650b vs 700c: Bicycle Tire and Wheel Sizes Compared

650b and 700c are different rim sizes and are not directly interchangeable. A 650b tire fits a rim with a 584 mm bead seat diameter. A 700c tire fits a rim with a 622 mm bead seat diameter. The 38 mm difference means a tire made for one rim cannot be mounted on the other. A wide 650b tire, however, can have about the same overall diameter as a narrower 700c tire. That is why some frames with disc brakes accept either wheel size. The most reliable way to tell them apart is the ETRTO number on the tire sidewall, such as 47-584 or 28-622.

What the numbers on a bicycle tire sidewall mean

A bicycle tire sidewall usually carries several markings for the same size, along with ratings that are not dimensions at all. The size markings use three systems: ETRTO/ISO, French, and inch. Only the ETRTO/ISO marking states the rim fit exactly.

ETRTO size markings (for example 47-584 or 28-622)

An ETRTO marking has two numbers, both in millimetres.

• First number: nominal tire width in mm. In 47-584, the tire is designed to be about 47 mm wide.
• Second number: bead seat diameter (BSD) in mm. This is the diameter of the rim surface where the tire bead sits. It must match the rim exactly. In 47-584, the tire fits a 584 mm rim.

A 650b tire always ends in 584. A 700c tire always ends in 622. If the second number matches the rim's bead seat diameter, the tire is the right diameter for that rim. You still need to check that its width suits the rim and fits the frame.

French markings (for example 700x28c or 650x47b)

The French system writes a nominal outside diameter in millimetres, then a width in millimetres, then a letter. Examples are 700x28c and 650x47b.

• 700 or 650 was originally the approximate overall tire diameter in mm. Today it is only a size name. A 700x23c tire and a 700x45c tire have very different overall diameters, yet both fit a 622 mm rim.
• 28 or 47 is the nominal width in mm.
• The letter (a, b or c) originally marked width classes that kept the overall diameter near the nominal figure. Each letter now simply identifies a bead seat diameter: 650b means 584 mm and 700c means 622 mm.

The letter matters. 650a (590 mm BSD) and 650c (571 mm BSD) are different sizes from 650b, and none of them interchange. A marking that says only 650 is ambiguous. Check the ETRTO number instead.

Inch markings (27.5, 28 and 29)

Inch markings such as 27.5 x 2.0 or 29 x 2.25 give a nominal diameter and a width in inches. These names map to the same rims as the French sizes.

• 27.5-inch is the same rim size as 650b: 584 mm BSD. A 27.5 x 2.0 tire is roughly 50-584.
• 29-inch and 28-inch are the same rim size as 700c: 622 mm BSD. A 29 x 2.2 mountain bike tire and a 700x32c road tire both fit 622 mm rims, if the rim width suits them.

The inch names are labels, not measurements. A 27.5-inch tire is not exactly 27.5 inches in diameter. Some older inch sizes with fractional widths, such as 26 x 1 3/8, belong to entirely different bead seat diameters.

Pressure, TPI and other ratings that are not dimensions

Other sidewall markings describe use and construction, not size.

• Pressure range is printed in psi and/or bar, for example 3.0–5.0 bar (45–75 psi). These are the manufacturer's minimum and maximum inflation limits. They are not a recommended single pressure, and the rim's own pressure limit also applies.
• TPI (threads per inch) describes the density of the casing fabric. It is a construction figure, not a width or diameter.
• Tubeless markings, such as TLR or Tubeless Ready, indicate the tire is designed for tubeless setup on a compatible rim.
• Rotation arrows show the intended rolling direction.
• E-bike ratings, such as E-25 or E-50, appear on some tires. They indicate the tire has been approved for e-bikes up to the stated speed in km/h. Some tires also print a maximum load.

Two tires with identical size markings can have different pressure ranges and construction ratings. None of these ratings change which rim a tire fits.

What is the difference between 650b and 700c?

The core difference is rim diameter. A 650b rim has a 584 mm bead seat diameter. A 700c rim has a 622 mm bead seat diameter. The 650b rim is therefore 38 mm smaller in diameter and 19 mm smaller in radius. Everything else depends on the tire width you mount: overall diameter, gearing feel, bottom bracket height and clearance.

