Wizarding currency

Currency in the wizarding world consists of three different coins. In decreasing order of value, they are: Galleon, Sickle and Knut. They are gold, silver, and bronze, respectively. According to Hagrid, there are 17 Sickles in a Galleon, and 29 Knuts in a Sickle. Around the edge of each coin is a series of numerals which represent a serial number belonging to the Goblin that cast the coin.

According to J.K. Rowling, the approximate value of a Galleon is  'About five [Great British] pounds, though the exchange rate varies!' . This is consistent with the "textbooks" Rowling wrote for charity (Fantastic Beasts and Where to Find Them and Quidditch Through the Ages), which states that US$250 million is equivalent to 34 million Galleons, which means that 1 Galleon ≈ US$7.35. US$7.35 was the same as about GB£5 at the time of publishing.

Converted into other currencies
With this information, we can calculate the value of wizarding currency in Muggle money. The amounts below are approximate, and were accurate as of 1 September, 2007.

As of mid-2008 1 GBP is close to 2 USD.

Note that the Galleon/Pound rate cited by Rowling is probably that offered by Gringotts and bears no relation to the precious-metal value of Wizarding coins.The "gold coins the size of hubcaps" mentioned in reference to the Quidditch World Cup would be much larger than the British five-pound Quintuple Sovereign today sold for its bullion value of hundreds of pounds sterling (though this hubcap reference may have been an exaggeration).

Fake Galleons
Galleons made of Leprechaun gold were common at Quidditch games where Leprechauns are the mascots for the Irish team. These galleons are occasionally in temporary circulation (they vanish a few hours after appearing), but goblin experts at Gringotts can differentiate them from real ones.

Hagrid used Leprechaun gold fake Galleons at a Care of Magical Creatures class, while teaching students about Nifflers' ability to hunt for shiny objects like coins.

Hermione Granger fabricated fake Galleons for members of Dumbledore's Army as a means of conveying clandestine communication about the time of future meetings.

Ludo Bagman used Leprechaun gold to pay Fred and George after their bet resulting in them blackmailing Ludo with no luck.

Währung in der Zaubererwelt