A stimulus of a given sound pressure sounds louder when it is heard with two ears than when it is heard with only one ear. This fact is demonstrated by a series of experiments designed to quantify the ratio of binaural to monaural loudness at various stimulus levels. The methods used included magnitude estimation, magnitude production, one‐vs‐two‐ear ratio production, monaural‐binaural loudness matching, and the cross‐modality matching of loudness to the apparent intensity of a vibration.The results indicate that (1) monaural loudness grows as a power function of sound pressure with an exponent of about 0.54, whereas binaural loudness grows with an exponent of 0.6, and (2) the ratio between binaural and monaural loudness increases as a power function of sound pressure with an exponent of about 0.066. This ratio is 2:1 at about 90 db SPL. At lower levels the ratio is smaller, and at higher levels it is larger.