A jig for determining a patient-adapted implant size of a femoral implant of a knee endoprosthesis. The jig has a main body, a probe part movable relative to the main body along a measuring direction. The probe part has an arm with a tip placeable on an anterior reference point of the femur's distal end, two contact pieces for placing on and referencing the medial and lateral posterior condyle of the femur's distal end, and at least one scale and pointer for indicating the probe part's position in the measuring direction relative to the contact pieces. The distance of the probe tip from the contact pieces is adjustable such that the distance is different with respect to the two contact pieces and, for each contact piece, a dedicated scale and an associated pointer are provided for the indication of the probe part's position relative to the respective contact piece.