Minecraft wiki entry for Smelting, associated text: When using planks to smelt logs to charcoal. However, value is instead 69 7⁄37 (~69.19) if using charcoal to smelt logs, assuming a plank or something of equal value is used to make the initial charcoal (640 seconds / 9.25 logs). In Bedrock Edition, if a wooden slab is used to make the first charcoal, then the value is 70 10⁄73 (~70.14) (640 / 9.125). If non-wood fuel is used to make the initial charcoal, the value is instead 71 1⁄9 (~71.11) (640 / 9). If charcoal is continually used to make more charcoal, then regardless of the initial fuel source, the average value approaches approach 71 1⁄9 (~71.11) as more is made.