Tire diameter calculator
Compare two sizes
Metric sizes (120/80-18) and inch sizes (3.00-21) are both understood. An R or ZR in the middle is fine.
How the numbers are worked out
A size like 120/80-18 reads as “120 mm wide, aspect ratio 80%, 18-inch rim”. The aspect ratio is the sidewall height as a percentage of the width, so the sidewall here is 120 × 0.80 = 96 mm. The overall diameter is the rim diameter plus two sidewalls.
Diameter = rim (inch) × 25.4 + width × aspect ÷ 100 × 2
For 120/80-18 that gives 18 × 25.4 + 96 × 2 = 649.2 mm, and a circumference of 2039.5 mm — roughly two metres per revolution.
Ride height changes by half the difference in diameter. A tire 32 mm larger in diameter raises the axle by 16 mm, because the bottom half is on the ground.
Inch sizes carry no aspect ratio. By convention they are treated as 100% (sidewall height equal to width), and that is what is used here. It is a convention for calculating, not a measurement of the actual tire.
What these numbers cannot tell you
Real tires vary between brands at the same size. What you see here is the size marking worked through the formula. Where a manufacturer publishes a measured diameter, that figure is the right one — differences of over 10 mm exist.
Diameter does not decide whether a tire fits your bike. Rim width, clearance at the fender and swingarm, and whether it fouls the chain or disc are all separate questions. A similar diameter is not evidence that a tire will fit.
The speedometer row shows the shift, not the error. A larger diameter covers more ground per revolution, so the needle reads lower than your true speed. Most bikes already read a few percent high from the factory, so this row tells you how far the reading moves from standard — not what the total error is. On bikes with ABS or other systems that use wheel speed, a change in diameter can affect more than the display.
Alpha sizes (MT90-16 and similar) cannot be calculated. Their widths are defined by a table rather than a formula. See the size cross-reference.
Related pages
- Tire size cross-reference — how metric, inch and alpha markings map to each other
- Rim widths, tubes and rim locks — whether a width suits your rim
- Motorcycle Tire Finder — tires that fit the OE sizes of your bike, compared across brands