Minecraft wiki entry for Time-saving_tips, associated text: If you need to build something of known dimensions, or place exactly some number of an item, adjust the size of the stack in your active slot before you start. This way, you can use the stack itself as a counter that automatically runs out at the desired measurement. For example, if you wanted to mark out a 40×50 rectangular area, fill an active slot with exactly 40 blocks. You can then place them without worrying about counting; once the stack runs out, you know you've placed exactly 40 blocks. In the other direction, fill the active slot with 49 blocks and place them. For lengths larger than 64 meters in one direction, either use stacks of 64 and then one final stack with the remainder, or size your stacks with round numbers of blocks. Use whichever method you find simplest.