Motorcycle Inner Tube Size Conversion Chart: Reading Tire Markings and Matching the Right Tube

The right motorcycle inner tube has the same rim diameter as the tire and a width range that includes the tire's width. You find both on the tire sidewall. In a size such as 110/90-18, the 18 means an 18-inch rim, so the tube must be an 18-inch tube. The 110 is the tire width in millimetres, so the tube's printed range must include 110 mm or its inch equivalent, for example a tube labelled 4.00/4.50-18 or 110/90-18 to 120/90-18. The valve type must also suit the rim. Load index and speed rating describe the tire. They do not change which tube size you need.

What size motorcycle inner tube do I need?

Match the inner tube to the tire size printed on the sidewall. Rim diameter must match exactly. Width only needs to fall within the tube's printed size range.

1. Read the full tire size on the sidewall. Examples are 90/90-21, 3.00-18, 120/80-17 or MT90-16. Write down the whole code, including any letters.
2. Confirm the tire is meant to run with a tube. Look for TT (tube type). TL means tubeless, and a tube in a tubeless tire is a matter for the tire and wheel maker's guidance.
3. Identify the rim diameter. This is the last number in the size, in inches: 21, 19, 18, 17, 16 and so on. The tube must carry the same number.
4. Identify the tire width. This is the first number. It is in millimetres for metric sizes and in inches for inch sizes.
5. Check the tube label. The tube's printed width range should include your tire's width, in either metric or inch form, at the same rim diameter.
6. Match the valve stem. Compare the valve code or shape with the old tube, and check that it suits the valve hole in your rim.

If the tire size and the tube range agree on rim diameter and the width falls inside the range, the tube is the correct size for that tire.

Motorcycle tire size markings explained

A motorcycle tire sidewall carries several kinds of information:

• Dimensions: width and rim diameter.
• A ratio: aspect ratio.
• Construction and use codes: such as -, B, R, TT, TL and M/C.
• Performance ratings: load index and speed symbol.

Only the dimensions decide inner tube size.

Metric tire sizes (for example 120/80-17 or 110/90-18)

A metric motorcycle tire size has three parts:

• 120 is the nominal section width in millimetres. This is the width of the inflated tire at its widest point on the specified measuring rim.
• 80 is the aspect ratio. It is the sidewall height as a percentage of the width, so 80 means the sidewall is 80% as tall as the tire is wide. It is a ratio, not a measurement in millimetres.
• 17 is the rim diameter in inches.

Metric motorcycle sizes mix units: width is in millimetres, while rim diameter is in inches. Keep this in mind when you calculate anything.

Inch tire sizes (for example 3.00-18 or 2.75-21)

An inch tire size gives the nominal section width in inches, followed by the rim diameter in inches. In 3.00-18, the tire is about 3.00 inches wide (76.2 mm) and fits an 18-inch rim. Inch sizes do not print an aspect ratio. This is why converting them to metric sizes is approximate rather than exact.

Alpha-numeric sizes (for example MT90-16)

Some tires, mainly on cruisers, use an alpha-numeric code:

• The first M stands for motorcycle.
• The second letter is a width code.
• The number is the aspect ratio.
• The final number is the rim diameter in inches.

MT90-16 is commonly listed as equivalent to 130/90-16. Because the width is coded by a letter, use the tire or tube maker's equivalence table rather than guessing the width. The rim diameter can still be read directly: 16 means a 16-inch tube.

Construction and tube-type letters

• - (dash) usually indicates bias-ply construction.
• B indicates bias-belted construction.
• R indicates radial construction, as in 120/70R17.
• TT means tube type: the tire is designed to be used with an inner tube.
• TL means tubeless.
• M/C marks a motorcycle tire.
• Reinf or Reinforced indicates a reinforced construction for higher load.

None of these letters changes the tube's size. Some tube makers do state whether a tube is suitable for radial tires, so read the tube packaging if your tire is radial.

Load index and speed rating

The load index and speed symbol are performance ratings, not dimensions. In a marking such as 58H:

• 58 is a load index code. It corresponds to the maximum load the tire can carry at its specified pressure.
• H is a speed symbol, meaning 210 km/h. Other examples are S = 180 km/h, T = 190 km/h, V = 240 km/h and W = 270 km/h.

These ratings let you compare tire capability. They do not affect which tube diameter or width range you need.

