What does 10d6 actually look like next to 6d10? How much does rolling twice really help? Type some dice, flip the switches, and the shapes redraw.
Both top out at 60, but 10d6 averages 35 and 6d10 averages 33, and 6d10 swings more: its spread (standard deviation) is 7.0 against 5.4. With two or more dice, about two rolls in three land within one spread of the average. Type up to four rolls, like 2d8+1d6+4, in the boxes; empty boxes are left out.
One bar per total. Above each hump: a dot at the average and a whisker one standard deviation (the spread) either side.
Read across at any total: the chance the roll reaches it.
Rolling two d20s and keeping the higher (fortune in Pathfinder, advantage in 5e) does not add a flat bonus: it helps most in the middle of the die and almost nothing at the ends.
Find the number you need on the die: each line is your chance to make it.
The gain (or loss) in chance compared to a single roll, in percentage points. Its peak is at 11.
Each column splits all your rolls at that bonus: crit at the bottom, then hit, miss, crit fail.
The odds are exact, worked out from every face of every die, with the same engine as the Strike Lab.