Roll any combination of dice - 3d6, 2d20, a single d100 - and see each individual die alongside the total. Modifiers are supported, as is rolling a d20 with advantage or disadvantage, so it works for tabletop games as well as for probability demonstrations.
NdS+M means roll N dice with S sides and add M. So 3d6+2 is three six-sided dice plus two, giving a range of 5 to 20. Omitting the count means one die: d20 is a single twenty-sided roll.
Both span a similar range, but multiple dice cluster around the middle. With 3d6 a total of 10 or 11 is roughly 12% likely each, while 3 or 18 come up under 0.5% of the time. A single die is flat - every face equally likely. This is why systems that want dependable outcomes roll several dice and systems that want drama roll one big one.
Rolling two d20s and keeping the higher lifts the average from 10.5 to about 13.8, which is roughly equivalent to a +3 to +5 bonus. The gain is largest in the middle of the difficulty range and smallest when you need a 2 or a 20, because there is little room left to improve.
Roll three six-sided dice, add them together, then add 2 to the total. The notation is NdS+M: N dice with S sides each, plus a modifier M.
You roll two d20s and keep the higher one. Disadvantage keeps the lower. Advantage lifts the average result from 10.5 to about 13.8, and is worth roughly a +5 bonus in the middle of the range.
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