Abstract: A depth calculating device is provided wherein the exact depth of a fisherman's bait presentation is calculated while that fisherman is trolling. The invention utilizes an electronic sensing device which determines the deflection of the fishing line away from the line formed between the tip of the fishing pole and the water's bottom. An additional electronic sensing device measures the length of line played out from the reel. Both measurements are processed by a central processing unit which in conjunction with the cosine function stored in the memory of the central processing unit then calculates and displays the exact depth of the bait presentation at any given moment.