Wheelchair Tire and Wheel Size Chart: Reading Sizes, Diameters and Widths

A wheelchair tire size is read from the numbers on the tire sidewall. An inch designation such as 24 x 1-3/8 gives the nominal wheel size and the tire width. An ETRTO size such as 37-540 gives the tire width in millimetres (37 mm) and the bead seat diameter of the rim in millimetres (540 mm). The ETRTO number is the most reliable way to match a replacement tire to a rim. The inch number is only a nominal label: two tires both called 24-inch can need different rims. Front casters are usually marked by outside diameter and width, for example 8 x 1 (inches) or 200 x 50 (millimetres). Mobility scooter tyres often use formats such as 3.00-4 or 9 x 3.50-4, where the last number is the rim diameter in inches.

What do the numbers on a wheelchair tire sidewall mean?

The sidewall markings on a wheelchair or scooter tire fall into two groups. Dimensions describe size: diameter, width and rim size. Ratings and use markings describe how the tire may be used: maximum pressure, load-related markings and rotation direction. A single tire often carries several size formats at once, such as an inch size and an ETRTO size, and these describe the same tire in different systems.

Inch size designations on rear wheels (for example 24 x 1-3/8)

On manual wheelchair rear wheels, an inch designation such as 24 x 1-3/8 means a nominal wheel diameter of about 24 inches and a nominal tire width of 1-3/8 inches (about 35 mm). The first number is a size name, not a precise measurement. It does not tell you the exact rim diameter, so it should not be used on its own to choose a tire.

Fractional widths (1-3/8, 1-1/4) and decimal widths (1.375, 1.75) belong to different traditional sizing families. A tire marked 24 x 1-3/8 and a tire marked 24 x 1.375 have the same nominal width written two ways, but they are normally made for rims of different diameters. That is why the ETRTO number matters.

ETRTO size markings (for example 37-540)

An ETRTO size is a two-part metric marking from the European Tyre and Rim Technical Organisation. In a marking such as 37-540:

• 37 is the nominal tire width in millimetres.
• 540 is the bead seat diameter in millimetres: the diameter of the rim at the point where the tire bead sits.

For a tire to mount on a rim, the bead seat diameter must match exactly. A 540 mm tire will not fit a 507 mm or 559 mm rim, even if both wheels are called 24-inch or 25-inch. The width can vary within the range the rim is designed for. Rims may carry their own ETRTO marking, such as 19-540, where 19 is the inner rim width in millimetres and 540 is the bead seat diameter.

Front caster markings

Front casters are usually sized by their overall outside diameter and width, not by a bead seat diameter. A caster marked 8 x 1 is about 8 inches in diameter and 1 inch wide. A caster marked 200 x 50 is about 200 mm in diameter and 50 mm wide, which is roughly 8 x 2 inches. Many casters have solid polyurethane or rubber tires moulded onto the wheel. In those cases, the size applies to the whole caster wheel rather than to a separate tire.

Mobility scooter tyre markings

Mobility scooter tyres use several formats. The final number after the dash is almost always the rim diameter in inches.

• 3.00-4: nominal section width of 3.00 inches on a 4-inch rim. With no aspect ratio given, the sidewall height is treated as roughly equal to the width. Retailers often list this size as 260 x 85 (overall diameter x width in millimetres) or 10 x 3 (inches).
• 9 x 3.50-4: overall diameter about 9 inches, section width about 3.50 inches, rim diameter 4 inches.
• 4.10/3.50-4: a tyre made to suit applications calling for either a 4.10 or a 3.50 section width, on a 4-inch rim.
• 120/90-10 (aspect-ratio format): section width 120 mm, aspect ratio 90 (the sidewall height is 90% of the width), rim diameter 10 inches.

Note the mixed units. Width may be in inches or millimetres, but rim diameter in these formats is in inches. The aspect ratio is a percentage, not a length.

Pressure, ply and other performance markings

Performance markings tell you how a tire may be used, not how big it is.

• Maximum or recommended pressure is shown in psi, bar or kPa. For conversion, 1 bar is about 14.5 psi, and 100 kPa equals 1 bar.
• Ply rating (PR), such as 4PR, is found on many scooter tyres. It is an indicator of load capacity and strength, not necessarily the actual number of layers.
• Maximum load may be printed in kilograms or pounds.
• A rotation arrow shows the direction the tire should turn when moving forwards.

What is the standard wheelchair wheel size?

