Some of the common dimensions for bathtubs and showers are standard because they're required by building codes, and others are a matter of comfort, efficiency or functional design.
Building codes specify minimum sizes for showers, and any shower compartment must adhere to these specifications in order to be compliant with the codes. The International Residential Code, on which most local building codes are based, lays out guidelines for both the horizontal and vertical dimensions for showers.
According to the IRC, the interior of any shower must be at least 30 by 30 inches, and an imaginary 30-inch-diameter disc must fit within the shower's interior space. The ceiling height within the shower must be at least 80 inches above the entirety of the minimum 30-by-30-inch interior space.
Space-saving neo angle shower stalls, because of the space lost by their angled corner, must be at least 32 by 32 inches to meet the code requirement.
The National Kitchen and Bath Association recommends an even larger interior dimension of at least 36 by 36 inches for showers.
Larger walk-in showers, including doorless showers, should be at least 36 by 36 inches, but a significantly larger minimum dimension of 42 by 60 inches works better. Showers of this size can usually be used comfortably by two people at once.
The standard size for a traditional bathtub, including tub surrounds with an integrated shower head, is 60 inches long by 30 or 32 inches wide, but space-saving tubs may be as small as 4 feet long. Garden tubs, jetted whirlpool tubs and soaking tubs, which may be designed to be used by more than one person at a time, can be more than 6 feet long, although some compact units are designed to fit in the same space as a traditional 5-foot tub.
The IRC requires that that walls of a shower or bath tub surround, including shower/tub combination enclosures, be covered by a waterproof material to a height of at least 72 inches above the bathroom's finished floor. The National Kitchen and Bath Association further recommends that the surround material extend at least 3 inches above the point where the shower head emerges from the surround wall.
The association also recommends that the walls of a shower enclosure never extend all the way to the ceiling because a fully enclosed shower provides no way for steam to escape the enclosure, which can lead to problems with mold growth.