How to Measure Bicycle Tire Size: From Sidewall Markings to Mounted Width

The most reliable way to find a bicycle tire's size is to read the ETRTO size on the sidewall, a pair of numbers such as 25-622 or 57-622. The first number is the nominal tire width in millimetres. The second is the bead seat diameter (BSD) in millimetres, which is the rim diameter the tire fits. If you need the tire's real width, mount it on your rim, inflate it to riding pressure and measure across the widest point with calipers. If the markings are worn off, you can work out the size from the rim's markings or from measurements of the tire and wheel.

This article explains how to read sidewall size markings, how to measure tire width on a road bike or mountain bike, and what calculated diameter and circumference can and cannot tell you.

How do I measure a bike tire size?

To identify a bike tire size, start with the markings and only measure when you need to confirm or replace missing information. The basic order is:

1. Find the ETRTO size on the sidewall. Look for two numbers joined by a hyphen, such as 28-622 or 54-559. This is the most precise size label on the tire.
2. Note the traditional size as well. You will usually also see a French size such as 700x28c or an inch size such as 29 x 2.25. These names are useful for shopping but less precise than ETRTO.
3. Check the rim. Many rims carry a marking such as 622x19, meaning a 622 mm bead seat diameter and 19 mm internal rim width.
4. Measure the mounted width if it matters. For frame clearance or comparison between tires, measure the inflated tire on your own wheel with calipers.
5. Measure diameter or circumference if needed. For a bike computer or a rough check of overall size, use a roll-out measurement rather than relying only on the printed size.

What do the numbers on a bicycle tire sidewall mean?

The numbers on a bicycle tire sidewall fall into two groups. Size markings describe dimensions: width and rim diameter. Other markings are ratings and specifications, such as pressure range, casing thread count and tubeless compatibility. Only the size markings tell you whether a tire matches a rim diameter.

ETRTO size: width and bead seat diameter

The ETRTO size, also standardised as ISO 5775, is written as width-diameter in millimetres. For example, 25-622 means a nominal width of 25 mm on a rim with a 622 mm bead seat diameter.

• Width (first number): the nominal width of the inflated tire, measured by the manufacturer on a reference rim. Your mounted width may be different.
• Bead seat diameter (second number): the diameter of the rim at the point where the tire bead sits. This number must match the rim exactly. A 622 mm tire will not fit a 584 mm rim, and vice versa.

ETRTO sizing removes most of the ambiguity of older naming systems. Two tires with the same BSD fit the same rim diameter, even if one is labelled in inches and the other in the French system.

French sizes: 700x25c, 650b and similar

French sizes such as 700x25c combine a nominal outside diameter (700 mm), a width (25 mm) and a letter. The letter originally marked a width class. Today, 700c effectively means a 622 mm bead seat diameter, and 650b means 584 mm. The 700 is a historical name rather than the real outside diameter of a modern tire.

Inch sizes: 29 x 2.25, 27.5 x 2.4, 26 x 1.75

Inch sizes such as 29 x 2.25 give a nominal wheel size and a width in inches. A 29-inch mountain bike tire uses the same 622 mm bead seat diameter as a 700c road tire. A 27.5-inch tire uses 584 mm, the same as 650b. A modern 26-inch mountain bike tire uses 559 mm.

Inch sizes have one well-known trap: a decimal width and a fractional width with the same nominal diameter can fit different rims. For example, a 26 x 1.75 tire uses a 559 mm bead seat diameter, while a 26 x 1 3/8 tire uses 590 mm. Always confirm the ETRTO number when the inch size uses a fraction.

Inch widths do not always convert exactly to the ETRTO width printed on the same tire. When the two disagree, use the ETRTO number.

Common bicycle tire size equivalents

Some names, especially 20-inch and 16-inch, are used for more than one bead seat diameter. In those cases, the ETRTO number is the only dependable guide.

