This mat is designed to lay the fish on to measure. The mat is Made from polystyrene sign plastic. Lay the fish with its lip at then end, fan the tail to get total length. The average weight for a bass that length is listed across the bottom of the mat. Example- If the bass you catch is 12.5 inches is should weight 1 pound. Split the difference between the 12/13 inch weights. Remember these are the average weights of bass in North America (think making a 75 on a test). Bass can be several pounds heavier than the weights listed. Bass under these weights are in poor condition.