Tan Leather is a Lunar spell which tans up to 5 hides in the player's inventory without the usual cost associated with tanning with an NPC. However, in order to cast the spell, the player must have completed all the easy, medium, and hard tasks of the Fremennik Diary.
With good timing the spell tans approximately 8000 hides per hour, awarding the player with ~130K Magic experience. It is also a good method for money making.
As cowhide can be tanned into either soft leather or hard leather, it is possible to toggle between the two by right-clicking the spell icon.
The formula below assumes you will be tanning 5 hides per spell, as well as buying the dragonhides, nature rune and astral runes while casting the spell with either a smoke/lava/fire/steam staff or a Tome of Fire.