Proper way to measure ancient coins?

[CPK]

Just curious, since ancients often tend to come in irregular shapes, what is the "proper" way to measure the diameter? Longest, shortest, or just try to average?

Thanks!

[Phil Davis]

If the shape is irregular enough that the differences matter, you'll sometimes see both given; i.e. 16mm x 10mm or the like. In less extreme cases, I tend to measure the longest dimension. I honestly don't know though if that's any kind of "norm" or simply personal preference.

[Spaniard]

Yes I agree with Phil....If the coin has a slight irregularity in diameter I just write "maximum diameter" xxxx which allows the viewer to roughly assess.

[Sulla80]

 

I agree with Phil - I don't think there is a strong norm.  I recently bought a coin that said clearly "16mm (max)" - I like the clarity of this, but don't usually see it.  I have usually put both max and min (e.g. 32mmx39mm) when the coin is very oval.  Most coins I try to record the avg diameter.  Here's one where I felt I needed to share both dimensions.

Caria, Alabanda, Circa 169-161 BC, AR Tetradrachm (32-39mm, 16.2g, 12h), in the name and types of Alexander III of Macedon, Dated CY 1 (168/167 BC)
Obv: Head of Herakles right, wearing lion skin
Rev: Zeus Aëtophoros seated left; in left field, Pegasos springing left; A (date) below throne.
Ref: Price 2460

[CPK]

Thanks for the replies!

I've been kind of averaging the measurements, but I like the idea of just measuring the maximum and labelling it as such.

 

[Severus Alexander]

43 minutes ago, Phil Davis said:

If the shape is irregular enough that the differences matter, you'll sometimes see both given; i.e. 16mm x 10mm or the like. In less extreme cases, I tend to measure the longest dimension. I honestly don't know though if that's any kind of "norm" or simply personal preference.

FWIW this is exactly what I do too.

[Heliodromus]

I always thought the convention is to measure width at widest point, as well as at narrowest point if coin is too far off round.

However, for late roman bronze at least, the only useful measurement is the diameter (from center to center) of the beaded/pearl border, sometimes called PRD (Pearl Ring Diameter).

The reason the PRD is useful is because this was controlled on the die and reflects the targetted coin size for that issue. It's quite common for two bronze issues to vary only in PRD (maybe only by 1mm) and average weight (# coins per pound weight), and given the large variabiity of individual coin sizes and weights, the PRD, being the only controlled parameter, is the only reliable way to tell such issues apart.

 

Edited June 17, 2022 by Heliodromus

[Roman Collector]

I always measure from 9:00 to 3:00 on the obverse with a caliper.

[zumbly]

I usually just go with the widest too, but as some have pointed out above, for some coins it's better to go with two measurements...

 

PHOENICIA, Arados
AR Shekel. 10.48g, 23.8mm x 15.4mm min.
Circa 420-400 BC. E&E-A Group III.1.1, C13; HGC 10, 29.
O: Laureate head of Ba’al-Arwad right.
R: Galley right, Pataikos on prow, above waves; M A (in Aramaic) above.