For single die rolls like a d20, you count the numbers on the dice that fall in the range. So for a roll of 1-5, there are 5 numbers that fall in that range: 1, 2, 3, 4, and 5, so that wight is 5. For a roll of 6-12, the numbers in the range are 6, 7, 8, 9,10, 11, 12 - counting them gives 7, so wight is 7. The quickest way to calculate this is: Take the top number Subtract the bottom number add 1 So a range of 6-12 would be Top number = 12, bottom number is 6, so 12 - 6 + 1 = weight 7. For multiple dice (2d6, 3d6, 1d20 with advantage, etc) it gets a lot more complicated, but for single die rolls like these its very simple once you know what you are doing.