Marking (example)	Type of information	Unit	Matters for tube size?
120 in 120/80-17	Section width	Millimetres	Yes: must be inside the tube's width range
80 in 120/80-17	Aspect ratio	Percent of width	Indirectly: tube ranges are often printed with the aspect ratio
17 in 120/80-17	Rim diameter	Inches	Yes: must match exactly
3.00 in 3.00-18	Nominal section width	Inches	Yes: must be inside the tube's width range
TT / TL	Tube type or tubeless	Code	Tells you whether the tire is designed for a tube
-, B, R	Construction	Code	No (but check tube suitability for radials)
58H	Load index and speed symbol	Code	No

Motorcycle inner tube sizes explained

A motorcycle inner tube label usually lists one rim diameter and a range of tire widths. A tube is a flexible bag that expands to fill the inside of the tire. One tube can therefore serve several neighbouring tire widths, but only one rim diameter.

How tube labels are written

Tube labels typically combine several widths with a single rim size. Common formats include:

• Inch range: for example 2.75/3.00-21 or 4.00/4.50-18. The tube covers tires from roughly 2.75 to 3.00 inches wide on a 21-inch rim.
• Metric range: for example 110/90-18 to 120/90-18.
• Mixed listing: inch and metric sizes printed together, plus a valve code such as TR4 or TR6.

Ranges differ between manufacturers, so the printed label on the specific tube is the reference to trust.

Why rim diameter must match exactly

The rim diameter on the tube must equal the rim diameter on the tire because the tube's circumference is built for that wheel size.

• A 17-inch tube in an 18-inch tire would be stretched around its circumference.
• A 19-inch tube in an 18-inch tire would have surplus material that can fold or crease.

Rim diameter is the one dimension where no range applies.

What the width range means

The width range describes how much tire volume the tube is designed to fill.

• Tube too narrow for the tire: it stretches more and its wall becomes thinner.
• Tube too wide for the tire: it can wrinkle or fold inside the tire, which can cause chafing.

Staying within the printed tube size range avoids both problems.

Valve stem compatibility

Valve stem compatibility is separate from size. A tube can be the right size and still have the wrong valve for your rim. Common motorcycle valve codes include:

• TR4 and TR6: straight valves.
• TR87: an angled valve, used where a straight valve is hard to reach.

Compare the code, valve angle and stem length with the old tube, and confirm the stem fits the rim's valve hole.

Motorcycle inner tube size conversion chart

Converting an inch tire width to millimetres is simple arithmetic: multiply by 25.4. Matching an inch size to a metric size is only approximate, because inch sizes do not state an aspect ratio. The chart below shows two things for common inch widths:

• The exact unit conversion.
• The metric sizes commonly cited as approximate equivalents.

Use it to recognise which sizes belong together. Then confirm with the range printed on the tube.

Inch width	Exact conversion (inches × 25.4)	Commonly cited approximate metric equivalents
2.75	69.9 mm	80/90 or 70/100
3.00	76.2 mm	90/90 or 80/100
3.25	82.6 mm	100/90
3.50	88.9 mm	100/90 or 110/80
4.00	101.6 mm	110/90 or 120/80
4.10	104.1 mm	110/90
4.50	114.3 mm	120/90 or 130/80
5.10	129.5 mm	130/80 or 140/80

The commonly cited metric equivalents are wider than the exact conversion. For example, 3.00 inches equals 76.2 mm, yet 3.00 is often grouped with 90/90. Inch widths are nominal size classes, and both tire and tube makers group sizes by practical fit rather than by strict unit conversion. Equivalents also vary between brands. This chart works only within the same rim diameter: a 3.00-18 tube relates to 90/90-18 tires, never to 90/90-17 or 90/90-21.

How width, aspect ratio and rim diameter change tire geometry

Three values set a tire's calculated size:

• Width and aspect ratio together determine sidewall height.
• Sidewall height and rim diameter together determine overall diameter.
• Overall diameter determines circumference.

With three formulas you can compare metric motorcycle tire sizes on paper.

Formulas for metric motorcycle tire sizes

• Sidewall height (mm) = width (mm) × aspect ratio ÷ 100
• Rim diameter (mm) = rim diameter (inches) × 25.4
• Overall diameter (mm) = rim diameter (mm) + 2 × sidewall height (mm)
• Circumference (mm) = overall diameter (mm) × π (about 3.1416)

Sidewall height counts twice in the overall diameter, once above the rim and once below. Small changes in width or aspect ratio therefore have twice the effect on diameter.

