Include the width and the depth of your chair seat and add about a half inch for the seam allowance.
How to make dining room chair covers for the seat.
Using a temporary fabric pen trace a line three inches out from the seat and cushion.
Cut new fabric place your foam and seat on top of your new fabric.
If you are making a simple cover with no cushion cut the fabric to the size needed and also cut fabric to be the liner on the bottom of the seat cover.
For this project the chair was 44 inches high x 20 inches wide x about 22 inches deep.
Add another inch if you plan to make cushioned seats.
Use a tape measure to determine the width of the chairs you want to cover.
If you have an oversize dining table and chair set and your dining room chair is wider you may consider using 60 inch wide fabric to guarantee adequate coverage.
Sew a slipcover out of fabric.
Then measure up the back of the chair from the floor to the top down the front of the backrest along the edge of the seat from back to front and from the seat down to the bottom of the legs.
Take measurements of the chairs.
Then pin around the chair seat and connect it to the fabric over the chair back.
You need about 3 yards of 45 inch wide fabric to cover one standard size dining room chair.