Last update: 7 June 2023
Numista produces an index to show the approximate rarity of coin, banknote, and exonumia types. This index, sometimes abbreviated as NRI, theoretically ranges from 0 to 100, with 0 representing extremely common items and 100 representing the rarest items. It is shown at the bottom of every Numista coin, banknote or exonumia page.
Rarity is a subjective matter. It is related to the mintage quantity, the number of known remaining examples (when coins were melted for example), the item price, and how hard it is to obtain. It is good to mention that these characteristics are not related to each other. Some items are expensive and still easy to obtain. Some items from obscure issuers can be relatively cheap, but hard to obtain because they are rarely sold.
This is further influenced by where the item comes from and where people who want them are based, as it is far easier to get local coinage than coinage from very far or from countries with a tough political situation. On top of that, rarity is also based on how desired the item is and how many are available on the market.
The Numista Rarity Index is computed from the number of members that have it in their collection and the number of members who offer it for swap. Note that collections from Numista users are not verified so they can't be fully trusted. Multiple entries by the same user are not taken into account to build the index.
The Numista Rarity Index provides a good indication of how easy it would be to obtain an item from another Numista member. So an item with an index lower than 10 is fairly easy to get, while an item with an index greater than 30 will be harder to get in a swap, as not many people have it in the first place.
To be short, the Numista Rarity Index is an indicator of how frequently a coin, a banknote or a piece of exonumia is found within the collection of Numista members. Nothing else.
From this we can draw a few conclusions: