Question 1

What does 700c mean in bike tire sizes?

Accepted Answer

700c refers to the approximate outer diameter of the tire in millimeters - roughly 700mm. The "c" is a legacy from the old French sizing system. In ETRTO terms, 700c corresponds to a 622mm Bead Seat Diameter (BSD). Most road, gravel, cyclocross, and 29er mountain bike wheels use this standard. The actual outer diameter varies depending on tire width - a 700×23c tire has a smaller total diameter than a 700×45c.

Question 2

What is the ETRTO tire sizing system?

Accepted Answer

ETRTO (European Tyre and Rim Technical Organisation) is the universal, unambiguous tire sizing standard. It uses two numbers separated by a dash: the first is tire width in mm and the second is the Bead Seat Diameter (BSD) in mm. For example, 25-622 means a 25mm wide tire on a 622mm BSD rim (700c). Unlike French or Imperial sizes, ETRTO eliminates confusion because one designation maps to exactly one tire size.

Question 3

How do I convert between French, Imperial, and ETRTO tire sizes?

Accepted Answer

Use an ETRTO conversion table or our calculator above. The key reference is the BSD: 622mm = 700c = 29", 584mm = 27.5" = 650b, 559mm = 26", and 406mm = 20". Once you know the BSD, match the tire width in mm to find the equivalent French (e.g., 700×25c) or Imperial (e.g., 28×1") designation. Note that some Imperial and French sizes are approximate and can overlap.

Question 4

How does tire width affect total wheel diameter?

Accepted Answer

Tire width directly increases the total outer diameter of the wheel. The total diameter equals the BSD plus twice the tire width (since the tire wraps around both sides of the rim). For example, on a 622mm BSD rim, a 25mm tire gives a total diameter of about 672mm, while a 50mm tire gives about 722mm - a 50mm difference. This affects frame clearance, speedometer calibration, and gearing.

Question 5

What tire widths fit my rims?

Accepted Answer

Tire compatibility depends on the internal width of your rim. A general guideline is that the tire width should be 1.4 to 2.4 times the internal rim width. For example, a rim with 19mm internal width works well with tires from 28mm to 45mm. Too-narrow tires on wide rims can cause poor handling and blowoffs, while too-wide tires on narrow rims create a light-bulb profile with poor cornering grip.

Question 6

What is tire clearance and why does it matter?

Accepted Answer

Tire clearance is the gap between the tire and the frame, fork, or brakes. You need at least 3-4mm of clearance on each side to prevent rubbing, especially when the wheel flexes under load or when mud and debris accumulate. Check clearance at the fork crown, seat tube, chainstays, and brake calipers. Running a tire that is too large for your frame can cause dangerous rubbing or lock the wheel entirely.

Question 7

Why do I need to know my wheel circumference?

Accepted Answer

Wheel circumference is essential for accurate bike computer and speedometer calibration. GPS-based computers may not need it, but sensor-based computers use circumference to convert wheel rotations into distance and speed. An incorrect circumference value can cause speed readings to be off by 5% or more. Our calculator provides the precise circumference for your specific tire and rim combination.

Question 8

Are 29er and 700c wheels the same size?

Accepted Answer

Yes, 29er mountain bike wheels and 700c road wheels share the same 622mm Bead Seat Diameter (BSD). The difference is in tire width: 700c road tires typically range from 23-45mm, while 29er mountain bike tires run 50-65mm or wider. This means you could technically mount a road tire on a 29er rim (if the rim internal width is compatible), though in practice the rim widths are usually optimized for different tire ranges.

Question 9

What is the difference between 27.5" and 650b wheels?

Accepted Answer

27.5" and 650b are the same wheel size with a BSD of 584mm. The "27.5 inch" name is used primarily for mountain bike wheels, while "650b" is more common in gravel and touring contexts. Both use identical rims. This size sits between 26" (559mm BSD) and 29"/700c (622mm BSD), offering a balance of agility and rollover capability.

Question 10

How do I measure my current tire size?

Accepted Answer

The easiest way is to read the ETRTO marking on your tire sidewall - it looks like "25-622" or "54-584". If the sidewall only shows a French size (e.g., 700×25c), use our converter table above to find the ETRTO equivalent. For total outer diameter, measure from the ground to the top of the tire while the bike is upright, then double the distance from the ground to the axle center. For circumference, mark the tire and ground, roll the wheel one full revolution, and measure the distance.

BSD (mm)	Common Name	Typical Use
622	700c / 29"	Road, gravel, cyclocross, 29er MTB
584	27.5" / 650b	Trail MTB, gravel, touring
559	26"	Classic MTB, touring, city bikes
406	20"	BMX, folding bikes, recumbents

BSD (mm)	Common Name	Typical Use
622	700c / 29"	Road, gravel, cyclocross, 29er MTB
584	27.5" / 650b	Trail MTB, gravel, touring
559	26"	Classic MTB, touring, city bikes
406	20"	BMX, folding bikes, recumbents

Tire Size Calculator

Understanding Bike Tire Sizes

Common Wheel Size Standards

Rim & Tire Compatibility

Internal Rim Width Guidelines

Why Compatibility Matters

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