How Do You Measure the Size of a Bicycle Wheel? (Complete Guide)

You need to buy a new tire or replace a wheel, but you’re not sure what size your wheels are. Is it 26 inches? 700c? 29er? The numbers on your tire are confusing, and you don’t know which measurement actually matters.

Measuring bicycle wheel size is essential for buying the right replacement parts, but there are multiple sizing systems that can be confusing. The good news: once you understand the different measurement methods, it’s straightforward.

In this guide, I’ll show you how to measure bicycle wheel size using multiple methods, explain the different sizing systems (ISO, traditional, and tire markings), and help you identify your wheel size quickly and accurately.

Understanding Wheel Size Systems

Bicycle wheels use several different sizing systems, which is why it can be confusing:

1. ISO System (ETRTO): Most accurate, measured in millimeters (e.g., 622mm)
2. Traditional Sizing: Measured in inches (e.g., 26”, 27.5”, 29”)
3. Tire Markings: Found on tire sidewalls (e.g., 700x25c, 26x2.1)

Key Point: The ISO measurement (bead seat diameter) is the most accurate way to identify wheel size, as it measures the actual rim diameter where the tire sits.

Method 1: Read the Tire Sidewall (Easiest)

The easiest way to find your wheel size is to check the tire sidewall:

Step 1: Locate the Tire Markings

Look at the sidewall of your tire. You’ll see numbers like:

• 700x25c
• 26x2.1
• 27.5x2.3
• 29x2.4

Step 2: Understand the Format

Format: [Diameter] x [Width]

• First number: Wheel diameter (700, 26, 27.5, 29, etc.)
• Second number: Tire width (25, 2.1, 2.3, etc.)

Examples:

• 700x25c = 700mm diameter, 25mm width
• 26x2.1 = 26-inch diameter, 2.1-inch width
• 27.5x2.3 = 27.5-inch diameter, 2.3-inch width

Step 3: Find the ISO Size (Most Accurate)

Many tires also show the ISO size (ETRTO), which is the most accurate measurement:

• Look for numbers like 622-25 or 559-26
• The first number is the ISO diameter in millimeters

Method 2: Measure the Rim Diameter (Most Accurate)

If your tire is missing or unreadable, measure the rim directly:

Tools Needed:

• Tape measure or ruler
• Calculator (for conversion)

Step-by-Step:

1. Remove the tire (if possible) or measure with tire installed
2. Measure the rim diameter:
   • With tire removed: Measure from one edge of the rim to the opposite edge (inside to inside, where the tire bead sits)
   • With tire installed: Measure the overall diameter, then subtract twice the tire height
3. Convert to ISO: The rim diameter in millimeters is your ISO size

Example: If you measure 622mm from rim edge to rim edge, your ISO size is 622mm (which is 700c or 29”).

Method 3: Measure Overall Wheel Diameter

If you can’t access the rim, measure the overall wheel diameter:

Step-by-Step:

1. Position the wheel: Stand the wheel upright or measure while it’s on the bike
2. Measure diameter: Measure from the top of the tire to the bottom (through the center)
3. Subtract tire height: Subtract approximately 2x the tire height to get rim diameter
   • Road tires (~25mm): Subtract ~50mm
   • Mountain bike tires (~50mm): Subtract ~100mm
4. Convert to standard size: Use the conversion table above

Note: This method is less accurate but works if you can’t remove the tire.

Common Wheel Sizes Explained

Road Bikes

• 700c (622mm ISO): Standard road bike size
• 650b (584mm ISO): Smaller road bikes, some gravel bikes
• 27” (630mm ISO): Older/vintage road bikes

Mountain Bikes

• 26” (559mm ISO): Older mountain bikes
• 27.5” / 650b (584mm ISO): Modern mountain bikes (mid-size)
• 29” (622mm ISO): Modern mountain bikes (larger)

Hybrid/Comfort Bikes

• 700c (622mm ISO): Most common
• 26” (559mm ISO): Some comfort bikes

Kids Bikes

• 20” (406mm ISO): Common kids bike size
• 24” (507mm ISO): Larger kids bikes

ISO System (ETRTO) Explained

The ISO system (also called ETRTO) is the most accurate because it measures the bead seat diameter—the actual diameter where the tire sits on the rim.

Format: [ISO Diameter] - [Tire Width]

Example: 622-25 means:

• 622mm: Rim diameter (bead seat diameter)
• 25mm: Tire width

Why ISO is best: It’s universal and doesn’t depend on tire width or inflation, making it the most reliable measurement.

Measuring Wheel Size: Step-by-Step Guide

Quick Method (Tire Markings)

1. Look at tire sidewall
2. Find numbers like 700x25c or 26x2.1
3. First number is your wheel diameter
4. Check for ISO size (most accurate)

Accurate Method (Rim Measurement)

1. Remove tire (or measure with tire installed)
2. Measure rim diameter from edge to edge (inside to inside)
3. Convert to ISO size using conversion table
4. Use ISO size when buying replacement parts

Alternative Method (Overall Diameter)

1. Measure overall wheel diameter (top to bottom)
2. Subtract approximate tire height (2x tire width)
3. Convert to standard size
4. Verify with tire markings if possible

Common Mistakes to Avoid

1. Confusing Tire Width with Diameter: The second number (e.g., 25 in 700x25c) is width, not diameter
2. Using Overall Diameter: Measure rim diameter, not overall wheel diameter with tire
3. Not Checking ISO Size: ISO size is most accurate—always check if available
4. Assuming All 26” Wheels Are the Same: Different tire widths can affect compatibility

How to Use Your Measurement

Once you know your wheel size:

Buying Tires:

• Match the ISO size (most important)
• Choose appropriate width for your rim
• Ensure tire width fits your frame clearance

Buying Wheels:

• Match the ISO size exactly
• Check hub spacing (front vs rear)
• Verify brake compatibility (rim vs disc)

Buying Tubes:

• Match the wheel diameter
• Choose tube width range that matches your tire width

Measuring Bicycle Wheel Size FAQs

Summary: ISO Size is Key

Measuring bicycle wheel size is straightforward once you understand the different systems. The ISO size (ETRTO) is the most accurate measurement because it measures the actual rim diameter where the tire sits.

Key Takeaways:

• Check tire sidewall for markings (easiest method)
• ISO size is most accurate (e.g., 622mm)
• Traditional sizes vary (700c = 29” = 622mm ISO)
• Measure rim diameter if tire markings are unclear
• Match ISO size when buying replacement parts

Whether you’re buying tires, wheels, or tubes, matching the ISO size ensures compatibility. Once you know your ISO size, you can confidently purchase the right replacement parts.