Attribute	650b	700c
Bead seat diameter (ETRTO)	584 mm	622 mm
Typical ETRTO marking	47-584, 50-584	28-622, 40-622
Equivalent inch name	27.5-inch	28-inch, 29-inch
Common use of the size	Wide road-plus and gravel tires, mountain bikes, some touring and randonneuring bikes	Road, gravel, cyclocross, hybrid and touring bikes, 29er mountain bikes
Rim radius difference	650b is 19 mm smaller than 700c
Tire interchangeable on the other rim?	No

Is 650b the same as 700c?

No, 650b is not the same as 700c. Their bead seat diameters differ by 38 mm, 584 mm versus 622 mm, so their tires and rims cannot be swapped. The two sizes are sometimes confused because a wide 650b tire can match the overall height of a narrower 700c tire. Matching outside diameter does not make the rim sizes the same.

650b is the same rim size as 27.5-inch. 700c is the same rim size as 29-inch and as the European 28-inch designation. A 27.5-inch mountain bike tire can therefore fit a 650b rim, and a 29-inch tire can fit a 700c rim, provided the tire width suits the rim's internal width.

How tire width and rim diameter affect overall diameter and circumference

A tire's overall diameter equals the bead seat diameter plus twice the tire's height above the rim. Bicycle tires have a roughly round cross-section, so the height is close to the width. That gives a simple estimate:

Estimated overall tire diameter (mm) ≈ bead seat diameter + 2 × tire width

Circumference (mm) ≈ π × overall diameter (π ≈ 3.1416)

Rim diameter is fixed by the size: 584 mm or 622 mm. Tire width is the variable that changes overall diameter. Each added millimetre of width adds roughly 2 mm of overall diameter, because the tire's height counts on both the top and bottom of the wheel.

Worked examples: 650b vs 700c tire size

Tire (ETRTO)	Calculation	Estimated overall diameter	Estimated circumference
700x28c (28-622)	622 + 2 × 28	678 mm	≈ 2,130 mm
650x47b (47-584)	584 + 2 × 47	678 mm	≈ 2,130 mm
700x32c (32-622)	622 + 2 × 32	686 mm	≈ 2,155 mm
650x40b (40-584)	584 + 2 × 40	664 mm	≈ 2,086 mm
700x40c (40-622)	622 + 2 × 40	702 mm	≈ 2,205 mm
27.5 x 2.1 (about 54-584)	584 + 2 × 54	692 mm	≈ 2,174 mm

Three comparisons stand out:

• 650x47b and 700x28c have about the same overall diameter, 678 mm by this estimate. The 650b tire achieves that height with a smaller rim and a taller, wider tire.
• At the same width, 650b is noticeably smaller. A 40 mm tire on a 650b rim is estimated at 664 mm in diameter. On a 700c rim it is 702 mm. The difference is 38 mm in diameter and about 5% less distance travelled per wheel revolution.
• Air volume rises quickly with width. Cross-sectional area scales roughly with width squared. A 47 mm tire therefore holds roughly 2.8 times the cross-sectional air area of a 28 mm tire, since (47 ÷ 28)² ≈ 2.8. This is why wide 650b tires are usually run at lower pressures than narrow 700c tires.

Aspect ratio and sidewall height on bicycle tires

Bicycle tires do not print an aspect ratio the way car tires do. The simple formula above treats sidewall height as roughly equal to width, an aspect ratio of about 100%. In practice, the inflated height of a bicycle tire is often somewhat less than its width. The exact ratio depends on the tire's design and the rim it is mounted on. Taller sidewalls on a smaller rim, as with wide 650b tires, are what allow 650b to reach a similar overall diameter to 700c.

Calculated geometry vs actual tire dimensions

The figures above are estimates from nominal markings, not measurements. Real tires differ for several reasons:

• Rim internal width changes tire shape. The same tire usually measures wider on a wider rim and narrower on a narrower rim. Its height also changes slightly.
• Nominal width is not measured width. A tire labelled 47 mm may measure more or less than 47 mm, depending on the brand, casing and tread.
• Tread and knobs add height and width. Knobby tires can exceed the casing dimensions that the label describes.
• Pressure and load matter. Inflation pressure and rider weight affect the tire's shape and its effective rolling circumference.

