For the left: Press space, enter a price, and press enter. It will tell you how many stacks on top, and the remainder on the bottom. The limit is 640 million galleons. Over the limit it displays stacks in decimals, and does not calculate the extra. For the right: Press the up arrow, enter a number of stacks, enter the amount of extra, then the total will be displayed. There is no limit for this one.
This was created for easy calculation of prices for the Minecraft server hp.knockturnmc.com, a Harry Potter themed server. I wanted an easy way to see how many galleons exactly are needed to pay for expensive things. Although it was originally for Galleons, it can be used for anything that has 64 items per stack. A background will be added for aesthetic purposes if this becomes popular.