Roman coin denominations: a UK detectorist's quick reference

Roman coinage in Britain ran from the conquest in AD 43 to the withdrawal in AD 410. Four denominations account for almost every Roman coin a UK detectorist will dig in their lifetime: the denarius, the sestertius, the antoninianus, and the late-Roman nummus. Here’s how to tell them apart.

The four denominations in one table

The denarius

The denarius was the standard silver coin of the Roman Republic and early Empire. UK detector finds tend to cluster in the 1st and 2nd centuries AD — conquest-era issues of Claudius, then the Flavians, Trajan, Hadrian, Antoninus Pius, Marcus Aurelius. A well-struck denarius shows a laureate emperor right on the obverse and one of the classical reverse types (a personification of a virtue, the emperor on horseback, a temple, a captive figure).

• Module 17–20 mm, weight 3–3.5 g.
• Silver. Even when toned dark grey, the silver tone is recognisable.
• Laureate, not radiate. A radiate crown means you’re looking at an antoninianus, not a denarius.
• Legend reads IMP CAES [emperor name] AVG on the obverse and a virtue or commemoration on the reverse.

The sestertius

The big brass workhorse of early imperial coinage. UK detector finds of sestertii are usually 1st–2nd century. The diagnostic is unmistakeable: a large 25–34 mm brass / orichalcum disc with S Con the reverse — Senatus Consulto, “by decree of the Senate”. That mark only ever appears on senatorial bronze (sestertius, dupondius, as), never on imperial silver or gold.

The antoninianus

Caracalla introduced the antoninianus in AD 215 as a double denarius, worth 2 denarii but containing only about 1.5 denarii of silver. The single most reliable diagnostic is the radiate (spiked) crownon the emperor — laurel for the denarius, radiate for the antoninianus. The denomination became the standard silver of the 3rd century and was progressively debased until the late Gallic Empire issues are essentially bronze with a thin silver wash.

The 3rd-century crisis produced enormous quantities of “barbarous radiates” — local British and Continental imitations of antoniniani struck during the breakaway Gallic Empire (Postumus, Victorinus, Tetricus I and II). These are extremely common UK finds, often crude in style, usually 14–19 mm, and almost always with traces of the radiate crown surviving.

• Radiate crown— confirms antoninianus.
• Module 20–24 mmfor well-struck issues; as small as 12–16 mm for crude barbarous radiates.
• Silver or debased silver on early issues; copper alloy with silver wash on late and Gallic Empire issues.
• Reece period 13 (260–275) is the typical British find context.

The nummus (late Roman bronze)

Diocletian’s reform of 296 introduced a new fractional bronze called the nummus or follis. Module starts at ~25 mm and shrinks steadily through the 4th century to ~13–15 mm by the Valentinianic period. These are the small bronzes that flood Romano-British sites — particularly Reece Period 17 (Constantinian, 330–348) and Reece Period 19 (Valentinianic, 364–378).

Common nummus reverse types

Reading the reverse type and matching it to the table almost always gives you the decade. Combine that with the bust style (diademed head, laureate, military) and the emperor name in the obverse legend (CONSTANTINVS, CONSTANTIVS, VALENS, etc.) and the attribution falls out.

Reading a mintmark

From the late 3rd century onwards, late Roman bronzes carry a mintmark in the exergue (the space below the reverse design). UK finds skew heavily to the western mints: Trier, Lyon, London, Arles. Eastern mints are scarce.

The Reece periods in one breath

Richard Reece’s chronological framework groups Roman coins into 21 (now 23) periods covering specific emperors and coin-type profiles. The two periods that dominate UK detector finds:

• Reece Period 13 (260–275)— the barbarous radiates of the Gallic Empire. Tiny, crude, radiate crown. Often illegible legends.
• Reece Period 17 (330–348)— Constantinian small bronzes. VRBS ROMA, CONSTANTINOPOLIS, GLORIA EXERCITVS. The single most common identifiable Roman coin from UK detecting.

Procedural identification — Roman coin

1. Module + weight→ ballpark denomination per the table above.
2. Crown type on the bust. Laureate = denarius or early imperial bronze. Radiate (spiky) = antoninianus or barbarous radiate. Diademed (pearl band) = late Roman 4th century.
3. Reverse type. Match to the table for late bronzes. For sestertii, look for S C in the field.
4. Legend. The emperor’s name in the obverse legend, even partially visible, narrows the date.
5. Mintmark on the late bronzes, in the exergue under the reverse design.