An efficient design method is proposed for robust low-boom design considering sonic boom loudness at both on-track and off-track positions. There exists an efficient design method for on-track low-boom design based on a reversed equivalent area distribution without using optimum design methods. The proposed method extends the existing method to consider off-track loudness. The characteristic of the proposed method is a conversion and a unification by weighted average. In the conversion, reversed equivalent area distributions at off-track positions are converted to that at the on-track position. In the unification by weighted average, converted distributions are unified into a single distribution based on loudness estimation using the F-function of the converted reversed equivalent area distribution so that sonic boom loudness at on-track and off-track positions can be balanced. Once the unified distribution is defined for the on-track position, the existing method can be applied. The proposed method is applied to a 50-seat class supersonic airliner, and the maximum loudness of front part of sonic-boom signature at both on-track and off-track positions is successfully reduced by 6.5dB in terms of Stevens Perceived Level Mark VII from 91.6dB to 85.1dB.