How can I tell what size my wheels are?

Knowing your wheel size is essential for choosing tires, rims, or replacements without compatibility issues. You can determine size quickly from tire markings, rim labels, or official vehicle specifications. This guide explains how to read those markings and, if needed, how to measure the wheel diameter accurately for bikes and cars.

Bicycle wheels

What the numbers mean

Bike wheels are sized primarily by the rim diameter (the bead-seat diameter, BSD) and the tire’s ISO/ETRTO measurement. The markings you’ll encounter on tires or rims tell you this size, but the format differs between older naming and the ISO standard.

• Look at the tire sidewall for the common name and the ISO/ETRTO size. Examples include 700x23C, 29x2.0, or 27.5x2.4. The ISO code may appear as something like 42-622, where 622 mm is the bead-seat diameter (BSD).


• Note that 700c and 29" wheels often share the same BSD (622 mm) even though the labels differ. 27.5" wheels (650B) use a BSD of about 584 mm, and older 26" wheels use about 559 mm.


• Rim labeling can also appear as 622x14-21, indicating the BSD (622) and a range of acceptable rim widths. If the marking is unclear, use the ISO number instead of the outer tire size for accuracy.


• When in doubt, check your bike’s manufacturer specifications or the frame’s clearance and brake compatibility to ensure the wheel will fit properly.

Conclusion: For bicycles, BSD (bead-seat diameter) is the key metric. Read the tire’s ISO/ETRTO size or check the rim label, and cross-check with your bike’s specs to confirm compatibility.

Car and truck wheels

Decoding tire and wheel markings

Vehicle wheels are determined by the wheel diameter encoded in the tire size, along with official placards and, for aftermarket wheels, the wheel’s own markings. The diameter shown on tires and in official documentation tells you the compatible wheel size in inches.

• Examine the tire sidewall for size markings such as P215/65R15, LT245/75R16, etc. The last number (e.g., 15 or 16) is the wheel diameter in inches.


• Consult the door jamb placard or the owner’s manual. It lists recommended tire sizes and the corresponding wheel diameters, along with other fit details like offset and bolt pattern.


• Inspect the wheel itself for stamped information. Many rims show markings like Dia 16, 7Jx16, or 16x7.0JJ, where 16 denotes the wheel diameter. The designation ET (offset) and PCD (bolt pattern) are also commonly listed and matter for proper fit.


• For aftermarket wheels, confirm compatibility by checking the manufacturer’s specifications or using the wheel’s model number, since some combinations with certain tires or suspension setups may require adjustments.

Conclusion: On cars, the wheel diameter is most reliably identified by the tire size’s last number and the placard. If markings are worn or unreadable, measure or consult official specs to avoid mismatches.

Practical tips for measuring when markings aren’t readable

Manual measurement methods

If you can’t read markings, you can estimate wheel size with a ruler or calipers, though accuracy improves with the right reference points. Remove the tire if you need to measure the rim, and compare your measurement to standard BSD values or OEM specs.

• For bikes: measure the bead-seat diameter directly by seating a caliper across the inside of the rim where the tire bead sits, or compare to known standards (622 mm for most road and many mountain bikes; 584 mm for 27.5" wheels; 559 mm for older 26" wheels).


• For cars: remove the tire to measure the rim diameter or rely on the wheel label if present. If unavailable, use OEM specifications from the vehicle’s manual or manufacturer site to determine the correct diameter and bolt pattern.

Concluding: Direct measurements can fill in gaps when markings are worn, but cross-checking with manufacturer data helps ensure you select the right tires and wheels.

Summary

Key takeaways: Bicycle wheel size centers on the bead-seat diameter (BSD), with common values like 622 mm for many road and 29" wheels and 584 mm for 27.5" wheels. Car and truck wheel size is determined by the wheel diameter indicated in tire sizes (the last inch value), and official placards or manuals provide the exact recommendations. When markings are unreadable, use measurements and confirm with the manufacturer specifications to avoid mismatches. Knowing these markers helps you shop confidently for tires, rims, and wheels that fit your bike or vehicle.

Are rims marked with size?

Yes, rim size is marked on the rim itself, usually on the back of the spokes or the inside surface. The markings often look like "17x8" where "17" is the diameter in inches and "8" is the width in inches, but other numbers like offset (ET) and a flange profile letter (e.g., J) will also be present.
 
