Bike Tire Conversion Chart | Match Sizes Without Guessing

One tire label can map to different rim sizes, so the bead seat diameter in millimeters is the number that tells you what truly fits.

Bike tire sizing gets messy fast. One sidewall might say 700x35C. Another says 37-622. A mountain bike tire may say 29×2.20, while a gravel tire on the next rack says 700x45C. They can fit the same rim diameter, yet the labels look like they belong to different worlds.

That’s why a good Bike Tire Conversion Chart is less about memorizing names and more about reading the one number that doesn’t lie: the bead seat diameter, often shown in the ISO or ETRTO format. Once you know that number, shopping gets easier, fit mistakes drop, and you stop wasting time comparing labels that sound bigger or smaller than they really are.

This chart is built to do one job well: help you convert the common inch, French, and ISO markings into a size you can buy with confidence. It also shows where riders get tripped up, since some labels that sound alike do not fit the same rim at all.

Why Bike Tire Labels Get Confusing So Fast

Bike tires are named in a few different ways. Older inch labels like 26×1.75 are still everywhere. Road and gravel tires often use French sizing like 700x32C or 650Bx47. Then there’s the ISO or ETRTO format, which shows width first and bead seat diameter second, such as 32-622.

The trouble starts when the casual label sounds plain but hides different diameters. “26 inch” is the classic trap. That name has been used for several rim sizes over the years. So if you buy by the inch label alone, you can land on a tire that looks close on paper and still won’t mount on your wheel.

The Number That Matters Most

The last three digits in an ISO or ETRTO size are the fit number. In 37-622, the “622” is the rim’s bead seat diameter in millimeters. If your old tire says 622 and the new tire says 622, you’re in the right family. Width can still change, but the tire is made for the same rim diameter.

That single number is why 700C road tires and 29er mountain bike tires can share the same rim diameter. The names differ. The fit number does not.

Why Width Still Matters

Diameter gets the tire onto the rim. Width decides whether it works well on your bike. A wider tire may rub the frame, fork, fenders, or brake bridge. It may also fall outside the rim maker’s allowed range. So diameter comes first, then width, then clearance.

REI’s tire-sizing notes and bike tire sizing overview line up with the same idea: learn the sidewall marking, then match the tire to your wheel diameter and riding style.

How To Use A Bike Tire Conversion Chart

Start with the tire already on your bike. Find the ISO or ETRTO marking if it’s there. It will look like two numbers split by a dash. The second number is your rim diameter. Match that first. Next, choose a width that your frame and rim can handle.

If your tire only shows an inch or French label, use the chart below to translate it into the bead seat diameter. Once you know the millimeter diameter, the whole picture gets cleaner.

Common Bike Tire Size Conversions

Common Tire Label	ISO / ETRTO Diameter	What It Usually Means
20 x 1.75	47-406	BMX, folding bikes, compact bikes
24 x 1.75	47-507	Youth mountain bikes, some cruisers
26 x 1.75	47-559	Common older MTB and commuter size
26 x 1 3/8	37-590	Older city and utility bikes; not the same as 559
650C	571	Older triathlon and some small road bikes
27.5 / 650B	584	Modern trail, gravel, and all-road size
700C	622	Road, gravel, hybrid, touring
29er	622	Mountain bike label for the same rim diameter as 700C
27 x 1 1/4	32-630	Older road bikes; larger than 700C

Conversions That Trip Up Riders Most Often

Some size names are cousins. Some are strangers wearing the same jacket. That’s where bad purchases happen.

700C And 29er Are The Same Rim Diameter

This surprises a lot of riders the first time they swap between road, gravel, and mountain parts. A 700C tire and a 29er tire both use a 622 bead seat diameter. The tire shape, width, tread, and marketing name may differ a lot. The rim diameter does not.

So a 700x45C gravel tire and a 29×1.75 tire can point to the same diameter family. The width and intended use are different. The fit number on the rim is the same.

27.5 And 650B Match Too

Another pair that causes confusion is 27.5 and 650B. Both sit on a 584 rim diameter. In plain terms, “27.5” is the mountain bike label and “650B” is the road and gravel label for the same wheel diameter. If you know that, a whole section of the tire wall suddenly makes sense.

“26 Inch” Is Not One Single Size

This is the old headache that still catches riders today. A modern 26-inch mountain bike tire is usually 559. Yet older 26×1 3/8 tires are often 590. Some roadster and specialty tires use other diameters too. If you order a tire just because it says “26,” you’re rolling the dice.

Continental’s ETRTO tire and rim notes make the same point in a stricter way: tire and rim compatibility has to be checked by size, limits, and maker guidance, not by a loose label alone.

Sidewall Markings You Can Translate At A Glance

If Your Tire Says	Search Or Buy As	What To Watch
700 x 25C	25-622	Road width; same diameter family as 29er
700 x 40C	40-622	Hybrid or gravel width on a 622 rim
29 x 2.25	57-622	Same rim diameter as 700C, much wider tire
650B x 47	47-584	Same diameter family as 27.5
27.5 x 2.10	54-584	Same rim diameter as 650B
26 x 1.95	50-559	Common older MTB size
26 x 1 3/8	37-590	Not the same as 26 x 1.75
27 x 1 1/4	32-630	Older road size; do not swap with 700C

How To Read Your Tire Before You Order

Most riders already have the answer sitting on the bike. It’s printed on the sidewall. You just need to read it in the right order.

Find The Full Marking

Clean the tire sidewall and rotate the wheel until you spot the size. It may show one format or two. A gravel tire might say 700x38C and 40-622. A mountain tire may say 29×2.20 and 57-622.

Match Diameter First

Use the last three digits in the ISO or ETRTO size as your first filter. That is your rim diameter. If the old tire is 622, the new tire must also be 622.

Then Check Width And Clearance

Next, compare the width. If your bike came with 32-622, there may be room for 35-622 or 38-622, but only if the frame, fork, fenders, and brakes leave enough space. Wider is not always better. It has to clear on both sides and at the top.

When You Can Change Width Safely

You can often move a little wider or a little narrower inside the same diameter family. That’s common on road, gravel, commuter, and mountain bikes. Still, there are limits. Rim width affects tire shape, and your bike’s clearances set the hard stop.

That means a 32-622 tire can sometimes become a 35-622 or 38-622, while a 47-584 may swap to a 50-584 on a bike with room for it. The bike decides what is possible, not wishful thinking or a close-sounding label.

Also check the rim and tire maker limits before making a big jump. Pressure limits and approved rim-width ranges matter. A tire that mounts is not always a tire that should be ridden on that wheel.

Mistakes That Waste Money Fast

• Buying by “26 inch” alone and skipping the ISO number.
• Assuming 27 inch and 700C are the same. They are not.
• Thinking 29er needs a different rim diameter than 700C.
• Jumping to a wider tire without checking frame and brake space.
• Reading only the front half of the size and missing the bead seat diameter.

What To Do Before You Hit Buy

Read the sidewall. Find the ISO or ETRTO number. Match the bead seat diameter first. Then choose a width your bike and rim can handle. If the old tire uses a vague inch label, translate it with the chart before you shop.

That simple routine turns a messy shelf of tire names into a clean yes-or-no fit check. Once you start using the millimeter diameter, bike tire sizing stops feeling random.