Bicycle Tire Air Volume: Estimation and Calculator

Bicycle tire air volume is the space inside an inflated tire, usually given in liters. You can estimate it from the two numbers in the tire's ETRTO size, the width and the bead seat diameter, by treating the tire as a torus (a ring with a round cross-section). With this method, a 700×25C road tire (25-622) holds about 1.0 liter, a 700×40C gravel tire (40-622) about 2.6 liters, a 29×2.4 mountain bike tire (61-622) about 6.3 liters and a 26×4.0 fat bike tire about 16 liters. These figures are geometric approximations. Real internal volume depends on rim width, casing thickness and how the tire actually inflates.

How much air does a bicycle tire hold?

A bicycle tire's internal volume ranges from about 1 liter for narrow road tires to more than 15 liters for fat bike tires. Volume rises roughly with the square of the tire width, so width matters much more than wheel diameter. Going from a 25 mm to a 28 mm road tire on the same 622 mm rim adds about 26% volume. Going from a 27.5-inch to a 29-inch wheel at the same 2.4-inch width adds only about 6%.

Volume is not the same as the amount of air. A tire's volume in liters stays roughly fixed once it is inflated. The amount of air packed into that space depends on pressure. This difference is explained further below.

Reading the numbers on the tire sidewall

A bicycle tire sidewall usually shows a size in one or more systems, a pressure range and several ratings that describe construction or intended use. Only the size numbers are needed to estimate air volume. The other markings help you understand what the size numbers do not tell you.

ETRTO / ISO size: the most useful marking

The ETRTO size, also standardized as ISO 5775, is written as two numbers in millimeters, for example 28-622 or 57-622.

• First number, for example 28: the nominal tire width in millimeters. This is the approximate inflated cross-section width on a suitable rim.
• Second number, for example 622: the bead seat diameter (BSD) in millimeters. This is the diameter of the rim where the tire bead sits. It is the rim diameter that must match exactly.

These two values are the inputs for a bicycle tire air volume calculation. The ETRTO size is also the reliable way to compare tires, because the same BSD appears under several different inch and traditional names.

Inch and French sizes: names, not measurements

Inch sizes such as 29×2.25, 27.5×2.4 and 26×2.10, and traditional sizes such as 700×28C or 650B, are size names rather than precise measurements.

• 700C, 28-inch and 29-inch wheels all use the 622 mm bead seat diameter. A 700C tire is not 700 mm in diameter when measured.
• 27.5-inch and 650B both refer to the 584 mm bead seat diameter.
• 26-inch decimal mountain bike sizes such as 26×2.10 use a 559 mm BSD. Some older fractional 26-inch sizes, such as 26×1-3/8, use a different BSD (590 mm) and are not interchangeable.
• The second inch number, for example 2.25, is the nominal width in inches. Multiply by 25.4 to get millimeters: 2.25 in ≈ 57 mm, 2.4 in ≈ 61 mm.

Pressure markings: ratings, not dimensions

Pressure markings give the minimum and/or maximum inflation pressure set by the manufacturer, usually in bar, psi and sometimes kPa. Conversions: 1 bar = 100 kPa ≈ 14.5 psi. These values are limits for the tire, not part of its geometry, and the rim may have its own, sometimes lower, maximum pressure. Pressure does not change the calculated volume much. It changes how much air is stored in that volume.

Other ratings and construction markings

Other common sidewall markings describe construction or intended use and are not dimensions:

• TPI (threads per inch): the density of the casing fabric. It relates to casing feel and weight, not to size.
• Tubeless Ready, TLR or similar: the tire is designed for tubeless setup on a compatible rim.
• E-bike markings: some tires state suitability for e-bikes or a speed category.
• Direction arrows and compound names: mounting direction and rubber type.

Bicycle tires generally do not carry an aspect ratio, unlike car tires such as 205/55 R16. Bicycle tire cross-sections are close to round, which is why the width alone can be used to estimate height and volume.

