Minecraft wiki entry for Coal, associated text: Wither skeletons have a 1⁄3 chance of dropping a single coal upon death. The maximum amount of coal is increased by 1 per level of Looting. The chance of a wither skeleton not dropping any coal can be found using the formula 2 / (Looting Level + 3). For example, Looting III gives a 1⁄3 chance of not dropping any coal. All other amounts have an equal chance of occurring.