Choosing the right tire for your bicycle can be confusing, especially with the variety of tire sizes and sizing systems used over the years. This is particularly true for vintage bicycles. Understanding your Bike Rim Dimensions is crucial for ensuring a safe and efficient ride.
This guide will explain how to accurately measure your bike rims to determine the correct tire size. While we recommend matching your new tire to the old one using the ISO/E.T.R.T.O. (European Tire and Rim Technical Organization) size number whenever possible, this article will provide a solution when those markings are illegible or the original tire is unavailable.
Matching the ISO/E.T.R.T.O. size number ensures tire compatibility with your rim. While tire width can vary, maintaining the correct ISO/E.T.R.T.O. diameter is essential. But what if you can’t rely on existing tire markings? Don’t worry! With a simple tape measure, you can determine the bike rim dimensions and find the ISO/E.T.R.T.O. Bead Seat Diameter of your rim. You can achieve this by measuring either the diameter or the circumference of your rim.
Measuring Rim Diameter
The outer diameter of the rim is typically about 10mm larger than the crucial ISO/E.T.R.T.O. bead seat diameter. This difference accounts for the height of the rim flanges.
To measure the diameter:
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Position your measuring tool: Lay a ruler or tape measure across the rim, spanning from one inner edge to the opposite inner edge.
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Find the maximum measurement: Ensure you are measuring the widest point across the diameter. Slide one end of the tape measure slightly back and forth to find the largest measurement between directly opposing points.
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Take multiple measurements: Measure across a few different diameters and calculate the average to account for minor imperfections in rim roundness. A slightly out-of-round rim (a millimeter or two) on an aluminum or steel rim can often be corrected by spoke tension. However, severely deformed rims should not be used, and deep-section carbon fiber rims are less forgiving.
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Convert to millimeters (mm): Once you have the diameter in inches, multiply by 25.4 to convert it to millimeters. For example, if your measurement is 20 7/8 inches (20.875 inches), multiplying by 25.4 gives approximately 530mm.
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Determine ISO Diameter: Refer to the size chart below. The measured outer diameter will be slightly larger than the ISO/E.T.R.T.O. bead seat diameter. In our 530mm example, this indicates a 520mm ISO diameter rim.
Measuring the diameter is easiest with an empty rim. However, if you have a built-up wheel, the hub can obstruct accurate diameter measurement. In such cases, measuring the rim’s circumference is a more practical approach. Circumference measurement can also serve as a verification for your diameter measurement.
Measuring Rim Circumference
Measuring the circumference is often easier for built-up wheels. It involves wrapping a flexible measuring tape around the rim’s inner circumference.
For accurate circumference measurement:
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Use a narrow metal tape measure: A 1/4 inch or 6 mm wide metal tape is ideal as it fits into the rim well (the deepest part of the rim bed where the tire sits). Wider tapes won’t conform accurately to the rim’s curve. Avoid fabric measuring tapes, as they can stretch and provide inaccurate readings.
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Secure the tape: Hook the tab at the end of the metal tape measure into the valve hole on the rim.
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Wrap and measure: Wrap the tape snugly around the rim, ensuring it sits in the rim well. Record the total circumference measurement.
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Calculate the diameter: Divide the circumference measurement by pi (approximately 3.142) to obtain the rim well diameter.
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Convert to millimeters (if necessary): If your tape measure is in inches, multiply the diameter in inches by 25.4 to convert it to millimeters.
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Account for rim flange height: Measure the height from the rim well to the bead seat (the edge where the tire bead sits). A simple method is using a spoke and ruler as shown:
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Add flange height: Add twice the measured flange height to the rim well diameter calculated in step 4. This gives you the approximate outer rim diameter.
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Estimate ISO Diameter: Subtract approximately 10mm from the outer rim diameter to estimate the ISO/E.T.R.T.O. bead seat diameter. For instance, if your calculated outer diameter is 534mm, the ISO diameter is likely around 520mm.
Let’s illustrate with an example: Suppose the measured circumference of the rim well is 64 1/8 inches (64.125 inches).
- Convert to millimeters: 64.125 inches * 25.4 mm/inch = 1629 mm
- Calculate rim well diameter: 1629 mm / 3.142 = 518.5 mm
- Add flange height (example: 16mm for twice the depth): 518.5 mm + 16 mm = 534.5 mm
- Estimate ISO Diameter: 534.5 mm – approximately 10mm = roughly 520 mm ISO diameter.
Rim Size Chart and ISO Compatibility
Once you have determined your rim’s ISO bead seat diameter, you can use the chart below to identify common rim sizes and ensure compatibility when purchasing new tires. Matching the ISO diameter is the most reliable way to guarantee tire fit, regardless of traditional sizing designations.