Marking example	Type	What it means	Use for volume estimate
28-622	Dimension (mm)	28 mm nominal width, 622 mm bead seat diameter	Yes: both numbers are inputs
700×28C	Size name	Traditional road size, equivalent to 28-622	Only after converting to ETRTO
29×2.25	Size name (inches)	622 mm BSD, about 57 mm nominal width	Only after converting to ETRTO (57-622)
Max 5 bar / 73 psi	Performance rating	Upper inflation limit for the tire	No; affects amount of air, not volume
120 TPI	Construction rating	Casing thread density	No
Tubeless Ready	Construction rating	Designed for tubeless setup	No

Bicycle tire air volume calculator: the torus formula

A bicycle tire air volume calculator models the inflated tire as a torus. The torus has a circular cross-section with a diameter equal to the tire width. That circle sits on the rim, so the ring's centerline lies half a tire width above the bead seat.

Formula (all lengths in millimeters):

• Cross-section radius: r = width ÷ 2
• Centerline radius: R = BSD ÷ 2 + r
• Volume: V = 2π² × R × r² (in mm³)
• Liters: V (L) = V (mm³) ÷ 1,000,000

The same formula can be read as cross-section area (π × r²) multiplied by the length of the ring's centerline (2π × R). In short form, V (L) ≈ 19.74 × R × r² ÷ 1,000,000, with R and r in millimeters.

How to calculate bicycle tire air volume step by step

1. Find the ETRTO size on the sidewall, for example 28-622. If only an inch size is printed, convert it: width in inches × 25.4 = width in mm, and look up the BSD for the wheel size (622 for 700C/29, 584 for 27.5/650B, 559 for 26-inch decimal sizes).
2. Halve the width to get r. For 28 mm, r = 14 mm.
3. Add r to half the BSD to get R. For 622 mm, R = 311 + 14 = 325 mm.
4. Calculate the cross-section area: π × 14² ≈ 615.8 mm².
5. Calculate the centerline circumference: 2π × 325 ≈ 2,042 mm.
6. Multiply: 615.8 × 2,042 ≈ 1,257,000 mm³.
7. Divide by 1,000,000: about 1.26 liters. If needed, 1 liter ≈ 61 cubic inches, so this is about 77 in³.

For a better estimate, measure the actual inflated width of your tire on your rim with calipers and use that value instead of the nominal width.

Volume of air in bike tyre by size

The table below lists the estimated volume of air in a bike tyre for common sizes, calculated with the torus formula from the nominal ETRTO width. Values are rounded geometric estimates, not manufacturer measurements.

Common size name	ETRTO	Estimated volume (liters)
700×25C road	25-622	≈ 1.0
700×28C road	28-622	≈ 1.26
700×32C road / touring	32-622	≈ 1.65
700×40C gravel	40-622	≈ 2.6
20×1.75 BMX / kids	47-406	≈ 2.5
26×2.10 mountain	54-559	≈ 4.4
29×2.25 mountain	57-622	≈ 5.4
27.5×2.4 mountain	61-584	≈ 5.9
29×2.4 mountain	61-622	≈ 6.3
26×4.0 fat bike	≈ 100-559	≈ 16.3

Two patterns stand out. First, width dominates: 29×2.4 has about 15% more volume than 29×2.25, even though the width difference is only about 4 mm. Second, rim diameter has a smaller effect: 29×2.4 has only about 6% more volume than 27.5×2.4.

How width, height and rim diameter change tire size

Tire width, sidewall height and rim diameter together set a bicycle tire's overall diameter and circumference. They also set its volume, but each value affects those results differently.

Sidewall height and the aspect ratio question

Because bicycle tire cross-sections are nearly round, a simple estimate assumes that tire height above the bead seat equals tire width. This works like an aspect ratio of about 100%. In practice, height on a given rim is often somewhat less than width, especially on wide rims that spread the tire's base. The assumption is a geometric convenience, not a manufacturer specification.

