Measure the back of the couch from the top down to the floor and from one side to the other. Record measurements as you proceed.
Take another measurement on the inside of the couch from the top down to the deck and another one from one side of the couch all the way to the other. Add 3 inches to the measurement for the top down to allow for tucking into the cushion.
Measure from the front to the back of each cushion. All cushions are not the same size. Typically, the middle varies slightly from the sides.
Measure the distance from the front to the back of each arm and the distance from the deck up to the top of the arm. Take a final measurement on the arm from the highest point down to the floor.
Take a measurement of the back cushions of the couch from one side to the other and top to bottom. Measure the boxing (thickness) of the cushion separately. Repeat for the seat cushions.
Add all of the measurements together in inches and divide the total by 36 to calculate how many yards of fabric you need for the project.