Are you tired of asking this question only to see hoards of confusing numbers pop up on your screen? Do you just want a simple guide to understanding your bike’s inner tube size? Well then, you’ve come to the right place.

Figuring out the inner tube size of your bicycle tires is a nightmare. That’s because you have to deal with not one, not two, but three different sizing systems. This makes it nearly impossible to have a universally accurate **bicycle inner tube size conversion chart**. So expect lots of trails and errors while changing your bike’s inner tube.

**Why Is This So Confusing?**

The short version? Because there isn’t a centralized system yet! Although there have been attempts to establish this with the ISO/ETRTO system, the results have been mixed. Let’s just say that, in the biking industry, everyone does whatever they want. Companies employ the system which they prefer for their own agendas, and it’s the common biker that suffers from the added complexity.

So, is there ever going to be a **bicycle inner tube size conversion chart** that is universally accepted? Well, maybe in the future, if manufacturers get along with each other. Otherwise, it’s most likely a lost cause. However, there are a few guidelines for accurately (almost) predicting the inner tube size of your bike tires. But you’re still going to need some tinkering around.

**How To Figure Out Your Bike’s Inner Tube Sizing**

The first thing you want to do is find out what sizing system your bike tires fall under. You can do this by taking a look at the wheels; just crouch down and look at the tire’s sidewall. If you don’t know what that is, it’s basically a rubber strip adjacent to the wheel’s rim. To be more simple, it’s that part of the tire that DOESN’T touch the ground.

What you want to look out for is a set of numbers displayed on your tire’s sidewall. This could read something like ’57-584, 27.5×2.25, 650B’. Why the long numbers? Well, there are three sizing systems that are used for bike tires; ISO (ETRTO) sizing, imperial sizing, and metric sizing. In this situation, 57-584 are the ISO (ETRTO) numbers, 27.5×2.25 are the imperial numbers, and 650B are the metric numbers.

You can also refer to them as ETRTO size, English size, and French size. While these systems do have their own ‘scientific’ terminologies, we can at least have some wiggle room in the naming. After all, anything that can help us better understand this complex mess is a welcoming element.

The first thing to understand is that the first set of numbers signifies the diameter of the tire, while the second signifies the tire’s width. However, this seems to be the other way around in the ETRTO sizing system. So, the first set will be the tire’s width, while the second will be the tire’s diameter. Go figure.

## Different Sizing System – Different Measurement’s System

Now, the second and most important aspect of the sizing system is what measurement each system uses. The ETRTO and metric systems use millimeters, while the imperial system uses inches. So converting between these systems is quite simple (Not really), with the occasional hit and miss of course.

So if we were to use the above tire size as an example, the first pair of metric numbers, 27.5 inches, is the same as 584 mm. Note that the ETRTO counterpart to the metric numbers is the second pair and not the first. Meanwhile, the second metric pair, 2.25 inches, represents 57 mm on the ETRTO scale.

Now, you will notice that while the first two examples come in a pair of two, the third only has one number and a letter. Don’t be confused; there’s a simple explanation for this.

The metric, A.K.A, the French system, goes on to add alphabets into their pairs as well. Sigh, they really love overcomplicating the simplest of things, don’t they? In this example, 650B represents the standard metric size for 27.5 inches. You might also come across metric sizes that read something like 700x32c. In this case, **700x32c tire size in inches** will be 27 ½’’ by 1 ¼’’ (give or take).

**A Simple Bicycle Inner Tube Size Conversion Chart**

So, considering everything we have discussed till now, the best **bicycle inner tube size conversion chart** we can come up with looks something like this.

ISO (ETRTO) Sizing System | Imperial Sizing System | Metric Sizing System |

37-622 | 28 x 1-5/8 x 1-3/8 | 700 x 35C |

32-597 | 26 x 1-1/4 | |

28-622 | 28 x 1-5/8 x 1-1/8 | 700 x 28C |

23-622 | 700 x 23C | |

40-635 | 28 x 1½ | 700B standard |

32-622 | 28 x 1-5/8 x 1-1/4 | 700 x 32C 700C Course |

40-622 | 28 x 1-5/8 x 1-3/4 | 700 x 40C |

25-622 | 28 x 1-5/8 x 1 | 700 x 25C |

38-622 | 28 x 1-5/8 x 1½ | 700 x 38C |

32-630 | 27 x 1-1/4 | |

28-630 | 27 x 1-1/8 | |

25-630 | 27 x 1 | |

47-622 | 28 x 1,75 | 700 x 47C |

37-590 | 26 x 1 3/8 | 650 x 35A 650A |

25-571 | 26 x 1 | |

32-590 | 26 x 1-3/8 x 1-1/4 | 650 x 32A |

40-590 | 26 x 1-3/8 x 1½ | |

40-584 | 26 x 1½ | 650B Standard |

32-541 | 24 x 1-3/8 x 1-1/4 | 600A 600 x 32A |

47-559 | 26 x 2 x 1-3/4 | |

37-584 | 26 x 1½ x 1-3/8 | |

40-540 | 24 x 1-3/8 x 1½ | |

37-540 | 24 x 1-3/8 | |

40-534 | 24-1½ | |

47-507 | 24-2 x 1-3/4 | |

37-501 | 22 x 1-3/8 B.S.R. | |

37-498 | 22 x 1-3/8 x 1-1/4 | |

40-489 | 22 x 1-3/8 x 1½ | |

37-489 | 22 x 1-3/8 | |

37-451 | 20 x 1-3/8 B.S.R. | |

40-438 | 20 x 1-3/8 x 1½ | |

37-438 | 20 x 1-3/8 | |

32-357 | 17 x 1-1/4 | |

57-406 | 20 x 2,125 | |

47-406 | 20 x 2 x 1-3/4 | |

40-432 | 20 x 1½ | |

47-355 | 18 x 2 x 1-3/4 | |

54-305 | 16 x 2 | |

37-288 | 14 x 1-3/8 | |

37-298 | 14 x 1-3/8 | |

47-305 | 16 x 2 x 1-3/4 | |

62-203 | 12½ x 2-1/4 | 12½ x 2-1/4 |

## Conclusion

Despite having a **bicycle inner tube size conversion chart** to refer to, you might still get the sizing wrong. So, we’d recommend not taking these figures at face value, and leaving a few inches here and there for interpretation.

As we mentioned before, this isn’t something we can fix until the so-called geniuses in the biking industry stop fighting each other. So until then, stick to your own sizing system. Unless you want to end up with a headache.