A mechanical calculator that computes payouts for combinations of roulette bets. A number wheel (50) has an array of payout numbers (54) for a combinations of two types of bets over a range of chips, such as up to 20 chips on a street bet and up to 20 chips on a line bet. A mask wheel (40) overlays the number wheel (50). Windows (44) on the mask wheel each reveal an individual number (54) on the number wheel, depending on the relative rotational positions of the mask and number wheels, which are manually selectable with knobs (28, 32). A cover plate (22) overlays the mask wheel, and has a window (26) that reveals a single combined payout number (54) on the number wheel under a mask window (1-20) and the cover plate window (26). Rotational index numbers (49, 68) indicate the number of chips selected for each bet.