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.

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.

• A proper wheelchair seat depth will afford the user increased pressure reduction.
• It will also enhance comfort and will promote improved sitting posture.
• As shallow a seat as possible will assure a shorter wheelchair frame (in a manual wheelchair) which will result in a lighter chair, decreased turning radius, and easier transporting and storage.
• A longer depth will serve to distribute pressure over a greater area (the surface area of the seat), thus reducing pressure in any given place.
This does not hold true if the user is sitting in the knees up position with only the ischials (sitting bones) making seat contact. The pressure will still remain on the ischials no matter how deep the seat is. If the thighs do not make contact with the seat they will not aid in pressure distribution.

• An excessively long seat will push against the back of the leg or in the area of the popliteal crease (fold on the back of the knee) causing the user to be pushed forward on the seat and into a sacral (slouched) position. If left unresolved in patients with insensate (no sensation or feeling) lower extremities this situation may also result in tissue trauma (sores, wounds) to the back of the leg.
• A overly deep seat will not allow the user to sit all the way back against the wheelchair back.
• An excessively long seat will push against the back of the leg or in the area of the popliteal crease (fold on the back of the knee) causing the user to be pushed forward on the seat and into a sacral (slouched) position.
• If the user is forced foward on the seat most of the weight will be located forward on the wheelchair making the chair harder to push.
• Long seats will also add weight to the chair.
• Excessively short wheelchair seat depths will result in increased pressure to the seating area (less surface area). This may increase the risk of tissue trauma.
• The amount of support offered by the seat will also diminish possibly causing poor posture.