Fit & measurements
How to measure a walker or wheelchair frame for an accessory
Measure the exact mounting location, tube shape and clearance before choosing a clip-on mobility-aid accessory.
By Mission Printpossible · September 25, 2026 · 4 min
From the initiative building this library. This guide describes accessory planning, not an assessment of your equipment.
Measure the mounting spot, not just the equipment name
A clip-on accessory needs to match the tube where it will sit. A brand or model name is useful context, but it does not describe every tube, bracket or obstruction on that equipment. Start by choosing a possible mounting location and checking it against the equipment manufacturer’s instructions.
Commercial wheelchair mounting systems use several tube sizes. KUPO, for example, lists 19, 22, 25, 28 and 32 mm options for one of its clamp systems. That range is a good reason to measure your frame rather than assume that “wheelchair size” means one diameter.
Record the outside diameter of a round tube
Use calipers across the outside of the tube, without compressing padding or including a loose cover. Record millimeters and the exact location. Measure in a second direction to check whether the tube is round. If the two measurements differ, describe the shape instead of averaging it into a round-tube size.
For an unobstructed round tube, you can estimate diameter by measuring its circumference with a narrow paper strip and dividing by π, approximately 3.1416. A circumference of about 79.8 mm corresponds to a diameter near 25.4 mm. This is an estimate, not a substitute for checking the actual fit. The DAESSY mounting handbook also describes circumference measurement and notes that it does not apply to square or rectangular tubing.
Check the space around the attachment
Record the available straight length and the direction of the tube. A holder designed for a horizontal bar may tilt on a diagonal one. Look at the space the accessory will occupy with the phone or container in it, not only the empty bracket.
An attachment must not obstruct brakes, wheels, folding joints, controls or the space used for transfers. If you cannot check these clearances yourself, ask someone familiar with your equipment to help. Do not drill or modify the mobility aid to match an example from our library.
Send a small, useful set of details
For a fit discussion, send the equipment brand/model if known, a close-up of the proposed mounting location, its shape and outside dimensions, and the everyday task you want to make easier. Keep faces, identifying paperwork and medical records out of the photo.
Our current library uses explicit prototype dimensions: 25.4 mm tubes and 180 mm bar spacing. These are starting assumptions, and a nominal tube size alone does not establish grip strength. We still need a physical fit review before agreeing that a print is suitable.