens being, for example, slightly over or under 17mm. In this case you see a split listing 'AE3/4'. Mint workers placed little importance in exact diameter or roundness; if anything was important it was weight. All four sizes were rarely produced together and the system fails to separate the two sizes of AE2 'centenionales'. However imperfect, this system will have a place in describing late Roman coins until all denomination systems are understood. Care should be taken to separate these listings from the millimeter scale used for Greek coins where AE20 is a coin of 20mm diameter. There should be no confusion here since the smallest AE coins are over 4mm diameter. 
This discussion has omitted gold issues, coins issued only in the Republic and some less frequently encountered Imperial denominations. Of these, the only one likely to be encountered by a collector of average means is the silver siliqua of the 4th and 5th centuries AD. These coins are much thinner than denarii or than the bronze coins of the same period. Our example of Arcadius (383-408 AD) weighs only 1.3g. It was common practice of the later period to trim away bits of silver from the edge of silver coins reducing the weight. While this could be considered a form of fraud, it served to balance the value of older coins with newer, lighter issues. Other, larger late Roman silver issues (argenteus, miliarense) exist but are not common. There are particularly deceptive fakes of these larger coins. Buyer beware.