Overall diameter and circumference

Estimated overall diameter = BSD + 2 × tire height. Estimated circumference = π × overall diameter. Using height ≈ width:

Tire	Estimated overall diameter (mm)	Estimated circumference (mm)
25-622 (700×25C)	622 + 2×25 = 672	≈ 2,111
28-622 (700×28C)	622 + 2×28 = 678	≈ 2,130
54-559 (26×2.10)	559 + 2×54 = 667	≈ 2,095
61-584 (27.5×2.4)	584 + 2×61 = 706	≈ 2,218
61-622 (29×2.4)	622 + 2×61 = 744	≈ 2,337

A practical example: a 700×25C road tire and a 26×2.10 mountain bike tire have similar estimated overall diameters, about 672 mm and 667 mm. Yet the mountain bike tire holds roughly four times the air volume, about 4.4 liters versus 1.0 liter. Overall diameter tells you little about volume. Width and cross-section tell you most of it.

Actual rolling circumference, which matters for bike computers, is usually measured by rolling the loaded wheel. It differs from these calculated values because of tread depth, real tire height, rim width and the tire flattening under load.

Volume versus amount of air at pressure

A tire's volume is its internal space in liters. The amount of air it holds depends on that volume multiplied by the absolute pressure. Absolute pressure is gauge pressure plus atmospheric pressure, about 1 bar.

As a simple estimate, which assumes constant temperature (Boyle's law), the equivalent amount of air at atmospheric pressure is volume × (gauge pressure in bar + 1). The pressures in these examples are illustrative, not recommendations:

• A 28-622 road tire (≈ 1.26 L) at 5 bar gauge contains roughly 1.26 × 6 ≈ 7.6 liters of atmospheric-pressure air. At about 1.2 grams per liter, that is roughly 9 grams of air.
• A 61-622 mountain bike tire (≈ 6.3 L) at 1.5 bar gauge contains roughly 6.3 × 2.5 ≈ 16 liters of atmospheric-pressure air.

This explains why a larger tire takes more pump strokes to reach even a modest pressure. It also explains why a fixed amount of gas produces a lower pressure in a high-volume tire than in a narrow one.

Why calculated volume differs from the real tire

The torus formula estimates the external envelope of an idealized round tire. Real internal tire dimensions differ for several reasons:

• Nominal versus actual width: the width printed on the sidewall is nominal. Measured width on a particular rim can be narrower or wider, and a wider rim generally makes the same tire measure wider.
• Non-circular cross-section: the rim cuts off the bottom of the circle, while the rim well adds some space below the bead seat. The real tire cross-section is neither a perfect circle nor exactly the assumed size.
• Casing and tread thickness: the outside width includes rubber and fabric, so the internal air space is somewhat smaller than the external dimensions suggest.
• Tubes and sealant: an inner tube's material, or the liquid sealant in a tubeless setup, takes up a small part of the space.
• Load and deformation: under the rider's weight the tire flattens at the contact patch, changing its shape slightly while riding.

For these reasons, treat calculated liters as a consistent way to compare sizes, not as a precise measurement of a specific tire on a specific rim.

Calculated geometry is not fitment compatibility

A calculated volume or diameter does not show whether a tire will fit your bike. Compatibility depends on separate conditions:

• Bead seat diameter must match the rim exactly. A 584 mm tire will not mount on a 622 mm rim, however similar the size names look.
• Rim inner width must suit the tire width. ETRTO and tire and rim manufacturers publish recommended combinations. Outside them, the tire may not seat or perform as intended.
• Frame and fork clearance: the real inflated width and height, plus room for mud and wheel flex, must fit within the frame, fork and brakes.
• Pressure limits of both tire and rim must be respected. The lower limit applies.

Use the sidewall numbers and the torus estimate to understand and compare how much space different tires offer. Use measured dimensions and the manufacturers' compatibility information to decide what actually fits.