It’s the tire’s size, shown in a common road/gravel format.

• 700c refers to the rim diameter the tire fits. In modern terms that means a 622 mm bead seat diameter (you’ll often see the ETRTO size listed as something like 40‑622).
• 38 is the tire’s nominal width in millimeters. Actual installed width can vary a bit with rim width and air pressure.
• “c” is a legacy French code that stuck around with 700c sizing; today it essentially points to the 622 mm standard.

If your current tire or rim lists 622 mm, you’re in the right diameter family. Always confirm frame/fender clearance for the chosen width.

They’re different ways of describing wheels that usually share the same rim diameter.

• 700c, 28-inch, and 29-inch commonly fit rims with a 622 mm bead seat diameter.
• 28" vs 29" often reflects marketing and tire volume: 29er tires are typically wider/taller (MTB style), which increases overall outside diameter.
• Fit depends on clearance at your frame, fork, and under fenders. Wider/higher-volume tires may not fit every bike.
• What to check: the bead seat diameter (622) on your current tire/rim, your bike’s available clearance, and brake setup.

Matching the 622 mm diameter and verifying space around the tire can help you choose confidently without assuming all 28"/29" tires will behave the same.

Think of it as roughly a 28 x 1.5" tire.

• Width: 38 mm ≈ 1.50" (since 25.4 mm = 1 inch).
• Common inch label: “28 x 1.50”. Some packages may also show “28 x 1 1/2”.
• Metric/ETRTO: You may see 38‑622 or 40‑622 listed as the precise metric size, indicating ~38–40 mm width on a 622 mm rim.
• Outside diameter varies: tread height and casing shape affect the mounted height, so total wheel diameter isn’t fixed.

Use the sidewall markings as your primary guide, and remember installed dimensions can change slightly with rim width and air pressure.

It’s an informal benchmark some riders use when changing sizes.

• Idea: keep overall tire diameter/circumference changes within about 3% of your current setup to maintain similar fit, handling, and gearing feel.
• Not an official standard: it’s a guideline, not a strict rule. Different tire models vary in true size.
• Practical steps: compare published dimensions when available, and physically check clearance at the chainstays, seatstays, fork crown, and under fenders.
• Example: moving from 700x35 to 700x38 may add a little height and width, but the actual change depends on casing and tread.

Use the 3% concept as a starting point, then confirm real-world fit on your bike before riding.

Look for tubes sized for 700c/28" wheels with a width range that includes 38 mm.

• Tube size: labels like 700 x 35–43 or 28 x 1.35–1.75 commonly fit 700x38c tires.
• Valve type: match your rim. Presta suits many road/gravel rims; Schrader requires a larger valve hole.
• Valve length: 40–60 mm works for most standard rims; deeper rims may benefit from longer valves.
• Tips: lightly inflate the tube before installation to reduce pinches, and avoid exceeding tire and rim pressure limits.

Always confirm the tube’s listed size range, your rim’s valve hole, and the pressure range printed on your tire’s sidewall.