ChairSmarts

Front wheelchair casters: measure these four things before you order

Last updated: October 27, 2026

A front caster is the small wheel at the front of the chair. It sits in a fork. The fork is the metal piece with two legs and a stem that swivels. A replacement wheel has to fit that fork, not just look like the picture in a catalog.

Write down four measurements: wheel diameter, tire width, hub width, and the axle bolt (the hole through the bearing). Then compare them with the size stamped on the tire, the chair's parts manual, and the seller's diagram. A ruler works for the diameter. Calipers are better for the hub and the bolt. If you do not have calipers, the dowel method and the drill-bit check below still work.

The short version

  1. Read the size printed on the old tire and in your chair's parts manual.
  2. Before you take the wheel off, take a photo of the spacers and washers in the order they sit.
  3. Write down the four measurements below and match them to the new wheel.

The sections after this explain each step and the bearing sizes. You do not need to read all of it to order a part.

Not medical advice: This is general information about measuring a wheelchair part. It is not medical advice, and it is not a fitting. Seat height, the angle of the front frame, and how easily a chair tips all change if the caster size changes. For seating, positioning, pressure sores, or anything about your health, talk with your doctor, therapist, or wheelchair provider. If the chair is under warranty or was supplied through insurance, check with that provider before you replace parts yourself.

Your chair wins over any list on this page. Order forms from Quickie, Ki Mobility, Invacare, and Drive show the sizes those companies offer. They do not list every chair ever built. Measure the wheel that is on the chair now, and read the manual or the numbers molded into the tire, before you order.

Words you will see

Checklist: four measurements

  1. Wheel diameter. Straight across the center of the wheel, outside of the tire to outside of the tire.
  2. Tire width. How thick the tread is, and whether that tread clears the fork legs and the top of the fork.
  3. Hub width. From the outside face of one bearing to the outside face of the other, compared with the gap inside the fork.
  4. Axle bolt / bearing bore. The diameter of the old bolt's shaft, or the hole through the bearing that the bolt passes through.

Also write down the chair's make and model, any part number on the fork or the wheel, and whether the two front casters match each other.

The order to do it in

Do this with the chair empty and on a flat, stable surface. If you are not sure how to tilt or lift the chair safely to take the weight off the front wheels, ask your dealer or check your manual. Never remove a caster while someone is sitting in the chair.

With the wheel still on the chair:

Then photograph the spacers and washers, and remove the wheel. Keep the pieces in the same order.

With the wheel off:

Where to put the ruler

Side view of a caster wheel A round tire with a vertical double-headed arrow through the center, from the outside of the tire at the top to the outside of the tire at the bottom. That arrow is the wheel diameter. The dark center is the hub, and the small gold circle is the bearing hole the axle bolt passes through. Wheel diameter across the center Outside of the tire bearing bore
Side view. Measure the diameter through the middle of the wheel. Do not measure along the outside edge, and do not stop at the rim if the tire stands proud of it. The gold dot is the bearing bore, where the axle bolt goes.
Front view of a caster in its fork A caster tire held between two fork legs. A horizontal arrow across the top marks the inside gap between the fork legs. Arrows below the wheel mark the tire width across the rubber and the hub width from the outer face of one bearing to the outer face of the other. The axle bolt passes through the bearing bore in the center. Inside of the fork Axle bolt Tire width Hub width, bearing face to bearing face
Front view, looking at the fork. The dark U is the fork. The pale block is the tire. The gold blocks are the bearings. Hub width is the outside of one gold block to the outside of the other. Tire width can be wider than the hub. The bolt runs through the bore in the middle. The inside-of-fork arrow is the gap the whole wheel has to live in, including room so the tire does not rub the legs or the crown at the top.

1. Wheel diameter

Measure from the outside of the rubber, through the center of the wheel, to the outside of the rubber on the other side. If a size is molded into the sidewall, read that too. A caster measuring guide from Service Caster says wear can remove as much as 1/2 inch of material, so a badly worn tire can measure smaller than the size it was made to. That guide is about casters in general, not one wheelchair brand. Treat it as a warning to check the sidewall or the chair manual instead of trusting only a worn measurement.