Other sidewall markings: ratings, not dimensions

Several other sidewall markings describe how the tire should be used rather than its size:

• Pressure range: usually given in psi, bar or kPa, for example 60–100 psi. 1 bar is about 14.5 psi, and 100 kPa equals 1 bar. Treat the maximum as a limit, not a target. The rim may also have its own maximum, and the lower of the two applies.
• TPI (threads per inch): describes how dense the casing fabric is. It is a construction detail, not a size, and TPI figures are not always directly comparable between brands.
• Tubeless markings (such as TLR or Tubeless Ready): show whether the tire is designed to run without an inner tube on a compatible rim.
• Hookless compatibility: some tires state whether they are approved for hookless rims. Hookless rims often carry pressure limits and minimum tire widths, so check both the tire and rim specifications.
• Rotation arrow: shows the intended rolling direction. Some tires have separate arrows for front and rear use.
• E-bike ratings: some tires carry markings showing approval for electrically assisted bikes or speed pedelecs. These are load and speed approvals, not size information.

Dimensions, ratios, units and ratings: what each tells you

Bicycle tire markings are easier to compare once you know which type of value you are looking at.

How to measure tire width on a bike

To measure bicycle tire width, use calipers on the inflated tire mounted on the rim you will actually ride. Mounted bicycle tire width is the number that matters for frame and fork clearance. It often differs from the printed width.

1. Mount the tire on your own wheel. The internal rim width affects the result, so measuring on a different wheel gives a different number.
2. Inflate to your normal riding pressure. Higher pressure makes the tire slightly larger.
3. Let the tire settle. New tires commonly stretch a little after mounting, so measuring after the tire has been inflated for a day or so gives a more representative number.
4. Close the caliper jaws on the widest part of the tire. This is usually above the rim, not at the rim edge. Hold the calipers square to the tire, without squeezing the casing.
5. Measure at several points around the wheel. Take three or four readings and use the average, because casings are not perfectly uniform.
6. Compare the result with the rim width. If the rim is wider than the tire at any point, as can happen with some wide road rims, note both numbers when checking clearance.

Without calipers, you can use a ruler held against two flat objects placed on either side of the tire, but the result is less precise.

How to measure tire width on a road bike

Measuring tire width on a road bike follows the same steps, but small differences matter more. Road bike tires are often only a few millimetres apart in nominal size, and road frames often have limited clearance. Measure the widest point of the casing on your own wheel at riding pressure. Then check the gap between the tire and the frame, fork and brake calipers, because the tire's height also affects clearance.

Mountain bike and gravel tires: casing versus knob width

On knobby tires, the casing width and the width across the outer knobs can differ. Measure both if clearance is tight. The knob width is what will touch the frame, while the casing width is closer to what the manufacturer means by the nominal size.

Why the mounted width differs from the sidewall number

The width printed on the sidewall is a nominal value measured by the manufacturer on a reference rim. Your mounted width can be larger or smaller for several reasons:

• Internal rim width: the distance between the inside edges of the rim walls. A wider internal rim spreads the beads further apart and usually makes the same tire measure wider. A narrower rim usually makes it narrower.
• Inflation pressure: higher pressure slightly increases the size of the casing.
• Casing stretch: new tires may grow a little after mounting.
• Manufacturer differences: tires with the same nominal size from different brands, or even different models from one brand, can measure differently.

To measure internal rim width, use the inside jaws of the calipers between the inner edges of the rim walls, or read it from the rim marking. In a marking such as 622x19, the 19 is the internal width in millimetres. Rim and tire manufacturers publish recommended ranges for tire width on each internal rim width. Staying within those ranges matters more than matching any single number.

How to measure tire size when the markings are worn

If the sidewall markings cannot be read, you can identify the bicycle tire size from the rim and from simple measurements.

