Choosing the right size tire for your bicycle can be perplexing, especially with the variety of sizing systems used over the years, particularly for vintage bikes. This guide clarifies how to accurately measure your bike rim to ensure compatibility with new tires, focusing on the ISO/E.T.R.T.O. standard for precision.
By Sheldon “ISO/E.T.R.T.O.” Brown and John “Measure Twice” Allen
“One measurement is worth 50 expert opinions.” – Howard Sutherland
Note: This article guides you on measuring rims for tire fit. For information on measuring rims for spokes, please refer to our article on how to measure rims for spokes.
Why Accurate Bike Rim Size Matters
Selecting the correct tire size is crucial for safety, performance, and comfort. Mismatched tires and rims can lead to:
- Tire slippage or blow-offs: If the tire bead doesn’t seat properly on the rim, it can detach under pressure, causing accidents.
- Poor handling: Incorrect tire width can negatively impact steering and stability.
- Reduced efficiency: Tires that are too wide or narrow for the rim can increase rolling resistance.
- Damage to rims or tires: Improper fit can stress both components, leading to premature wear or damage.
Ideally, matching your new tire to your old one using the ISO/E.T.R.T.O. size number is the most straightforward approach. This international standard ensures compatibility between tires and rims.
Decoding ISO/E.T.R.T.O. Bike Rim Sizes
The ISO/E.T.R.T.O. (International Organization for Standardization / European Tire and Rim Technical Organization) system provides a clear and consistent way to understand Bike Rim Sizes and tire compatibility. It uses two numbers:
- Bead Seat Diameter (BSD): This is the diameter of the rim where the tire bead sits. It’s the most critical measurement for tire compatibility.
- Rim Width: Indicates the inner width of the rim, influencing the range of suitable tire widths.
If you can find the ISO/E.T.R.T.O. size on your current tire or rim, matching it ensures a proper fit. However, if these markings are illegible or absent, measuring the rim directly becomes necessary.
Measuring Bike Rim Diameter: A Step-by-Step Guide
Measuring the diameter is most effective with an empty rim. Here’s how to do it:
- Gather your tools: You’ll need a measuring tape or ruler.
- Position the tape: Lay the tape across the rim, spanning from one inner edge to the opposite inner edge.
- Find the widest point: Slide the tape end back and forth to find the largest diameter across the rim. Ensure you are measuring across points directly opposite each other.
- Take multiple measurements: Measure across a few different diameters and calculate the average to account for minor irregularities in rim roundness.
- Convert to millimeters: If your measurement is in inches, multiply by 25.4 to convert to millimeters.
Alt text: Measuring bike rim diameter using a tape measure across the inner edges of the rim.
Example: If your measured diameter is approximately 20 7/8 inches (20.875 inches), converting it to millimeters: 20.875 inches * 25.4 mm/inch = 530 mm. Referring to a rim size chart, a 530mm outside diameter suggests a 520mm ISO/E.T.R.T.O. bead seat diameter rim.
Note: The measured rim diameter is typically slightly larger (around 10mm) than the ISO/E.T.R.T.O. bead seat diameter due to the rim flanges.
Measuring Bike Rim Circumference: An Alternative Method
Measuring circumference is useful for built-up wheels where the hub obstructs diameter measurement.
- Tools needed: A narrow, non-stretch metal measuring tape (around 1/4 inch or 6mm width is ideal to fit into the rim well). Avoid fabric tapes as they can stretch and provide inaccurate readings.
- Locate the valve hole: This will be your starting point.
- Hook the tape: Insert the tab of the metal tape measure into the valve hole.
- Wrap the tape: Carefully wrap the tape around the rim’s circumference, ensuring it sits at the bottom of the rim well.
- Record the circumference: Note the measurement where the tape completes a full circle.
- Calculate the diameter: Divide the circumference measurement by pi (approximately 3.142) to get the diameter of the rim well.
- Convert to millimeters (if needed): If your circumference is in inches, multiply the diameter by 25.4 to convert it to millimeters.
- Account for rim flange height: For hook-edge rims, measure the depth from the rim well to the bead seat area. Add twice this depth to the diameter calculated in step 6 to get the approximate outer diameter. Then, estimate and subtract the typical flange height to approximate the bead seat diameter.
Alt text: Measuring bike rim circumference by wrapping a metal tape measure around the rim and inserting the hook into the valve hole.
Example: If the measured circumference of the rim well is 64 1/8 inches (64.125 inches), then:
- Circumference in mm: 64.125 inches * 25.4 mm/inch = 1629 mm
- Diameter of the well: 1629 mm / 3.142 = 518.5 mm
- Adding twice the well depth (estimated at 16mm in the original article’s example): 518.5 mm + 16 mm = 534.5 mm
- Estimating and subtracting flange height to approximate BSD: Around 520mm BSD.
Bike Rim Size Chart: Common Sizes and ISO/E.T.R.T.O. Equivalents
This table outlines common bike rim sizes, their applications, and corresponding ISO/E.T.R.T.O. Bead Seat Diameters and Circumferences for rim measurement reference.
| 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, 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 | 559 mm | 1756 mm |
| 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 |
Tools for Precise Bike Rim Measurement
While tape measures and rulers are sufficient, specialized tools can enhance accuracy and efficiency:
- Calipers with Depth Gauge: Calipers provide precise external and internal measurements. A depth gauge attachment is particularly useful for accurately measuring rim well depth.
Conclusion: Measure Twice, Ride Right
Accurately determining your bike rim size is essential for selecting compatible tires, ensuring safety and optimal cycling performance. By understanding the ISO/E.T.R.T.O. system and employing these measuring techniques, you can confidently choose the correct tires for your bike and enjoy a smoother, safer ride. Remember, taking the time to measure properly is a worthwhile investment in your cycling experience.
