Coins are Terraria's main unit of currency. They are primarily used for NPC transactions, though other players may also accept them as payment in multiplayer. Coins can also be used as ammunition for the Coin Gun. There are currently 4 types of coins: Copper Coins, Silver Coins, Gold Coins, and Platinum Coins.
Coins can be obtained by killing enemies, interacting with NPCs, or finding them in multiple forms of loot. Despite coins appearing as the materials "Copper", "Silver", "Gold", or "Platinum", they are unrelated to their respective ore types and cannot be crafted from them.
The coin value an enemy drops upon death is calculated from fixed values in the game code, with some random variation.
Coins can be "crafted" by hand, which combines or splits them into other denominations. Each coin is worth 100 of its lesser denomination (Copper -> Silver -> Gold -> Platinum).
When picked up, Coins are automatically combined into their higher denomination, if possible. For example, if the player picks 100 Silver Coins, they'll end up with one Gold Coin in their inventory instead. Similarly, a player holding 96 silver coins picking up another 15 will give them a gold coin and 11 silver coins.
However, Coins do not auto-combine when manually transferred into a Piggy Bank or other storage item; if there is no room in the bank or chest, some will be left in the player's hand or coin slots. stacks of 100 Coins should then be manually crafted into the next denomination. (See also the warning below regarding Deposit All.)
Upon a player's death in Softcore Mode (the default mode), half of all of the stacks of coins in a player's inventory are dropped, rounded up to the nearest whole coin. For example, if the player possesses 1 Gold Coin in one stack, it will be dropped. If it is instead 10 coins, 5 will be dropped.
Upon player death in Mediumcore or Hardcore Mode, all coins are dropped, regardless of how the player died.
Being killed by a player-wielded weapon will not cause the player to drop any coins in Softcore Mode, even if recently damaged by an enemy.
Dying from the player's own projectile after having it reflected back by an enemy (such as a Selenian) will also result in coins being dropped.
In Expert mode, enemies can pick up coins dropped by enemies or players. Coins are often lost permanently on death, as enemies who pick up dead players' coins will often despawn once off-screen. Enemies holding dropped coins emit a sparkle effect based on the color of the best coin they're holding.
In Expert Mode, Softcore players drop 75% of their coins upon death instead of 50%.
Inside the player's inventory, coins have their own special slots, although it is possible to move them into normal inventory slots.
In the early game, before receiving the piggy bank, it may be inevitable to store your coins in a chest. However, quick stack does not place coins in chests. Deposit All does place coins, but if the chest is already full when you press Deposit All, higher denominations may be destroyed if a stack for that coin doesn't exist.
Coins can be sold on the Wii U and Xbox editions. Doing so will do nothing, however.
Coins have a sell price equal to the amount in their stack, but, unsurprisingly, cannot be sold (on the Mobile version, Platinum Coins will sometimes show a higher sell price, but this is most likely due to bugs).