| Traditional Designation | Applications/Notes | ISO Bead Seat Diameter | Bead Seat Circumference (Rim measurement) |
|---|---|---|---|
| 36 inch | Mostly unicycles | 787 mm | 2472 mm |
| 32 inch | Mostly unicycles | 686 mm | 2155 mm |
| 28 x 1 1/2, 700 B | English, Dutch, Chinese, Indian Rod-brake roadsters (Also marked F10, F25, 700 B) | 635 mm | 1995 mm |
| 27 x anything except “27 five” and rare Danish 27 x 1 1/2 | Older road bikes, went out of fashion in the early 1980s | 630 mm | 1979 mm |
| 700 C, 28 x 1 5/8, 29 inch (28 x 1 1/2 F.13 Canada) | Most newer adult bikes for road use use this size, including most road bikes and hybrids. 29 inch are fat tires, same rim diameter. Also 700C tubular. | 622 mm | 1954 mm |
| 27 x 1 1/2 | Rare Danish size | 609 mm | 1913 mm |
| 26 x 1.25, x 1.375 | Very Rare U.S. size, 1940s and older. Not available. | 599 mm | 1881 mm |
| 26 x 1 1/4 EA.1,26 x 1 3/8 (S-6), 650 | Schwinn “lightweights“, older English “club” bikes | 597 mm | 1875 mm |
| 26 x 1 3/8 (E.A.3), 650 A | Most 3-speeds except Schwinn; department-store or juvenile 10 speeds | 590 mm | 1853 mm |
| Mavic 26″ tubular | Older Mavic 26″ tubulars | 587 mm | 1844 mm |
| 650B, 26 x 1 1/2,26″ tubular, “27 five” | French utility bikes, tandems, and loaded-touring bikes; some mountain bikes.This size is currently undergoing something of a renaissance. “27 five” are fat MTB tires. Some 26″ tubulars. | 584 mm | 1834 mm |
| 700 D | Oddball size formerly used on some GT models. Not available. 584 mm tires may work. Note: 700D is often incorrectly identified as 587mm bead seat. | 583mm | 1832 mm |
| 26 x 1, 650 C,26″ tubular | Triathlon, time-trial, high-performance road bikes for smaller riders. Some 26″ tubulars. | 571 mm | 1793 mm |
| 26 x 1 3/4 | Schwinn cruisers | ||
| 26 x 1.00- x 2.125,and wider on fatbikes | Most mountain bikes, cruisers, fatbikes | 559 mm | 1756 mm |
| 24 x 1 1/4, 24 x 1 3/8 (S-5) | Rare British or Schwinn juvenile | 547 mm | 1718 mm |
| 600A | French juvenile, very close to the nearest British size. | 541 mm | 1699 mm |
| 24 x 1 1/8, 24 x 1 3/8 (E.5) | British juvenile, most wheelchairs. French 600A is 541 mm, close enough. Saavedra 25″ tubular | 540 mm | 1696 mm |
| 24 x 1, 24 x 1 1/8, 24″ tubular | High-performance wheels for smaller riders; Terry front, most 24″ tubulars. | 520 mm | 1633 mm |
| 24 x 1.5- x 2.125 | Juvenile mountain bikes, BMX cruisers | 507 mm | 1593 mm |
| 22 x 1 3/8 | Wheelchair | 501 mm | 1573 mm |
| 550 A, 22 x 1 3/8 | European juvenile, folding bicycles | 490 mm | 1539 mm |
| 550C, 22 x 1 1/4″,22″ tubular | European juvenile and racing bicycles (rare); 22″ tubular | 470 mm | 1477 mm |
| 22 x 1.75; x 2.125 | Rare juvenile size…Schwinn | 457 mm | 1436 mm |
| 20 x 1 1/8; x 1 1/4; x 1 3/8 | Juvenile lightweights, BMX for light riders, some recumbents, Bike Friday Pocket Rocket | 451 mm | 1417 mm |
| 500 A | European juvenile, folding | 440 mm | 1382 mm |
| 20 x 1 3/4, 20″ tubular | Rare Schwinn juvenile, specialty racing bicycles, older Easy Racers recumbents | 419 mm | 1316 mm |
| 20 x 1.5- x 2.125 | Most BMX, juvenile, folders, trailers, some recumbents | 406 mm | 1275 mm |
| 18 x 1, 18 x 1 3/8 | Wheelchair | 400 mm | 1257 mm |
| 450 A | European juvenile | 390 mm | 1225 mm |
| 17 x 1 1/4, 18″ tubular | Alex Moulton AM series, 18″ tubular for specialty racing bicycles. | 369 mm | 1159 mm |
| 18 x 1.5- x 2.125 | Birdy folding bikes | 355 mm | 1115 mm |
| 16 x 1 3/8 | Older Moulton, Brompton & other folders, recumbent front, Greenspeed trikes, juvenile | 349 mm | 1096 mm |
| 400 A | European juvenile | 340 mm | 1068 mm |
| 16 x 1 3/8 | Very rare mystery tire | 337 mm | 1059 mm |
| 16 x 1 3/8 | Very rare Polish juvenile | 335 mm | 1052 mm |
| 16 x 1 3/4 | Rare Schwinn juvenile. Probably the same rim diameter as 16″ tubulars. | 317 mm | 996 mm |
| 16 x 1.75- x 2.125 | Juvenile, folders, trailers, some recumbents | 305 mm | 958 mm |
| 12 1/2 X anything, deicmal or fraction. | Juvenile, scooters, trailers. | 203 mm | 638 mm |
| 10 x 2 | Wheelchair casters | 152 mm | 478 mm |
| 8 x 1 1/4 | Wheelchair casters | 137 mm | 431 mm |
Sophisticated Tools for Precise Measurement
While this guide emphasizes common household tools, specialized tools like calipers with depth gauges can streamline the measurement process for professionals or those who frequently work with bicycle rims.
Math and Measurement: It’s Simpler Than You Think
Measuring rims involves basic math. Don’t let math anxiety deter you! As Albert Einstein famously said, “Do not worry about your difficulties in Mathematics. I can assure you mine are still greater.” This guide aims to make the process accessible and straightforward, demonstrating that determining your bike rim dimensions is achievable with simple tools and calculations.
Sutherland’s Rim Measuring System
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