Caleb MorganBy Caleb Morgan

Updated November 2026

A bike wheel size conversion chart lets you translate the names printed on tires and rims, such as 26″, 700c or 29″, into the millimeter measurements that actually matter when you shop. Once you understand the three systems in use, you can confirm that a tire will fit your rim before you order it. This guide walks you through the conversions and the checks that prevent a wrong purchase.

Why Bike Wheel Sizes Use Three Different Systems

Wheel sizes come from three overlapping naming systems. Marketing names like “26-inch” or “700c” are historical and often inconsistent. The ETRTO (European Tyre and Rim Technical Organisation) code is the most precise because it gives the tire width and bead seat diameter in millimeters, such as 47-559. Many manufacturers also print the actual outside diameter in millimeters. When two of these disagree, trust the ETRTO code.

Shopping for one? See our guide: 5 Best Road Bike Shoe Size Charts (2026): Expert Picks.

Conversion Chart for Common Wheel Sizes

Use the table below as a quick reference. Values are the nominal figures manufacturers commonly publish; always confirm against the tire sidewall or rim label.

Common name ETRTO bead seat Approx. outside diameter
20 in 406 about 508 mm
24 in 507 about 610 mm
26 in 559 about 660 mm
27.5 in (650b) 584 about 690 mm
700c / 28 in 622 about 700 mm
29 in 622 about 700–711 mm

Notice that 29-inch and 700c share the same ETRTO code (622). They are the same bead seat size, so a 29″ tire fits a 700c rim, though the mountain bike and road versions differ in tread and width.

How to Convert and Verify a Wheel Size

Follow these steps before buying a tire or replacement wheel.

  1. Read the ETRTO code printed on the tire sidewall (for example, 42-622).
  2. Find the same number on the rim, usually stamped on the inside of the rim bed.
  3. Match the first number (width or rim internal width range) and the second number (bead seat diameter).
  4. Check the tire’s maximum and minimum rim width if listed, to confirm the rim is in range.
  5. Only use the inch name as a secondary check, since it is not precise enough alone.

Converting Inches to Millimeters

To estimate a conversion yourself, multiply inches by 25.4. A 26-inch wheel is about 660 mm across, and a 27.5-inch wheel is about 699 mm. Keep in mind that inch figures are often rounded, so the ETRTO code remains your most reliable reference.

Tools and Checks You Need

  • A flexible tape measure or caliper for rim width
  • A printed copy of this chart or a saved reference photo
  • Your current tire sidewall, photographed for comparison
  • The bike manufacturer’s spec sheet if you have it

Common Mistakes to Avoid

  • Assuming “26-inch” always means the same rim: older mountain bike and cruiser rims can differ.
  • Ignoring tire width, which affects clearance in the frame and fork.
  • Matching only the inch name and skipping the ETRTO code.
  • Forgetting that a wider tire may need a wider rim to seat properly.
  • Buying a tire without measuring your brake reach if you change to a larger diameter.

FAQ

Is a 29-inch wheel the same as a 700c wheel?

Their bead seat size is the same (ETRTO 622), so the rims and tires are interchangeable in diameter. Tread patterns and intended use differ, so choose the tire for your riding.

Can I use the inch name alone to buy a tire?

It is risky. Two bikes labeled 26″ can use different bead seats, so match the ETRTO code instead.

Where do I find the ETRTO code?

It is printed on the tire sidewall and often stamped inside the rim bed. Look for two numbers separated by a hyphen, such as 37-584.

Conclusion

Use the ETRTO code as your primary reference and the inch name only as a shortcut. Compare the bead seat diameter and rim width before ordering, and check clearance if you change tire width. A few minutes of measuring prevents an expensive return.

Ready to choose one?

Caleb Morgan

About Caleb Morgan

Caleb Morgan is part of our editorial team and reviews and edits our cycling for beginners guides, checking product details against manufacturer specifications and owner feedback.