Bike Tire Size Conversion Chart | Match Sizes Right

The ISO bead seat diameter is the safest way to match inch, 650, 27.5, 28, 29, and 700C tire labels across brands.

A Bike Tire Size Conversion Chart helps only when you know which number matters on the sidewall. That number is not always the big inch label. It’s the ISO or ETRTO size, and it tells you the bead seat diameter that must match your rim.

That’s why tire shopping feels messy. One bike may wear 700x38C, another may say 28 x 1.50, and a mountain bike tire may say 29 x 2.10. Some of those labels point to the same rim diameter. Others look close and still won’t fit.

If you want the short version, start with the last number in the ISO code. A tire marked 40-622 fits a 622 rim. A tire marked 47-584 fits a 584 rim. Once that diameter matches, you can sort out width, frame clearance, rim width, tread, and pressure.

Why Bike Tire Labels Get Confusing

Bike tire names come from a few old systems that never lined up neatly. Inch sizing is common on mountain, BMX, and kids’ bikes. French sizing shows up on road, touring, and gravel bikes. ISO sizing cuts through both by giving one clear diameter in millimeters.

Take 26-inch tires. A modern mountain bike 26 x 2.0 tire usually fits a 559 rim. A 26 x 1 3/8 tire often fits a 590 rim instead. Both say 26 inch. They are not the same fit.

The same thing happens at the larger end. Many 700C, 28-inch, and 29er tires share the same 622 rim diameter. The tire shape and width may differ a lot, though the rim size matches.

How To Read The Numbers On Your Sidewall

Most tires show at least one size line, and many show two. The ISO line usually looks like this: 37-622. The first number is the tire’s stated width in millimeters. The second is the rim diameter in millimeters.

• 37-622 = about 37 mm wide, fits a 622 rim
• 47-584 = about 47 mm wide, fits a 584 rim
• 25-571 = about 25 mm wide, fits a 571 rim

That second number is the one to trust first. Schwalbe’s tire sizing page lays out why the ETRTO format removes the guesswork. Park Tool’s fit standards article makes the workshop rule plain too: match the bead seat diameter before anything else.

Width still matters, just later. A wider tire may rub the frame, fork, fenders, or brakes. It may also sit differently on a narrow rim than it does on a wide one. So diameter gives you the yes-or-no fit. Width decides whether the tire will work well on your bike.

Bike Tire Size Conversion Chart By ISO Number

Use this chart to match the old label on your bike to the ISO diameter that actually decides fit. The middle column includes common pairings you’ll see in shops and on sidewalls.

Common Label	Typical ISO Size	Usually Converts To
16 x 1 3/8	37-349	349 mm rim
20 x 1.75 / 20 x 2.0	47-406 to 54-406	406 mm BMX or kids’ rim
24 x 1.75 / 24 x 2.0	47-507 to 54-507	507 mm rim
26 x 1.75 / 26 x 2.0	47-559 to 54-559	559 mm MTB rim
26 x 1 3/8	37-590	590 mm rim
27.5 x 2.1 / 27.5 x 2.4	54-584 to 61-584	584 mm rim
650B x 47 / 650B x 42	47-584 to 42-584	Same 584 mm rim as 27.5
700C x 25 / 700C x 40	25-622 to 40-622	622 mm rim
28 x 1.50	40-622	Same 622 mm rim as 700C
29 x 2.1 / 29 x 2.25	54-622 to 57-622	Same 622 mm rim as 700C
650C x 23	23-571	571 mm rim
27 x 1 1/4	32-630	630 mm rim

Where Riders Get The Match Wrong

The most common mistake is buying by the old label only. A rider sees “26 inch” and grabs the first 26-inch tire on the shelf. That can fail fast if the old tire was a 590 size and the new one is a 559.

Another slip happens when people hear that 29er and 700C share a diameter, then assume any 29er tire will fit any 700C bike. The rim diameter may match, yet the tire can still be too wide for the frame, fork, or rim.

Road and triathlon riders run into a similar trap with 650B and 650C. The letters matter there. 650B is 584. 650C is 571. Close names, different fit.

Three Checks That Save Time

1. Read the ISO size printed on the old tire.
2. Match the second number to your rim.
3. Pick a width your frame, brakes, and rim can handle.

If the old tire is missing, check the rim sticker, the bike maker’s spec sheet, or the tube you pulled out. Tube packaging often lists the tire size range it fits, which can point you back to the right diameter.

Common Conversions That Work And Ones That Do Not

This second chart shows the conversions riders ask about most often. It’s built around the rim diameter first, then the real-world catch that decides whether the swap will work on the bike.

Your Current Size	Usually Matches	Watch Out For
700C	28 inch, many 29er rims	29er tires may be too wide for road frames
29er	700C, many 28 inch rims	Frame and fork clearance
27.5 inch	650B	Width and tread height
26 x 1.75	Other 559-based 26 inch tires	Do not mix with 26 x 1 3/8
26 x 1 3/8	Other 590-based tires	Do not mix with 559-based 26 inch MTB tires
650C	Only other 571-based tires	Do not mix with 650B
27 inch	Only other 630-based tires	Not the same as 700C

How To Buy The Right Replacement Tire

Start with the tire you already have. If it reads 40-622, your replacement tire must end in 622. Then choose the width range that fits your bike. A small jump, like 35 mm to 38 mm, often works if there is room. A big jump may not.

Check Rim Width And Clearance

A tire’s printed width is only a starting mark. Mounted width can grow or shrink based on the rim’s internal width. A tire on a wider rim usually measures wider than the same tire on a narrow rim.

Look for room at four spots: chainstays, seatstays, fork legs, and brake area. Mudguards and fenders cut that room down even more. If your old tire clears with only a few millimeters to spare, stay close to the old width.

Match The Tube Too

If you run tubes, the tube must match the same rim diameter and sit within the tire width range. Valve type matters too. Most road, gravel, and many MTB wheels use Presta valves. Many kids’ and city bikes use Schrader.

Know What Does Not Change The Fit

Tread pattern, puncture belt, folding bead, wire bead, tubeless-ready build, and casing feel all matter on the ride. None of them change the bead seat diameter. A fast slick and a chunky trail tire can share the same rim size while riding like two different bikes.

One Simple Rule To Keep

When labels clash, trust the ISO number. The last number tells you whether the tire fits the rim at all. The first number tells you how wide it is meant to be. Once you read those two numbers, inch labels stop feeling like a puzzle.

That makes the whole buying job easier. Match the diameter. Pick the width your bike can clear. Then choose the tread and casing that fit the way you ride.