If you want to prolong the life of a couch, you can recover existing cushions or make complete replacement cushions for your couch. However, don't make the mistake of buying any type of couch cover or foam slab labeled as a general couch cushion replacement. Many factors impact the size of couch cushions, and proper sizing is key before moving forward on a project.
The first factor in the size of the cushions on a sofa is how large the sofa itself is. Even sofas that are ostensibly the same size, such as a three-person sofa, can range several inches in length. That additional length will be reflected in the size of the cushions. A standard sized three-person sofa is 76 inches long. The size of the cushions also depends on the thickness of the arms of the sofa.
Couches now come in a number of styles. Sometimes the couch has the same number of cushions on the back and on the bottom, others have different numbers of back and bottom cushions, while others have a solid back and cushions on the seat. Obviously, a couch with three back cushions and four seat cushions has different dimensions than a couch the same length with only two couch cushions. This is why knowing the style of the couch is important when determining seat size.
Standard couch cushions are square. On a normal three person couch with smaller cushions, cushions are typically 18 inches or 20 inches square. However, larger couches certainly have cushions that are 24 inches square or larger. The issue becomes more complicated when you consider rectangular-shaped couch cushions or those that have cutout shapes in order to fit around the arms of couches. However, these sizes give you a very good approximation.
Depth is a little easier to predict than the other dimensions of a sofa cushion. Most standard sofa cushions are around 4 inches to 6 inches deep. A few manufacturers might make pieces with deeper cushions for added comfort, but this is certainly the norm.