Bike Tire Size Calculator

1. Find the ISO/ETRTO size on your tire sidewall, such as 28-622. Enter the first number as the tire width in mm.
2. Pick the wheel size that matches the second number (the bead seat diameter). For a size not listed, type it in the optional field.
3. Keep the reference tire at 700 x 28C, or set it to the tire your bike computer is calibrated for. Read the diameter, circumference and the difference.

Here is what each result means:

An ISO/ETRTO bike tire size is a pair of millimetre numbers: the nominal tire width and the bead seat diameter (BSD) of the rim it fits. In 28-622, the tire is about 28 mm wide and sits on a rim whose bead seat measures 622 mm across. The same width-diameter format is known as the ETRTO size, after the European Tyre and Rim Technical Organisation that aligns tyre and rim standards in Europe, and it is published internationally as ISO 5775-1.

It exists because the traditional names are ambiguous. The Sheldon Brown tire sizing guide notes that "29 inch" is a marketing name for wide 622 mm tires, the same rim size as 700C road tires, and "27.5" means 584 mm. Tires marked 26 inch have been made on several different rims, including 559, 571, 584, 590 and 597 mm, according to the same Sheldon Brown size tables.

This size helps anyone who buys tires, sets up a bike computer or checks frame clearance. It is not a measurement of the inflated tire on your bike, which varies with rim width, pressure and brand.

It adds the tire height to both sides of the bead seat diameter, then multiplies by pi for the circumference.

1. Assume the tire is about as tall as it is wide, so the height is 100% of the width unless you change the advanced field.
2. Add twice the height to the bead seat diameter, because the tire sits above the rim on both the top and bottom of the wheel.
3. Multiply the diameter by pi for the circumference, then divide by 25.4 for inches (1 in = 25.4 mm exactly, NIST SP 811) or by 1,000 for metres.
4. Divide 1,000,000 mm or 1,609,344 mm (one mile) by the circumference for revolutions per km or per mile.

This is the same approach Sheldon Brown and John Allen describe for bike computers: add the BSD to twice the width, then multiply by pi. The circle calculator shows the diameter-to-circumference step on its own.

Take a common road tire, 700 x 28C, which is 28-622 in ISO terms.

Calculation: outer diameter = 622 + 2 x 28 = 678 mm, or 678 / 25.4 = 26.69 in. Circumference = pi x 678 = 2,130 mm, or 83.86 in. Each wheel turn moves the bike 2.130 m, so it takes about 469 turns to cover a kilometre and about 756 to cover a mile.

Now compare a 47-584 gravel tire. Its diameter is 584 + 2 x 47 = 678 mm, exactly the same as the 28-622 road tire. So a wide 650B tire can keep about the same outer diameter as a narrow 700C tire.

A wider 40-622 tire gives 702 mm and 2,205 mm, which is 3.54% more circumference than 28-622. A bike computer still set for the 28 mm tire would read about 3.42% low until you update it.

The estimate is only as close as the tire is to its label, so a few real-world factors move it.

Actual Versus Nominal Width

The ISO width is nominal. The same tire can measure wider on a wide rim and narrower on a narrow one, and brands differ. If you measure your inflated width with calipers, enter that instead.

Tire Profile

Bike tires are close to round in section, which is why height is set to 100% of width. Some tires are a little taller or shorter than they are wide. Use the advanced height field if you have measured the height.

Load and Pressure

A rider squashes the tire where it meets the road, so the distance rolled per turn under load is a little shorter than the unloaded estimate. Sheldon Brown and John Allen note that tread thickness, pressure and rider weight all affect the effective size, which is why they measure roll-out with the rider aboard.

Tread and Knobs

Tall knobs on mountain bike tires add to the outer diameter, but the rolling distance depends on how the knobs deform, not only on their tips.

Traditional names describe a wheel family, while the ISO number fixes the rim size. This table matches the presets in the calculator.

Formula Estimate vs Published Chart vs Roll-out Test

The chart value is only about 0.28% above the formula, which shows how close the simple estimate gets.

Setting a Bike Computer

Most computers ask for the wheel circumference in mm. Enter the circumference from this tool, or measure a roll-out for the best accuracy. The mph to km/h converter helps if you switch display units.

Choosing Wider Tires

Going from 28-622 to 40-622 adds 24 mm of diameter, so the axle and bottom bracket sit about 12 mm higher and the tire needs more room in the frame and fork. Check clearance before you buy.

Swapping Wheel Sizes

Compare a 650B setup with a 700C one to keep the outer diameter, and so the bike handling, close to the original. For commuters, the commute cost calculator helps weigh the running costs of a new setup.

1. Trusting the Inch Name Alone

A 26 inch tire can fit several different rims. Always match the ISO bead seat number, not the inch label.

2. Using a Car Tire Method

Car sizes use a separate aspect ratio and rim size in inches. For bikes, enter the width and bead seat diameter in mm and leave height at 100%.

3. Mixing Diameter and Circumference

Bike computers want circumference, not diameter. Entering 678 instead of 2,130 makes every speed and distance wildly wrong.

4. Mixing Mm, Cm and Inches

Check which unit your computer expects. 2,130 mm is 213.0 cm and 83.86 in.

5. Forgetting to Update After a Tire Change

A wider or narrower tire changes the circumference. Keep the old tire as the reference to see how far your readings would drift.

The math is exact for the sizes you enter; the estimate usually lands close to real tires, but it is still an estimate.

Yes, for rim size. Both use a 622 mm bead seat diameter, so the rims are the same. The 29 inch name is used for wider mountain bike tires, which makes the outer diameter larger than a narrow road tire on the same rim.

It is the French-style name for a road tire about 28 mm wide on a 622 mm rim. In ISO terms it is 28-622, which gives an outer diameter of about 678 mm.

Enter the circumference in mm, for example about 2,130 mm for a 28-622 tire. For the most accurate readings, mark the tire, roll the bike one full turn with you seated, and measure the distance.

Often, if the frame and brakes allow it. A 47-584 tire has the same outer diameter as a 28-622 tire, so the handling stays close. Check brake reach and frame clearance first.

It changes it by about the same percentage as the circumference. Going from 28-622 to 40-622 adds 3.54% circumference, so a computer left on the old setting reads about 3.42% low.

Bike tires have a nearly round cross-section, so height is close to width. It is a simple, widely used estimate. If you have measured a different height, change the advanced height field.

Type the bead seat diameter in the optional field, such as 571 or 590. It overrides the dropdown and the results update right away.

Car sizes use a width, an aspect ratio and a rim size in inches. Bike sizes use width and bead seat diameter in mm, with height close to width. The car method gives the wrong answer for most bike tires.

Yes. ISO 5775-1 covers tubeless, tubeless-ready and tube-type clincher tires with the same width and bead seat designations, so the estimate works the same way. Real inflated width can still vary by rim and brand.

No. The calculation runs in your browser and nothing is sent to our servers. If you press Save, the result is stored only in this browser.

• ISO 5775-1:2023 Bicycle tyres and rims, tyre designations and dimensions (International Organization for Standardization).
• European Tyre and Rim Technical Organisation (ETRTO).
• Cyclecomputer and GPS calibration (Sheldon Brown and John Allen).
• Tire sizing systems and ISO cross reference (Sheldon Brown).
• NIST Guide to the SI, Appendix B.9 conversion factors (NIST SP 811).