How to find the size on the rim

• Look for a stamp: The size is typically cast or machined into the metal on the rear face of the spokes or the inside of the barrel. 
• Understand the numbers: A common format is Diameter x Width, for example, 17x8. 
  • Diameter: The first number is the wheel's diameter in inches (e.g., 17). 
  • Width: The second number is the wheel's width in inches (e.g., 8). 
• Look for other markings: You may also find other information, such as:
  • Offset (ET): A number in millimeters that indicates the wheel's position relative to the hub mounting surface (e.g., ET38). 
  • Flange profile: A letter that describes the rim's flange (e.g., "J"). 

What if you can't find the markings?

• Check the door jamb: Some cars have a sticker on the driver's side door jamb that lists the original tire and wheel size. 
• Use your tire's sidewall: The last two digits of your tire's size designation indicate the rim diameter it's designed for (e.g., a tire with "R20" on the sidewall fits a 20-inch rim). 

Does R17 mean 17 inches?

Yes, "R17" means that the tire is designed for a 17-inch diameter wheel rim. The "R" stands for "radial," which refers to the tire's internal construction, while the "17" is the diameter of the wheel it will fit.
 

• R: This letter indicates the tire's construction type is radial.
• 17: This number is the diameter of the wheel rim, measured in inches, that the tire is designed to fit.

How do I tell what size wheels I have?

You can determine your wheel size by looking for a series of numbers stamped on the back of the wheel, which will be in a format like 18x7.5, where 18 is the diameter and 7.5 is the width. Alternatively, check the door jamb sticker or the sidewall of your tires for the manufacturer's information.
 
This video explains how to read the numbers on the side of the tire to find the rim size: 50sHelpful DIYYouTube · Jan 8, 2024
Method 1: Check the wheel itself

• Look for the size marking: Find the numbers stamped or cast into the back of one of the spokes. 
• Read the markings: The numbers will usually be in a format like 18x7.5. 
  • The first number (18) is the wheel's diameter in inches. 
  • The second number (7.5) is the wheel's width in inches. 
• Find other information: You may also find other details like the offset (e.g., ET35), which is measured in millimeters. 

Method 2: Check the tire

• Read the tire's sidewall: Look at the numbers printed on the side of the tire currently on the wheel. 
• Identify the rim size: If the tires are the original factory size, the last two digits of the tire's size code will indicate the rim diameter. For example, in a P245/40R18 tire, 18 is the rim diameter. 

Method 3: Check the vehicle's door jamb 

• Locate the sticker: Open the driver's side door and look for a sticker on the door or door frame. 
• Read the tire information: This sticker will often list the original tire size and wheel size. 

How to tell rim size without a tire?

You can determine a rim's size by looking for stamped numbers, which often include the diameter and width (e.g., "18x7 ½"), or by measuring the diameter and width using a tape measure. To measure, find the diameter by measuring from bead seat to bead seat across the front, and measure the width from one bead seat to the opposite bead seat. 
This video demonstrates how to measure the diameter and width of a wheel: 1mBoosted MediaYouTube · Jan 14, 2017
1. Look for stamped markings 

• Often, the size is stamped directly onto the rim, typically on the inside of the barrel or behind the center cap. 
• Look for a marking that looks like 18x7 ½. The first number is the diameter, and the second is the width. 
• The size can be in inches or millimeters. 

2. Measure manually

• Diameter: Measure the distance from one bead seat to the other across the face of the rim. This is the diameter of the wheel. It is crucial not to measure from the outer lip to outer lip, as this can be misleading. 
• Width: Measure the distance from the inside of one bead seat to the inside of the opposite bead seat. This measurement is typically in increments of a half-inch. 
• Bolt pattern: If the size isn't stamped, you can also determine some information by measuring the bolt pattern. 
  • For 4 or 6 lugs: Measure from the center of one bolt hole to the center of the one directly across from it. 
  • For 5 lugs: Measure from the outer edge of one bolt hole to the center of the opposite bolt hole. 

This video demonstrates how to measure the bolt pattern of a wheel: 1mJEGS PerformanceYouTube · Oct 8, 2015