Use the calculation to compare sizes and narrow down choices. For an exact rolling circumference, for example to calibrate a bike computer, measure the distance the wheel travels in one revolution with the bike loaded.

700c vs 650b wheel size: practical effects of the geometry

Whether a 650b or 700c wheel feels different depends mainly on overall diameter and tire width, not on the rim size label alone.

• Same overall diameter, such as 650x47b vs 700x28c: bottom bracket height, effective gearing and computer calibration stay roughly the same. The main change is a much wider, higher-volume tire on the 650b wheel.
• Same width, smaller overall diameter, such as 650x40b vs 700x40c: the bottom bracket drops by half the diameter difference, about 19 mm in this example. That can increase the risk of pedal strike. Each pedal revolution also covers a little less ground, about 5% here. The bike computer must be set to the new circumference.
• Frame geometry changes with height, not label. Trail and handling are affected by overall wheel diameter. Swapping to a wheel with a similar overall diameter keeps those changes small.

Are 650b and 700c interchangeable?

Tires: no. A 650b (584 mm) tire cannot be mounted on a 700c (622 mm) rim, and a 700c tire cannot be mounted on a 650b rim. The bead seat diameter must match exactly.

Complete wheels: sometimes. A bike can accept both 650b and 700c wheels only if four things are true. The frame and fork have clearance for the chosen tire width and height. The brakes do not depend on rim diameter. The hubs and axles match the frame. The resulting bottom bracket height is acceptable. Many gravel and some all-road frames are designed for both sizes. Many road frames are not.

Brake compatibility

Brake type is usually the deciding factor in a 650b and 700c swap.

• Disc brakes: the rotor mounts to the hub, so rim diameter does not affect braking. With the same rotor size, hub standard and axle type, a disc-brake bike can often take either wheel size if clearance allows.
• Rim brakes (calipers, cantilevers, V-brakes): the pads must meet the rim's braking surface. A 650b rim's braking surface sits about 19 mm closer to the hub than a 700c rim's. Most rim brakes built for 700c cannot reach that far. Rim-brake bikes designed for 650b exist, but swapping wheel size on a rim-brake bike usually requires different brakes or brake mounts.

Frame and fork clearance

Frame and fork clearance must accommodate both the tire's width and its overall diameter, with room to spare for mud and wheel flex. A 650b tire may fit height-wise where a 700c tire of the same width would not. But a wide 650b tire may be too wide for the chainstays, seatstays or fork blades, and front derailleur clearance can also limit width. Frame manufacturers usually state maximum tire sizes for each wheel size. Treat those figures as the limit, not the calculated dimensions.

How to check whether a bike can take the other wheel size

1. Read the ETRTO number on your current tire to confirm the rim size: -584 for 650b or -622 for 700c.
2. Check the brake type. If it uses rim brakes, a swap usually is not practical without changing brakes.
3. Look up the frame and fork maker's stated tire clearance for 650b and for 700c.
4. Confirm that the new wheels match the bike's hub spacing, axle type (quick release or thru-axle size) and rotor mount, plus the drivetrain freehub standard.
5. Estimate the new overall diameter with BSD + 2 × width and compare it with your current tire. A similar figure keeps bottom bracket height and handling close to the original. A much smaller figure lowers the bottom bracket.
6. Choose a tire width suited to the new rim's internal width, then confirm actual clearance once the tire is mounted and inflated.

How to tell which tire size you have

To identify a tire, look for the ETRTO marking on the sidewall: two numbers separated by a hyphen. The second number tells you the rim size.

1. Find the ETRTO number, for example 40-584 or 32-622. It is often molded in smaller print near the brand name or the French or inch size.
2. Read the second number. 584 means 650b or 27.5-inch. 622 means 700c, 28-inch or 29-inch.
3. Read the first number for the nominal tire width in mm.
4. If only a French marking is visible, check the letter. 650b is 584 mm, while 650a and 650c are different sizes.
5. If the tire markings are worn away, check the rim. Many rims print their ETRTO size, written as internal width x BSD, for example 622x21 or 21-622.

When buying a replacement, matching the bead seat diameter is mandatory. Matching or changing the width is a choice limited by rim width, frame and fork clearance, and the overall diameter you want.