Duru or rounds are a central aspect of calculating moves in a group of mancala games. It is shown that the ability to calculate multiple duru is a prerequisite to mancala (bao) expertise in Zanzibar, while duru-related optimization strategies explain the accomplishments of Maldivian mancala (ohvalhu) players. Since bao masters define duru calculation skills as part of bao expertise, their performance on duru calculation tasks is contrasted with that of novice players. The results show that only game-specific duru calculation skills distinguish novices from master players. Maldivian players of ohvalhu solved their mancala game by identifying a winning opening. Their optimization strategy includes a minimization of the number of duru for a move choice. Duru calculations have a central role in our understanding of mancala expertise and both game-specific and general aspects of duru inform us about problem-solving and decision-making processes of mancala experts.