When you look things up in different places and find different answers to your questions, it can get very frustrating. Some time ago I set out to settle the question for myself, trying to match up the figures quoted in different dictionaries, Bibles, etc. Since you mention specifically the cor, here’s what I found in the case of liquid measure. The table doesn’t show up properly in the format they use here for these comment threads. The three figures in each line are supposed to appear under the column headings, amphorae, librae, litres.
In the Torah, the amount of flour to be offered as a sacrifice is defined as an issaron (עשרון), meaning a tenth (Numbers 15.4). The term is invariably understood to mean a tenth of an ephah. Earlier, Moses had decreed that the daily ration of manna in the desert was to be an omer (עומר), literally a sheaf, i.e. a unit of capacity corresponding to as much grain as an average sheaf of wheat would yield (Exodus 16.16). A few verses later it is explained that an omer was a tenth of an ephah. Issaron and omer are therefore held to be two names for one and the same unit of capacity, which forms part of a decimal series: 10 issaron or omer = 1 ephah, 10 ephah = 1 cor. (The ephah also has an alternative name, the bath.) This decimal series is held to be of very early origin, possibly Egyptian, in contrast with the duodecimal scales characteristic of Greek and Roman weights and measures.
In the hellenistic period the Judean units of liquid measure became fully integrated with the Greek and Roman systems. All or most of the Judean units had become closely aligned with the corresponding Roman units. The seah, for example, one-third of an ephah, was equal to the Roman urna, one-half of an amphora. The amphora, also known as the quadrantal, was defined in two ways, as a cubic foot and also as the volume of 80 pounds of wine or water.* Assuming a specific gravity of 1, the equation works out at a little over 25.9 litres. The metric equivalent of the amphora is usually stated as approximately 26.2 litres. Whatever the exact present-day equivalent, in antiquity the amphora was held to be two-thirds of the Greek metretes, which was the same as the ephah. Several of these larger units of capacity, in fact, give a neat match with the notional weight of water expressed in Roman pounds:
Liquid measure, selected units Amphorae Librae Litres
1 issaron or omer (J) 0.15 12 3.94
1 seah (J) or urna ® 0.5 40 13.1
1 amphora ® 1 80 26.2
1 ephah (J) or metretes (G) 1.5 120 39.4
1 cor (J) 15 1 200 394
1 culeus ® 20 1 600 525
G: Greek, J: Judean, R: Roman
*This is a further instance of Roman tolerance of what would nowadays be considered unacceptable inaccuracy: “The Romans were aware that there is a difference in the specific gravity of wine and of water, and in the different sorts of each, but, for the sake of simplicity, they regarded them as of the same specific gravity; when, however, they wished a very exact determination, they used rain water” (Smith, DG&RA, p. 979a, footnote).