A trimmable heater mat has a plurality of heater mat segments that together comprise a length of the heater mat. Each of the heater mat segments has a segment heater element connected to a power bus for generating heat. One or more of the heater mat segments can be severed from the others and removed from the heater mat to trim the length of the heater mat to a desired length. Heater mat segment boundary indicia on the heater mat indicate locations where one of the heater mat segments can be severed from the other remaining heater mat segments of the heater mat without interfering with the segment heater elements of the remaining heater mat segments or with the connections of the remaining segment heater elements to the power bus.