Coins
The Galleon is the largest and most valuable coin in the British wizard currency. It is gold, round and larger than the other coins are.
Around the rim of the Galleon is inscribed at least one serial number, which identifies the goblin who was responsible for minting the coin. In Harry Potter and the Order of the Phoenix, Hermione enchants fake Galleons to show the time and date of the next Dumbledore's Army meeting instead of the serial number.
As explained in Harry Potter and the Philosopher's Stone there are 17 silver Sickles to a gold Galleon, and 29 bronze Knuts to a Sickle.
According to Pottermore, the wizards never changed their units of measurement (i.e. feet to meters, ounces to grams, galleons to pounds, etc.) because they can do the calculations with magic, so strange number conversions don't bother them.