The most common standard wheelchair wheel size for adult self-propelled manual wheelchairs is a 24-inch rear wheel, frequently fitted with a 24 x 1-3/8 (37-540) or 24 x 1 (25-540) tire. Other rear wheel sizes are also used:

• 20-inch and 22-inch rear wheels are used for children and smaller users, and on some lower seat heights.
• 25-inch and 26-inch rear wheels are used for taller users and on many active and sport chairs.
• Small rear wheels, often around 12 inches or smaller, are typical of attendant-propelled transit chairs.

Common front caster sizes range from about 3 to 8 inches. Larger casters, around 6 to 8 inches, are common on standard chairs. Smaller casters, around 3 to 5 inches, are typical on lightweight active chairs.

How wide are the wheels on a wheelchair?

Wheelchair rear tires are typically about 1 to 1-3/8 inches wide, or roughly 25 to 37 mm. A narrower tire, such as 25 mm (24 x 1), is common on lightweight and active chairs used mainly indoors and on smooth surfaces. A wider tire, such as 37 mm (24 x 1-3/8), is common on standard chairs. Wider off-road or all-terrain rear tires on 507 mm or 559 mm rims can be about 1.75 inches (around 47 mm) or more.

Front caster widths commonly run from about 1 inch to 2 inches (roughly 25 to 50 mm). Tire width is a property of the tire and wheel only. It is not the same as the overall width of the wheelchair, which depends on seat width, camber and push rims.

Wheelchair tire size chart

The table below lists common wheelchair tire sizes with their ETRTO equivalents. Overall diameter and circumference are calculated using the simplified assumption explained further below: bead seat diameter plus twice the tire width. Real tires vary by model and inflation, so treat these as comparison values, not measurements.

Inch designation	ETRTO size	Bead seat diameter	Nominal width	Calculated overall diameter	Calculated circumference	Typical use
20 x 1.75	47-406	406 mm	47 mm	500 mm (19.7 in)	1571 mm	Children's chairs
20 x 1-3/8	37-451	451 mm	37 mm	525 mm (20.7 in)	1649 mm	Children's and smaller chairs
22-inch (various)	Check sidewall	More than one in use	Varies	Varies	Varies	Smaller users, lower seat heights
24 x 1	25-540	540 mm	25 mm	590 mm (23.2 in)	1854 mm	Lightweight and active chairs
24 x 1-3/8	37-540	540 mm	37 mm	614 mm (24.2 in)	1929 mm	Standard adult chairs
24 x 1.75	47-507	507 mm	47 mm	601 mm (23.7 in)	1888 mm	Some active and all-terrain setups
25-inch or 26-inch label (manufacturer-dependent)	25-559	559 mm	25 mm	609 mm (24.0 in)	1913 mm	Active and sport chairs
26 x 1-3/8	37-590	590 mm	37 mm	664 mm (26.1 in)	2086 mm	Taller users

The table shows why the inch name is not a dependable guide. Under these calculations, a 24 x 1-3/8 tire (614 mm) comes out larger overall than a 25-559 tire (609 mm), and the two need completely different rims. For front casters, a simple size chart follows the marking itself: 4 x 1, 5 x 1, 6 x 1-1/4, 8 x 1 or 200 x 50 each describe outside diameter by width.

Wheelchair tire size conversion: inches, millimetres and ETRTO

To convert wheelchair tire sizes, use the ETRTO number as the common reference and convert units by dividing millimetres by 25.4 to get inches. You cannot reliably convert an inch name like 24-inch into a rim size by arithmetic, because the name is nominal. Instead, look up or read the ETRTO bead seat diameter.

1. Find the ETRTO size on the sidewall. It is the marking with two numbers joined by a hyphen, such as 37-540. If it is worn off, look for a marking on the rim.
2. Keep the bead seat diameter fixed. Any replacement tire must have the same second number, for example 540.
3. Choose a width the rim can accept. The first number may change, for example from 37 to 25. The rim's inner width limits how narrow or wide a tire it can safely hold, so check the rim and tire manufacturers' compatibility guidance.
4. Convert units only where needed. For example, 37 mm divided by 25.4 is about 1.46 inches, and 540 mm divided by 25.4 is about 21.3 inches. The 540 mm rim is still called a 24-inch wheel, because the name refers to the approximate outside diameter with a tire fitted.
5. Check clearances and settings. A different width or overall diameter can affect frame clearance, mudguards and wheel lock position.

