Choose the sectional sofa's location in your home. Many sectionals can be put together in several ways and have an asymmetrical shape. Figuring out how you want the sofa to fit into your living space will help you choose an overall shape and size for your sectional before you start having to take specific measurements.
Sketch the sofa's shape before adding the measurements to the drawing. This will help you keep track of how the pieces fit together in your design plans. If you are comparing measurements of different sofas, you need to keep track of their shapes to help you compare them.
Measure the length of the sofa at its longest point, from the outsides of the armrests. Measure the width of the sofa from the back to the front edge of the seat's cushions. Measure the width at both the sofa's shortest and longest points. Many sectionals have a large end piece that is considerably bigger than the rest of the seating areas.
Include the height measurement of the sofa, from the floor to the top of the sofa's back, when measuring the length and the width. This will give you the most complete picture of your sofa's size if you need to determine where to put it in your home in relation to accent tables and windows.