Motorcycle Tire Size Calculator

1. Copy the size from the sidewall of your current tire, such as 180/55ZR17 or 150/70-17, into the first box.
2. Type the size you are thinking of fitting into the second box. Keep it on the same wheel: front with front, rear with rear.
3. Enter a speed you often see on the speedometer. Read the diameter, revs per mile, diameter change and the true speed on the new tire.

Here is what each result tells you:

A metric motorcycle tire code states the section width in millimetres, the aspect ratio in percent, the construction and the rim diameter in inches. In 180/55ZR17, the tire is 180 mm wide, the sidewall is 55% of that width, ZR marks a radial tire in the Z speed category, and it fits a 17 inch rim.

These parts follow the same pattern that NHTSA explains for car and light truck tires. The first number is the width from sidewall edge to sidewall edge in millimetres. The two-digit aspect ratio gives the height-to-width ratio. The rim diameter code is the wheel diameter in inches, and an R stands for radial.

Motorcycle tires often add a dash in place of the R. A code such as 150/70-17 means a bias (cross-ply) tire, and a B means bias-belted. After the size you will usually find a service description, such as 73W. The number is the load index and the letter is the speed symbol. This calculator reads the size part and ignores the service description.

In the United States, motorcycle tires fall under Federal Motor Vehicle Safety Standard 119. It requires the size designation to be marked on each sidewall, so the code you type in is always printed on the tire. Older inch sizes such as 4.60-17 and letter sizes such as MT90-16 use a different system, which this tool does not decode.

The tool turns the code into a sidewall height, adds two sidewalls to the rim diameter, and divides a mile by the circumference.

1. Read width, aspect ratio, construction and rim size from the code, even with spaces or a load and speed code after it.
2. Multiply the width by the aspect ratio to get the sidewall height in mm.
3. Convert the rim size to mm with 1 in = 25.4 mm exactly, then add the sidewall twice: once below the hub and once above it.
4. Multiply by pi for the circumference, and divide one mile (1,609,344 mm exactly, from NIST) by it for revolutions per mile.
5. For a new tire, divide its diameter by the current one. A speedometer that reads from this wheel is off by the inverse of that ratio.

The mm to inches converter shows the metric to inch step on its own, and the circle calculator shows how diameter becomes circumference.

A sport bike owner wants to swap a 180/55ZR17 rear for a wider 190/50ZR17.

Current tire: sidewall = 180 x 0.55 = 99 mm. Diameter = 17 x 25.4 + 2 x 99 = 431.8 + 198 = 629.8 mm, or 24.80 in. Circumference = pi x 629.8 = 1,979 mm, so the wheel turns about 813 times per mile.

New tire: sidewall = 190 x 0.50 = 95 mm. Diameter = 431.8 + 190 = 621.8 mm, which is 8 mm or 1.27% shorter. It turns about 824 times per mile.

What it means: the wider tire is not taller. It is a little shorter, so the rear of the bike sits about 4 mm lower. A speedometer that reads from the rear wheel would show about 1.29% high: at 60 mph shown, the true speed is about 59.2 mph. A 190 also needs a rim wide enough for it, which the code alone does not tell you.

The code gives a nominal size. Real tires differ from it for several reasons.

Profile Shape

Motorcycle tires are rounded so the bike can lean. Two tires with the same code can have different crown shapes, so the measured height and the contact patch at lean differ by brand and model.

Rim Width

A tire mounted on a narrower or wider rim than its design rim changes its section width and profile. The code assumes the rim the tire was designed for.

Load, Pressure and Wear

Rider weight, luggage and low pressure flatten the tire where it meets the road, so the rolling distance per turn is a little shorter than the unloaded figure. A worn rear tire can lose several millimetres of tread depth, which also shortens the diameter.

Where the Speedometer Reads

Some bikes take speed from the front wheel, others from the rear wheel or the gearbox output. Changing a tire on a wheel the sensor does not read leaves the speedometer unchanged.

The letter or dash between the aspect ratio and the rim size tells you the construction. It does not change the size math.

Front vs Rear Size Changes

Ride height changes by half the diameter change, because the axle sits at the centre of the wheel.

Choosing a Wider Rear Tire

Riders often assume a wider tire is taller. The aspect ratio decides that. Compare the two codes here before you order, and check the rim width chart from the tire maker.

Correcting a Speedometer Reading

After a size change, the diameter change tells you how far the speedometer drifts. Check the reading against GPS and use the km/h to mph converter if your display shows km/h.

Sorting Out Ride Height and Handling

A shorter front tire lowers the front and quickens steering, and a taller rear tire does the same from the other end. The diameter change in mm shows the size of that shift.

1. Mixing Front and Rear Sizes

Front and rear tires are built for their own position. Compare a new rear only with the old rear, and a new front with the old front.

2. Reading the Aspect Ratio as Mm

The 55 in 180/55 is a percentage of the width, not a height in mm. The sidewall of that tire is 99 mm.

3. Ignoring the Rim Size

A 17 inch tire will not fit a 16 inch wheel. The tool flags a code with a different rim size.

4. Assuming Wider Means Better Grip

A wider tire on a rim that is too narrow pinches the profile and can hurt handling. The approved size list from the bike maker matters more than width.

5. Treating the Result as the Rolling Size

The tool gives the unloaded size. The tire rolls a little shorter under a rider, so a GPS check is still the final test.

The size math is exact for the code you enter; the real tire on your bike will be within a few millimetres of it.

No. A 190/50ZR17 is about 621.8 mm tall and a 180/55ZR17 is about 629.8 mm, so the 190 is 8 mm shorter even though it is 10 mm wider.

ZR marks a radial tire in the Z speed category. The exact speed symbol, such as W or Y, appears in the service description after the size, for example 73W.

The dash means bias, or cross-ply, construction. The size math is the same as for a radial: 150 x 0.70 gives a 105 mm sidewall on a 17 inch rim.

About 854. Its diameter is 599.8 mm and its circumference about 1,884 mm, and 1,609,344 mm divided by that gives roughly 854 turns per mile, unloaded.

Only when the speedometer takes its signal from the rear wheel or the gearbox. A bike that reads speed from the front wheel shows no change after a rear tire swap.

The size math shows the 120/60 is 24 mm shorter than a 120/70, which lowers the front about 12 mm. Whether it is approved for your bike is a question for the maker fitment list.

Tire makers usually measure revolutions with the tire loaded, so it rolls a little shorter than its unloaded size. Their figure is the better one for speedometer work when it is available.

Yes, when the size uses the metric width/aspect-rim format, such as 90/90-21 or 130/70-12. Inch sizes such as 3.50-10 need a different method.

Divide the diameter in mm by 25.4. A 629.8 mm tire is 24.80 inches tall, and its 180 mm width is 7.09 inches.

No. The calculation runs in your browser and nothing is sent to our servers. Pressing Save stores the result only in this browser.

• Tires: tire label guide (width, aspect ratio, R, rim diameter code) (U.S. National Highway Traffic Safety Administration; the page may block automated requests, confirmed 200 in a browser).
• 49 CFR 571.119, FMVSS 119: tires for motorcycles and heavier vehicles, marking rules (eCFR, NHTSA).
• 49 CFR 571.139, FMVSS 139: definition of section width (eCFR, NHTSA).
• NIST Guide to the SI, Appendix B.8: inch and mile conversion factors (NIST SP 811).