Sofas come in different standard and non-standard sizes, each designed to seat a specific number of people comfortably. Most sofas for the home are for three-people and, although they may be different lengths, all have three seat cushions. However, you can have cushions of any width for any sofa, as long as each cushion is a comfortable size for a person to sit on. This is usually a minimum width of 20 inches and a depth that allows the cushion to sit firmly on the sofa, lodged against its back and coming out flush to the front of the frame.
Standard measurements for three-seat sofas are 76 inches in length and a depth of 30 inches. However, this distance includes the frame and is not all sitting space. Three evenly sized cushions form the seat of this sofa. Measure the width of your sofa inside the frame and divide the measurement by three to get the width of each cushion, and then measure the distance from the front of the frame to the back for the depth of the cushion. For example, a sofa with a sitting-space width of 63 inches needs three cushions of 21 inches width. Use the same method to calculate the cushion sizes of larger sofas.
Loveseats are smaller sofas that usually seat two people. The smallest loveseats are about 45 inches wide, medium-size ones are around 60 inches and larger ones are almost the same size as a three-seat sofa. With about 40 inches of sitting-space, cushions of smaller love seats are about 20 inches wide, and the depth of the sofa. For larger love seats, consider using three cushions instead of two.
Futons are sofas intended for use as a bed as well as a couch. These sofas generally have just one sitting cushion, which acts as both the seat cushion and the back of the couch, unlike the cushions of a regular sofa. The width of the futon cushion is the width of the sitting space of the futon, but its depth is the depth from the front to the back of the sitting space plus the height of the back of the futon.
Throw cushions, or pillows, provide both a decor accent for your sofa as well as extra comfort to lean against. The size of these cushions should be in proportion to the size of the sofa. Use 12 inch or 16 inch square throw cushions for smaller love seats, working up to 24 inch square cushions for a larger three- or more person sofa. Consider using cushions of different sizes for added interest, perhaps using two small and one large cushion on a family sofa.