Changing the diameter changes the chair, not just the wheel. On the Catalyst 5 form, each caster size has its own row of front seat heights, and Ki Mobility picks the fork and stem from the caster size and that seat height. Motion Composites says a larger diameter allows a higher front seat-to-floor height and may need a greater front-frame angle, and a smaller diameter does the opposite. Sunrise Medical says a longer fork is used for a larger caster, and a longer fork increases the turning radius. On the Quickie M6 form, the 17 inch seat height with a 5 inch caster uses a different axle and fork than the other 5 inch setup, and the 8 inch caster is paired with a 20 inch seat height.

Order the same diameter for both front casters. Those order forms choose one caster size for the chair. If one front wheel is taller than the other, that side of the front frame sits higher. Invacare's Tracer IV caster sheet sells some wheels as a pair and tells dealers to order tires in pairs after a color change. That sheet is not about diameter, but the habit is a good one: replace both sides together so they match, even when only one is worn.

2. Tire width

Tire width is the width of the tread, not the width of the metal hub. Measure the widest part of the rubber. Then look at the fork: the tire has to clear the inside of both legs and the crown, the curved part at the top, while the wheel spins. A wider tire can rub even when the hub width is correct, because the rubber can stick out past the hub or hit the fork above the axle.

Motion Composites describes the practical difference: a narrower caster usually rolls with less resistance on smooth floors, and a wider caster usually copes better with rough or soft ground. That is a reason to choose a width, not a reason to skip the fit check. On the Catalyst 5 form, Frog Legs suspension forks are offered only with 4, 5, or 6 inch casters, and not with the 2 inch poly or 2 inch pneumatic options. A wider or larger wheel is sometimes a different fork, not just a different tire.

Service Caster's measuring guide makes a useful distinction: the tread width tells you which wheel you are looking at, but the hub length is what has to fit between the fork at the axle. Check both.

3. Hub width

The hub is the center of the wheel, where the bearings sit and the axle bolt passes through. Measure from the outside face of one bearing to the outside face of the other bearing. That distance has to agree with the inside width of the fork at the axle holes.

Service Caster's method, if a tape will not fit through the hole: slide a dowel or a pencil through the bore, hold one end flush with one side, mark the other side, pull it out, and measure the mark. The same guide says the hub should fit the fork with minimal play. Too much side-to-side play lets the wheel flop. A hub that is too wide will not go into the fork, or it will spread the legs.

Hub widths vary a lot. Listings range from about 1 inch to over 2 inches, so do not pick one from a list. Measure the old hub and the inside of the fork. Examples are near the end of this page.

If the hub is a little narrower than the fork, washers or spacers fill the gap between the bearing face and the fork leg. New Mobility describes a standard fork the same way: the caster sits between the two arms on a bolt and spacers. An older Alco parts page shows a 6 by 1-1/4 inch caster with a 1 inch hub that includes spacers to fit forks up to 1-3/4 inch wide. Invacare's Tracer IV drawing lists a hub spacer, an axle washer, and an axle spacer as separate pieces. Keep the old spacers in order as you take the wheel off, or photograph the stack first.

The spacer should take up the empty space so the fork legs are not pulled inward onto the bearing when you tighten the nut. The bolt clamps the inner ring of the bearing, which is the ring with the hole. The outer ring is the part that turns with the wheel. After the nut is tight, the wheel should still spin freely by hand, with almost no side play. If the wheel locks up, the stack is pressing on the ring that should turn, or the hub is too wide for the fork. Stop and put the old wheel back on rather than forcing it.

4. Axle bolt and bearing bore

The bore is the hole through the bearing. The axle bolt passes through that hole. Measure the smooth shaft of the old bolt, not the threads at the end and not the head. Or measure the hole in the bearing. Service Caster suggests calipers for this, or trying drill bits until one fits the hole without slop. A ruler is a rough check. This size is small, so a 1/16 inch mistake is a wheel that will not go on.

