For the best end result, keep the following tips in mind;
Determining correct length
Measure the top and bottom lengths of the desired cushion.
Always try to round down to half-centimetres. Millimetres can be ignored. Will the cushion be between or inside something? Then make sure you leave about a centimetre. This prevents the cushion from being too tight, the fabric from wrinkling or the filling from not being tight. Do you want the cushion to overlap slightly? Then you may just take the size one centimetre larger.
Seat cushion thickness
For chair/(dining) bench: between 4 and 6cm
For lounge cushion: between 8 and 12cm
Including seat cushion, the height is between:
Normal seat height: approx. 47 to 49cm
Lounge seating height = approx. 42cm
Be careful not to make the cushions too thick, or you will end up sitting too high.