Tire Size Converter
Convert motorcycle and vehicle tire sizes into useful dimensions. Enter metric tire sizes such as 120/70-17 or inch-style sizes such as 4.00-18 and get diameter, sidewall, circumference, and estimated equivalent sizing.
Convert Tire Size
Choose metric or inch format, enter your tire size, and see the converted measurements instantly.
Common Motorcycle Tire Size Examples
Tire Size Converter Guide
A tire size converter helps you understand what the numbers on a motorcycle, scooter, car, or trailer tire actually mean. Tire sizes are often printed in a compact format, such as 120/70-17, 180/55ZR17, 90/90-21, or 4.00-18. Those numbers are not always easy to compare by eye. A converter turns the tire code into useful measurements such as sidewall height, overall diameter, circumference, and estimated revolutions per mile. These details help when comparing replacement tires, changing tire size, checking fitment, or understanding how a size change may affect handling and speedometer reading.
For motorcycle riders, tire size is especially important because the tire shape affects steering feel, lean behavior, stability, ride height, and clearance. A small change in width, profile, or rim diameter can change how quickly a bike turns, how much room is available under the fender, and how the tire sits on the rim. This tool is designed to make the numbers easier to understand before buying a tire or comparing two popular sizes.
How Metric Tire Sizes Work
A metric tire size such as 120/70-17 has three main parts. The first number, 120, is the tire width in millimeters. The second number, 70, is the aspect ratio. That means the sidewall height is 70 percent of the tire width. The final number, 17, is the rim diameter in inches. So a 120/70-17 tire is about 120 mm wide, has an estimated sidewall height of 84 mm, and fits a 17-inch rim.
The overall diameter is calculated by adding the rim diameter to two sidewalls, because the sidewall exists above and below the rim. Since the rim is measured in inches and the tire width is measured in millimeters, the converter changes the units automatically. This gives a practical estimate of total tire height, circumference, and revolutions per mile. These values are useful when comparing a stock tire with an alternative size.
How Inch Tire Sizes Work
Older motorcycles, classic bikes, cruisers, scooters, and some off-road tires may use inch-style sizing such as 4.00-18. In this format, 4.00 usually refers to the tire width in inches, and 18 is the rim diameter in inches. Unlike modern metric sizing, the aspect ratio is not always printed. Many classic inch-size tires are treated as having a tall sidewall, often close to 100 percent of the width, but this can vary by tire type and manufacturer.
Because inch sizing does not always include a visible aspect ratio, any conversion from inch format to metric format is an estimate. This tool lets you choose a profile assumption, such as 100 percent classic bias, 90 percent lower profile, or 80 percent low profile. The result gives an approximate metric equivalent and dimensional measurements. Always check the tire manufacturer’s specification chart before buying a replacement tire.
Why Overall Diameter Matters
Overall diameter is one of the most important values when changing tire size. If the diameter increases, the bike or vehicle may sit slightly higher, gearing may feel taller, and the speedometer may read differently. If the diameter decreases, the vehicle may sit lower, gearing may feel shorter, and engine RPM may be slightly higher at the same road speed. On motorcycles, diameter changes can also affect front-end geometry, rear ride height, and lean-in behavior.
Clearance is another major concern. A taller or wider tire may contact a fender, swingarm, chain guard, fork brace, brake line, mudguard, or luggage bracket. Even if the tire fits while parked, it may rub when the suspension compresses or when the tire grows slightly at speed. Before using a non-stock size, check clearance carefully and consider the manufacturer’s recommended rim width range.
Why Circumference and Revs Per Mile Matter
Circumference tells you how far the tire travels in one full rotation. Revolutions per mile is the opposite idea: it estimates how many times the tire turns to travel one mile. These numbers matter because many speedometers, odometers, ABS systems, traction control systems, and gearing calculations are influenced by wheel speed. A large tire size change can make the displayed speed or distance less accurate.
For example, a tire with a larger circumference turns fewer times per mile. This can make the speedometer read lower than before if the system is not recalibrated. A smaller tire turns more times per mile, which can make the speedometer read higher. On modern motorcycles with ABS or traction control, extreme tire size changes can also affect how the electronics interpret wheel speed differences.
Using This Tire Size Converter
To use the metric mode, enter the tire width in millimeters, the aspect ratio percentage, and the rim diameter in inches. The tool will show the formatted metric size, sidewall height, overall diameter, circumference, and estimated revolutions per mile. You can use the preset buttons for common motorcycle sizes such as 120/70-17, 180/55-17, 90/90-21, and 150/70-17.
To use the inch mode, enter the tire width in inches, choose a profile estimate, and enter the rim diameter. The converter will estimate the closest metric-style size and show the same dimensional results. This is useful for classic motorcycle owners who want to compare old inch sizes with modern metric sizes, but it should be used as a guide rather than a final fitment decision.
Tire Size Tips for Motorcycle Riders
- Start with the tire size recommended by the motorcycle manufacturer.
- Check the tire maker’s approved rim width range before changing width.
- Consider clearance at full suspension compression, not only while parked.
- Remember that a wider tire does not always improve handling.
- Compare overall diameter when changing profile or rim size.
- Use matching front and rear tire types when recommended by the manufacturer.
Final Thoughts
A tire size converter is a helpful first step when comparing replacement options, but it cannot replace a full fitment check. The numbers printed on the sidewall describe the tire’s basic size, while the real mounted shape depends on the tire design and rim. Use this tool to understand width, sidewall, diameter, circumference, and estimated equivalent sizes. Then confirm the final choice with the tire manufacturer, your vehicle manual, or a qualified mechanic before installing a non-standard size.