Bolt sizes are not interchangeable. The common ones are 8 mm (about 5/16 inch), 3/8 inch, and 7/16 inch, and some parts use 11 mm. Do not guess from a photo. Measure your bolt.

Easiest path: if you are replacing the whole wheel, buy a wheel that already comes with the right bearing for your bolt. If you cannot measure the bolt, take the old wheel to a dealer or compare it with the seller's diagram.

If you are replacing only the bearings

Three bearing sizes show up again and again. They are not interchangeable.

Do not stop at those three. Drive's Cruiser III page lists a replacement caster with 11 mm bearings (part STDS4S2432) beside an 8 inch caster chair. Eleven millimeters is close to 7/16 inch and is not the same part. Measure the bolt.

Match the whole bearing, not only the hole. A 608 and an R6 can look similar and still differ in bore, outside diameter, and width. Read the code on the old bearing, or measure bore, outside diameter, and width.

Quick reference

What to write down before you order a front caster
Measurement Where to measure What has to match
Wheel diameter Outside of the tire, straight across the center. Also read the sidewall. The original diameter, unless the manual allows another size with a matching fork and seat height. Replace both front casters with the same size.
Tire width Widest part of the tread. Spin the wheel in the fork. Clears both fork legs and the crown. Width examples on order forms include 1 inch, 1-1/4 inch, 1-1/2 inch, and 2 inch, plus narrower rollerblade casters and a few odd sizes such as 1.4 inch.
Hub width Outside face of one bearing to the outside face of the other. Compare with the inside of the fork. The fork gap, using the original spacers if the hub is narrower. Listings include 1 inch, 1-1/2 inch, 1-3/4 inch, and 2-3/8 inch hubs. Other widths exist.
Axle bolt / bore Shaft of the old bolt, or the hole in the bearing. Calipers or a drill bit that fits without slop. 8 mm (608, often labeled 5/16 inch), 3/8 inch (R6), or 7/16 inch are the sizes most of the sources we checked use. Some Drive parts use 11 mm. Measure yours.

Before you order

  1. Read the size on the old tire and in the parts manual. If the measured diameter is smaller than the stamp, the tire is probably worn. Then confirm the size against your chair's manual or spec sheet, because a previous owner or dealer may have fitted a different size than the chair shipped with.
  2. Write down diameter, tire width, hub width, inside fork width, and bolt or bore diameter. Keep the spacers in order.
  3. Buy the same size for the left and right front casters.
  4. Leave the diameter, the fork, and the stem alone unless the chair's order form or manual shows that combination. A different diameter can change front seat height, the angle of the caster housing, and how large a turn the front of the chair needs.
  5. Use the hardware and locknut that came with the new wheel, or the original hardware if it is in good shape, and tighten it the way your chair's manual says. The wheel should spin freely by hand with almost no side play, and the tire should not rub when you turn the fork. A loose axle bolt is one of the most common ways a caster causes trouble after a swap, so check it again after a few days of use, and any time the wheel starts to wobble. If you cannot find a spec in the manual, ask your dealer or the chair's manufacturer.

Signs a front caster needs replacing

If you see wobble or binding, check that the axle bolt is tight before assuming the wheel is the problem.

Shop for a replacement caster

Affiliate link: the button below goes to SpinLife through Sovrn. If you buy through it, SpinLife may pay ChairSmarts a commission. It does not change the price you pay. We do not choose the measurements on this page based on that commission. Read how we earn money.

Browse replacement wheelchair casters, wheels, and tires at SpinLife (affiliate link)

That link opens SpinLife's wheels, tires, and tubes section in a new tab. Use your four measurements there. A photo of an 8 inch caster is not enough if the hub or the bolt is different.

Examples from order forms and parts pages

This part is optional. It shows how much sizes vary. These are sizes printed on order forms and parts pages, dated where we know the date. They change, and your chair may not match any of them, so use them as examples, not to choose a part.

Wheel diameter

Tire width

Hub width

Widths printed on real listings, not a complete set:

Where these details come from

Did a part not fit, or did something on this page not match your chair? Tell us at contact@chairsmarts.com. We will fix the guide, and we will apologize if it cost you a return.