How to calculate motorcycle tire tube size geometry step by step

A motorcycle tire tube size calculator does the following steps. You can do them by hand for a size such as 120/70-17:

1. Multiply width by aspect ratio: 120 × 70 ÷ 100 = 84 mm sidewall height.
2. Convert rim diameter to millimetres: 17 × 25.4 = 431.8 mm.
3. Add two sidewalls: 431.8 + 2 × 84 = 599.8 mm overall diameter.
4. Multiply by π: 599.8 × 3.1416 ≈ 1,884 mm circumference.
5. Repeat for the size you want to compare, then subtract one result from the other. To express the difference as a percentage, divide it by the original value and multiply by 100.

Worked examples on a 17-inch rim

Tire size	Sidewall height	Calculated overall diameter	Calculated circumference	Diameter difference vs 120/70-17
120/70-17	84 mm	599.8 mm	≈ 1,884 mm	Reference
110/80-17	88 mm	607.8 mm	≈ 1,909 mm	+8 mm (≈ +1.3%)
130/70-17	91 mm	613.8 mm	≈ 1,928 mm	+14 mm (≈ +2.3%)
120/80-17	96 mm	623.8 mm	≈ 1,960 mm	+24 mm (≈ +4.0%)

The examples show three effects:

• Raising the aspect ratio from 70 to 80 at the same 120 mm width adds 12 mm to each sidewall and 24 mm to the overall diameter.
• Raising the width from 120 to 130 mm at the same 70 aspect ratio also makes the tire taller, because the aspect ratio is a percentage of the width.
• A narrower tire can still be taller: 110/80-17 is narrower than 120/70-17 but calculates slightly taller, because its higher aspect ratio more than compensates.

The percentage change in circumference equals the percentage change in diameter. A tire that is 4% larger in diameter covers about 4% more distance per revolution.

Calculating inch sizes requires an assumption

Inch tire sizes do not state an aspect ratio, so any diameter calculation for them depends on an assumed profile. For example, if you assume a 3.00-18 tire is as tall as it is wide (100% profile):

• Sidewall height: 76.2 mm.
• Rim diameter: 18 × 25.4 = 457.2 mm.
• Overall diameter: 457.2 + 2 × 76.2 = 609.6 mm.

A 90/90-18 tire calculates to 457.2 + 2 × 81 = 619.2 mm. That 9.6 mm gap shows the limit of the comparison: change the assumed profile and the result changes too. Treat inch-size geometry as a rough estimate. Use published tire dimensions when the comparison matters.

Calculated geometry vs actual dimensions vs vehicle fitment

Calculated tire geometry is a nominal estimate, not a measurement. It does not confirm that a tire fits a motorcycle.

Why actual tire dimensions differ from the calculation

The width and aspect ratio in a tire size are nominal values. Real overall diameter and width vary with several factors:

• Brand and tread pattern.
• Construction (bias or radial).
• Tread depth, which changes as the tire wears.
• The width of the rim the tire is mounted on.
• Inflation pressure.

Two tires of the same marked size from different makers can measure differently. Many tire manufacturers publish measured width and diameter for each size. Those figures are more reliable than any calculation.

Why a matching size calculation is not a fitment approval

Two tire sizes can calculate to a similar diameter and still differ in ways the numbers do not show:

• Clearance to the swingarm, fork, fender or chain.
• The tire's approved rim width range.
• Load and speed ratings.
• Handling characteristics.

The motorcycle maker's specified sizes and the tire maker's approvals determine fitment. Calculated geometry only helps you understand what a size difference means.

What an inner tube does and does not change

The inner tube follows the tire's shape. It does not change the tire's diameter, width or circumference. Choosing a tube from the upper or lower end of its printed range has no effect on tire geometry. What matters is that the tire's width falls within the range and the rim diameter matches exactly. If you change tire size, choose the tube for the new tire size, not the old one.

Motorcycle inner tube size guide: quick reference points

• Rim diameter: the last number in the tire size, in inches. The tube must match it exactly.
• Tire width: the first number, in millimetres for metric sizes or inches for inch sizes. It must fall inside the tube size range.
• Aspect ratio: sidewall height as a percentage of width. It affects tire height and is often printed in tube ranges, but the tube is chosen mainly by width and rim diameter.
• Metric and inch conversions: multiplying by 25.4 gives exact unit conversion. Size equivalence between inch and metric tires is approximate and varies by brand.
• Load index and speed rating: tire performance ratings that do not affect tube size.
• Valve stem: a separate compatibility check against the rim and the old tube.