For casters and scooter tyres, conversion is more direct. A 200 x 50 mm caster is about 7.9 x 2.0 inches. A scooter tyre listed as 260 x 85 mm is about 10.2 x 3.3 inches. The rim size, however, still has to match exactly.

How do width, aspect ratio and rim diameter change overall diameter?

The overall diameter of a tire equals the rim diameter plus two sidewall heights, and circumference equals overall diameter multiplied by pi (about 3.1416). Width affects overall size only through the sidewall height. The sidewall height depends on the width and, where stated, the aspect ratio.

The basic formulas

• Sidewall height = width x aspect ratio / 100. For bicycle-type wheelchair tires and for scooter tyres with no stated ratio, such as 3.00-4, the sidewall height is roughly treated as equal to the width.
• Overall diameter = rim diameter + 2 x sidewall height. Use the same unit for every value; 1 inch = 25.4 mm.
• Circumference = overall diameter x 3.1416. This is the distance covered in one wheel revolution.

Example: two wheelchair rear tires on the same 540 mm rim

Compare a 25-540 tire (24 x 1) with a 37-540 tire (24 x 1-3/8):

• 25-540: 540 + 2 x 25 = 590 mm overall; circumference about 1854 mm.
• 37-540: 540 + 2 x 37 = 614 mm overall; circumference about 1929 mm.

The wider tire is about 24 mm larger in calculated diameter, which means about 12 mm more height at the axle-to-ground radius. It covers about 75 mm more distance per revolution. On the same rim, then, changing only the width changes the overall size of the wheel.

Example: aspect ratio on a scooter tyre

Compare 120/90-10 with a hypothetical wider 130/70-10 on the same 10-inch (254 mm) rim:

• 120/90-10: sidewall = 120 x 0.90 = 108 mm; overall = 254 + 216 = 470 mm; circumference about 1477 mm.
• 130/70-10: sidewall = 130 x 0.70 = 91 mm; overall = 254 + 182 = 436 mm; circumference about 1370 mm.

In this example the wider tyre is actually 34 mm smaller in diameter, and it covers about 7% less distance per revolution, because its lower aspect ratio gives a shorter sidewall. A bigger first number does not mean a bigger tyre overall.

Example: three-number scooter sizes

In a size such as 9 x 3.50-4, the aspect ratio can be worked backwards. The overall diameter is 9 inches and the rim is 4 inches, so the two sidewalls together are 5 inches, or 2.5 inches each. Dividing 2.5 by the 3.50-inch width gives an aspect ratio of about 71%. In the same way, 13 x 5.00-6 gives sidewalls of 3.5 inches each and a ratio of about 70%.

Calculated size vs actual tire vs fitment

A calculated diameter is an estimate from nominal markings. The actual tire dimension is what you measure, and fitment is whether the tire works safely on your specific chair or scooter. These three things can differ.

• Calculated geometry uses nominal numbers. For example, 3.00-4 calculates to about 254 mm (10.0 inches) overall, yet the same size is commonly listed as 260 x 85 mm. The difference comes from tread profile and real construction.
• Actual dimensions vary with tire brand, tread, inflation pressure, rim width and whether the tire is pneumatic, foam-filled or solid. Two tires with identical markings can measure a few millimetres apart.
• Fitment compatibility requires an exact rim or bead seat match, a width suited to the rim, and enough clearance at the frame, fork, mudguards and brakes. For casters, it also depends on the bearing size, axle bolt and fork width, which the diameter-by-width marking does not show.

A change in overall diameter has practical knock-on effects. On a manual wheelchair, wheel locks are positioned for a particular tire diameter and may need adjustment after a size or width change. On a scooter, a different overall diameter changes the distance travelled per revolution and the ground clearance. When a size change rather than a like-for-like replacement is being considered, the chair or scooter manufacturer's specifications are the reference for what is permitted.

How to identify the tire size you have

The simplest way to identify a wheelchair or scooter tire size is to copy every size marking from the sidewall, with the ETRTO size first. If the markings are unreadable, measure carefully.

1. Read all sidewall markings, including the inch size, ETRTO size, pressure and any rim marking.
2. If unreadable, measure the overall diameter of the inflated tire across its centre, in millimetres or inches.
3. Measure the tire width at its widest point.
4. Check the rim for a stamped size, such as 19-540. Alternatively, with the tire removed, measure the bead seat diameter of the rim.
5. Match your figures to a chart, remembering that measured overall diameter is only an approximation. The bead seat diameter is the figure that must match exactly.