Stand facing the chair and place the end of the tape measure at the inside left edge of the back of the chair. Extend the tape measure straight across to the inside right edge of the back of the chair. Write down the measurement.
Measure the seat of the chair from the left edge to the right edge. Make sure you take this measurement inside the arms -- do not include the arms in this measurement. Write down this measurement.
Place the end of the tape measure at the back of the seat and stretch the tape measure to the front of the seat to measure the seat depth. Write down this measurement.
Measure a wing if your chair is a wing chair. Place the end of the tape measure at the back of the wing, along the back of the chair, and extend the tape measure straight across the widest point of the wing. Note the measurement and write it down.
Place the end of the tape measure at the top back of the chair and extend it straight down to the floor. Note the measurement and write it down.