25-540. 37-540. 25-559. 25-590. What?

If you've ever looked at the side of a wheelchair tire and wondered what all those numbers mean, you're definitely not alone.

Wheelchair tire sizing can be confusing because the familiar inch size, such as 24", doesn't always give you enough information to choose the correct replacement tire.

Let's Decode the Numbers

Take this common wheelchair tire size:

25-540

The first number represents the nominal tire width in millimeters.

25 = approximately 25 mm wide

The second number identifies the bead seat diameter, or BSD, in millimeters.

540 = 540 mm bead seat diameter

That second number is especially important because it identifies the rim diameter the tire is designed to fit.

Same Rim, Different Tire Width

For example:

25-540 = 24" × 1"

37-540 = 24" × 1 3/8"

Both use a 540 mm bead seat diameter, but they have different tire widths.

So while the tires aren't the same width, that 540 tells you they're designed to fit the same 540 mm rim diameter.

Don’t Order by “24-Inch” Alone

Here's where wheelchair tire sizing gets tricky.

The inch measurement is a familiar way of describing the tire, but it isn't precise enough by itself.

For example:

24" × 1" → 25-540

25" × 1" → 25-559

26" × 1" → 25-590

Notice how the first number stays 25, while the second changes from 540 → 559 → 590.

Those tires are approximately the same width, but they're designed for different rim diameters.

That means you shouldn't assume two tires will fit the same wheel just because the inch size sounds similar.

What About Inner Tubes?

Inner tubes have sizing information too.

When replacing a tube, check the size range printed on the existing tube or tire and make sure the replacement is intended for the correct tire diameter and width.

You'll also want to check the valve type, because not every wheel uses the same valve configuration.

Where Do I Find My Tire Size?

Start with the sidewall of your existing tire.

Look for both the traditional inch measurement and the metric/ISO-style designation when available.

For example, you might see something like:

24 × 1 (25-540)

If the markings are worn or difficult to read, take clear photos of anything that's still visible before removing or discarding the tire.

Before Ordering a Wheelchair Tire

Check:

• Full tire size printed on the sidewall
• ISO/ETRTO-style number
• Tire width
• Rim diameter
• Pneumatic or solid tire
• Valve type if using an inner tube
• Wheelchair make and model

Most importantly, don't assume another tire will fit simply because the inch size sounds the same.

Know the Numbers Before You Order

Those numbers on your wheelchair tire aren't random.

For 25-540:

25 = tire width

540 = bead seat diameter

Once you know how to read them, identifying replacement wheelchair tires becomes much easier.

Check the sidewall. Match the numbers. Know your wheel before you order.