1. Check the rim for an ETRTO marking. Many rims show a size such as 584x25 on a sticker or engraved near the valve hole. The first number is the bead seat diameter.
2. Measure the tire width with calipers, using the steps above.
3. Measure the outside diameter. With the tire inflated, measure from the ground to the top of the tire with the wheel upright and unloaded, or measure across the tire from edge to edge through the axle.
4. Estimate the bead seat diameter. Subtract twice the tire height from the outside diameter. For most bicycle tires, height is close to width, so outside diameter minus two times width gives a rough figure.
5. Match to the nearest standard BSD. Compare your estimate with standard values such as 622, 584, 559, 507 or 406 mm. Because the estimate is rough, confirm it with the rim marking or a known-good tire before buying.

How width, aspect ratio and rim diameter affect tire diameter and circumference

The overall diameter of a bicycle tire depends on two things: the bead seat diameter and the height of the tire above the bead seat. A larger BSD, or a taller tire, makes the wheel bigger. Circumference follows directly from diameter.

What aspect ratio means for bicycle tires

Aspect ratio is the tire's height divided by its width. Car tires print it as part of the size, for example the 45 in 225/45R17. Bicycle tires do not print an aspect ratio. A typical inflated bicycle tire has a roughly round cross-section, so its height is close to its width. For estimates, it is common to treat the height as equal to the width, while remembering that the real height can be somewhat less, depending on the tire and rim.

Simple formulas for calculated geometry

• Tire height (sidewall height) ≈ tire width, in mm.
• Outside diameter ≈ bead seat diameter + 2 × tire height, in mm.
• Circumference ≈ π × outside diameter, in mm, where π ≈ 3.1416.

Example: a 25-622 road tire gives an outside diameter of about 622 + 2 × 25 = 672 mm, and a circumference of about 3.1416 × 672 ≈ 2111 mm.

Worked comparisons

These figures show some practical points:

• Width changes diameter by twice the height difference. Going from 25 mm to 32 mm on the same 622 mm rim adds about 7 mm of height on each side, so about 14 mm of diameter and about 44 mm of circumference.
• Rim diameter has a bigger effect than modest width changes. The 29 x 2.25 tire is about 64 mm larger in diameter than the 700x25c, even though both use a 622 mm bead seat diameter, because the mountain bike tire is so much taller.
• A wide tire on a smaller rim can match a narrow tire on a larger rim. The 26 x 2.1 mountain bike tire comes out close to the 700x25c road tire in estimated outside diameter, despite a 63 mm smaller BSD.

How to measure real circumference with a roll-out

A roll-out measures the actual rolling circumference of a bicycle tire, which is more accurate than a calculation for setting up a bike computer.

1. Inflate the tire to riding pressure.
2. Turn the wheel so the valve is at the bottom and mark the floor at that point.
3. Sit on the bike, or apply your normal weight, and roll forward in a straight line for exactly one wheel revolution, until the valve is at the bottom again.
4. Mark the floor again and measure the distance between the two marks in millimetres.

The roll-out result is usually a little smaller than the calculated circumference, because the tire flattens under load and its real height is often less than its width.

Calculated geometry versus actual dimensions and fitment

A calculated bicycle tire diameter is an estimate, an actual measurement is a fact about one tire on one rim, and fitment is a separate question again.

• Calculated geometry uses the nominal ETRTO numbers and assumes height equals width. It is useful for comparing sizes and seeing how changes in width or rim diameter move the overall diameter.
• Actual dimensions depend on the specific tire model, the internal rim width, the pressure and casing stretch. Only measuring the mounted tire tells you these.
• Fitment compatibility depends on the bike: the tire's BSD must match the rim, the tire width should fall within the rim's recommended range, and the mounted tire needs enough clearance from the frame, fork, brakes and any fenders. A size comparison on paper does not confirm any of these.

In practice, use the ETRTO size to confirm rim compatibility, use simple calculations to understand how much bigger or smaller a different size will be, and use calipers and a roll-out to find out what the tire actually measures on your bike.