A minimal design for an easy experience, the NAT-20 Roller uses all the standard dice found in table-top RPG's like Dungeons & Dragons, Call of Cthulhu, Pathfinder, and more.

Just select the number of dice you want to roll, add the bonus to that roll, then just tap the die you want to roll and see your result in the top window. Comes complete with a percentile option for even more versatility.