Wheelchair seat depth is measured from the front to back of the seat. To determine the existing size you need only measure the distance from the front edge of the seat to the back where the seat meets the back upright. This will give you a depth dimension of the existing seat.
Measuring to the back rest upholstery may result in an incorrect measurement. Most back upholstery is stretched and hammocked, so measuring in the center of the seat to the center of the back upholstery will result in a longer and incorrect dimension. Some wheelchairs have add-on backs attached. Many of these mount forward of the back uprights thus making any seat surface located behind the add-on back unusable.
To determine the appropriate seat depth for a user- The user should be placed in the best possible seated position that can be obtained. A measurement is taken from the back of the pelvis (furthest part of the buttocks), forward to the back of the knee. Make sure that each leg is measured separately. There are more leg length discrepancies (differences in leg lengths) than you may think. Deduct 2" from the measured length for clearance.
If you have decided on an add-on back you will need to allow for this. If the decided on back takes up seat space in front of the back uprights, you will have to add that amount back into the formula. If not you will be scratching your head wondering why the seat appears so short on the finished product. How about some formulas about now?
With no add-on back; (measured length) – (2" clearance) = depth.
For chairs with an add-on back; (measured length) – (2" clearance) + (overlap of back on seat) = depth.
Source listing of add-on wheelchair back suppliers- www.usatechguide.org/techguide.php?vmode=1&catid=205
If there is a leg length discrepancy (differences in leg length), custom upholstery, seat pans, or inserts can be ordered from the wheelchair manufacturer (sometimes) or from aftermarket suppliers. Many cushion manufacturers will also custom make their products to accommodate these differences.