Single: length: 80-95, depth: 85-90; Cushion height: 35-42; Back height: 70-90
Double: length: 126-150; Depth: 80-90
Three person: length: 175-196; Depth: 80-90
Four person: length: 232-252; Depth 80-90
Sofa is upholstered most of the time. The final dimension of a sofa is really affected by the design and material, also the human weight. For example, the density and thickness of different foams can lead to different height of the seat; the depth of a sofa with cushions can be larger than a sofa without cushions; the height of the seat can also be different due to various purposes and functions.
If you are a beginner and not familiar with the actual size of a sofa, I highly recommend you to visit some furniture brands websites, find some products that are similar to your design concept, check the dimension of that product as a reference to give yourself a general idea. Besides, a full-scaled cardboard mock up can always help you figure out the size instantly.
- National Standard Size of Furniture, new edition, 2016. https://www.docin.com/p-1617757039.html