How to Read Bike Tire Sizes | Sidewall Numbers Explained

Bike tires carry their sizes printed or embossed on the sidewall, and working out how to read bike tire sizes comes down to spotting the right format among the three you’ll encounter. The ISO/ETRTO pair is the one that matters most, because it states the tire’s width and — critically — the rim’s bead-seat diameter in millimeters. Inch and French labels are common too, but they describe the tire more loosely.

Here’s what those sidewall markings mean and how to use them before you buy a replacement.

The Three Tire Size Formats You’ll See

Manufacturers label tires in one of three ways, and the same tire can appear under different names depending on the format.

ISO/ETRTO

This is the standardized pair like 37-622. The first number is the tire width in millimeters; the second is the bead-seat diameter of the rim in millimeters. It’s the most accurate identifier because it removes guesswork about fit.

Inch Size

Labels like 29 x 2.25 use inches. The first number is the approximate outer diameter of the tire; the second is the width in inches.

French Size

Labels like 700 x 25C use millimeters. The first number is the approximate outer diameter; the second is the width. The letter historically related to rim width but is not reliably used for modern fit decisions, so don’t treat it as a precise specification.

How to Find and Interpret The Numbers

The size lives on the tire’s sidewall, often embossed into the rubber. Rotate the wheel until you can read it clearly — it’s usually near the bead on both sides.

• A pair like 25-622 means 25 mm wide with a 622 mm bead-seat diameter. That tire fits a 700c wheel.
• A label like 29 x 2.25 corresponds to ISO 58-622, which also fits a 700c rim despite the “29” name.
• A label like 650 x 40 corresponds to ISO 40-584.

Once you have the ISO pair, match the second number exactly to your rim’s bead-seat diameter. That number is non-negotiable for compatibility. The width can vary within safe rim and frame limits, which is why the ISO system separates the two.

Common Mistakes That Ruin A Tire Purchase

Misreading the sidewall is the main reason a new tire doesn’t fit. These four errors cause most of the confusion.

Confusing outer diameter with bead-seat diameter. The ISO number refers to where the tire locks onto the rim, not the tire’s outside dimension. A 700c tire’s outer diameter is roughly 700 mm, but its bead-seat diameter is 622 mm. Those are different numbers, and the second one is what determines fit.

Assuming “700” is literal. French and 700C-style sizing is approximate, not exact. Use the ISO number to verify fit instead of trusting the outer-diameter label.

Using the French letter as a fit guide. The letter in a label like 700 x 25C once related to rim width, but it’s not a reliable indicator today. Ignore it and go by the ISO pair.

Measuring the rim strip. Rim-strip width does not equal tire width. The sidewall label is the standard source of truth, so read it before measuring anything.

A tire that doesn’t match the rim’s bead-seat diameter can be unsafe or impossible to mount correctly. Width changes also affect frame and fork clearance, so verify that clearance before moving to a wider tire. And remember that printed width is an approximation — the actual width depends on the rim choice and inflation pressure.

If you’re shopping for a replacement and want to see what works well on electric bikes specifically, our roundup of the best ebike tires for your build covers tested options that fit standard 622 and 584 rims.