Understanding 700x42c Tire Size In Inches

Your bicycle tire comes with a series of numbers and, very often, alphabets that all seem to show different dimensions and figures of your tire’s size. The universal 700c figure seems to be present across tires of varying sizes, and it can muddle your understanding of the exact dimensions of your tire when you’re more familiar with the metric system that uses inches. So, how do you express a 700x42c tire size in inches without miscalculating?

Discover below how you can decipher any bike tire size and learn to convert any bike tire’s value into inches or any other unit you prefer.

700x42c tire size in inches

How To Convert 700x42c Into Inches Correctly

The correct way of converting 700x42c in terms of inches is by reading the ISO and US/UK dimensions given on the tire. The 700 stands for diagonal measurement of the tire from one tire surface to the opposite surface. Since 700c is an estimated rounded value, the value in inches may come to 27.5 or 29 inches which are common standard sizes. The 42c is normally the width of the tire expressed in millimeters. So, a 700x42c can mean a tire that measures 27.5 or 29 inches across its diameter and is 1.65 inches thick.

The main advantage with having different metric measurements on your tire is that you can use each value as a reference for the other. Most tires come with three of the prevailing systems of indicating tire sizes – the UK/American metric, the older French system, and the ISO measurement.

The 700x42c Belongs To The French System

The 700x42c belongs to the French system of bygone days. Mostly used during the 1970s and 80s, this system used variants like 700a, 700b, 700c, and 700d to indicate tires with the same rims sizes but different widths. Eventually, they settled on the 700c as a rounded representation that could represent varying sizes around the same range (Hence the inaccuracy when calculating exact values).

The UK/American metric values uses inches as the accepted unit for denoting the tire’s size. This makes sense too because the US continues using the metric scale and even tools like wrenches come in inch-wise sizes. So, if your tire carried a value that showed 700 x 35c in the French system, but carried the numbers 28×15/8x13/8, that implies that the tire’s size in inches is actually 28 inches (not 27.5) in diameter.

So, instead of manually converting the 700c (mm) into inches, simply referring to the 20 or 30 figure given on the tire should reveal the dimensions in inches.

The Most Reliable Measurement

However, the most reliable measurement of your tire should be the ISO value given beside these two (French and UK/American) values. The ISO value is the measurement set by the International Standards Organization, which serves as the accepted value as far as accurate measurements go.

As opposed to the estimate figure given by the French system (700c), the ISO standard will show you the exact measurement of diameter in millimeters. So, a 700x42c tires size in inches may be 26, which means the ISO value will show it as 660mm (26 inches). This 660 value will be the real measure of the tire’s diagonal distance, instead of the 700c which is an estimate. Also, when converted to inches, the 660mm will appear as 26 inches (as depicted by the UK/American measurement on the tire).

Today, almost every manufacturer of bicycle tires include the ISO measurements along with the older French system and the UK/American values. This means you shouldn’t have any trouble locating the ISO numbers on your tire. In all tires, the values should be impressed on the side of the tire. So, if you view it from the side where the tire appears as an ‘O’, these values should all be there on the tire’s surface below the treading.

You can also check out a comprehensive bicycle inner tube size chart if you want more details on what each standard and particular size mean.

What Do 700c Values Mean?

When you see the 700x42c figure on your tire, you should be able to tell right off the bat that these are the tire’s dimensions in a certain unit. The unit in question here is millimeters.

Don’t let the conspicuous ‘c’ fool your eye, this value is supposed to be your tire’s measurements in millimeters. A lot of beginners can misread this symbol and take it as centimeters. This is an outrageous figure because a tire measuring 700cm (22 feet) would be as tall as your average two-story house. A 700mm tire, on the other hand, could be describing a tire that’s just over two feet tall, which is a much more reasonable size for a bike tire.

At first glance, the 700c is the indicative value of the distance between one side of the tire to the opposite side when measured diagonally. However, the number 700 isn’t normally an accurate measure of the tire because the diagonal measurement for any tire can change depending on the shape or treading, even though the rim is the same size.

This 700 figure is used only to provide an approximation of what the tire’s dimensions can be. So, you shouldn’t take it literally as an exact value. The wiser thing to do is use it as a rough indicator of the actual size.

Why Converting 700c Directly May Go Wrong

Any conversions you make with the 700x42c figures may be inaccurate because it’s often an estimate of the dimensions and not an exact measurement.

We already know that 700c is supposed to be 700mm. So, a direct conversion of 700x42c tire size in inches would give us about a figure of 27.5×1.65 inches. Now, 27.5 inches is a fairly common size for mountain bikes made by many manufacturers. So, if the 700c would be an accurate value if the tire indeed spanned over 27.5 inches exactly. However, a tire measuring 29 inches could also very well have a 700c value imprinted on its side. This is because the 700c is an approximate indicator of the diameter and not the actual value. In this instance, you’d find yourself choosing the wrong tire size for your bike which actually runs a different size.


Tiring sizing methods have been evolving over the years, but some standards have persisted even if their utility isn’t that obvious today. An example of this is the French system that indicates tire dimensions in estimates instead of accurate numbers. However, we have reliable ways of deducing the correct dimensions thanks to the other standards that come printed on all tires – the UK/US metric and the ISO standard.

Converting and expressing 700x42c tire size in inches is super easy if you use either of these values to find your dimensions. However, to be truly on the safer side of measurements, going for the ISO dimensions remains your best bet if you’re thinking of replacing your old tires with new ones of the same size.

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