Few things in cycling cause as much head-scratching as tyre sizing. Pick up three tyres from three different decades and you might find one labelled 700x28c, another 27 x 1 1/4, and a third simply 28-622 — and depending on which numbers you trust, you could end up with a tyre that physically will not seat on your rim. The good news is that behind the confusing labels there is one measurement that never lies, and once you learn to read it, buying the right tyre becomes almost trivial. This guide walks through why the mess exists, how to decode the modern standard, which sizes are genuinely interchangeable, and how to pull the rolling circumference your bike computer needs.

Why tyre sizing is so confusing

The chaos comes from history. Different countries developed their own systems before anyone agreed on a global standard, and all of them are still printed on tyres today.

  • Inch sizing (e.g. 26 x 2.10, 27 x 1 1/4): describes the approximate outer diameter and width in inches. The trouble is that the stated diameter is nominal, not measured — and crucially, two tyres marked "26 inch" can have completely different bead-seat diameters.
  • French sizing (e.g. 700x25c): the number is the nominal outer diameter in millimetres, the second figure is width, and the letter (a, b, c) once referred to a width/diameter class. Only the "c" survives in common use, and it no longer means much on its own.
  • ETRTO / ISO sizing (e.g. 25-622): the modern, unambiguous standard. This is the one to trust.

The single most important fact is this: the outer diameter does not determine fit. The bead-seat diameter — the diameter of the rim where the tyre bead locks in — is what decides whether a tyre fits a rim. Two tyres that look the same size can have different bead seats and will never be compatible.

How to read ETRTO

An ETRTO code is written as width-beadseat, both in millimetres. So 28-622 means a tyre 28 mm wide that fits a rim with a 622 mm bead-seat diameter. That second number is the one that matters for compatibility:

  • If the bead-seat number matches your rim, the tyre will fit (subject to width and clearance, covered below).
  • If it does not match, no amount of pumping or wrestling will make it safe — walk away.

This is why ETRTO is so valuable: it strips away marketing names and decades of regional quirks and leaves you with two honest measurements. When in doubt, find the ETRTO code moulded into the sidewall and ignore everything else.

The common sizes table

Here are the bead-seat diameters behind the names you will actually meet, grouped by the modern standard. Notice how several familiar names share a single bead seat — that is what makes them interchangeable.

  • 622 mm bead seat — known as 700c and as 29er. Yes: a 700c road wheel and a 29-inch mountain wheel are the same bead-seat diameter. A 29x2.3 MTB tyre and a 700x25c road tyre fit the same rim; only width and clearance differ.
  • 584 mm bead seat — known as 650b and as 27.5". Again, two names, one bead seat. Popular for gravel and all-mountain bikes.
  • 559 mm bead seat — classic 26" mountain bike. Still vast in number worldwide.
  • 630 mm bead seat — old 27 x 1 1/4 road wheels. Easy to confuse with 700c but NOT compatible (622 vs 630).
  • 406 mm bead seat — 20" as used on many folding bikes, BMX and cargo bikes. Note that some 20" tyres use 451 mm instead, so check carefully.

The 622-vs-630 and 406-vs-451 pairs are exactly the traps that catch people out. Both look like "the same size" by name, yet differ by 8 mm and 45 mm of bead seat respectively. If you only remember one rule from this article: match the bead-seat number, not the name. For a fuller picture of how these diameters relate to wheel standards, see our guide to understanding bike wheel sizes.

What actually fits your rim

Once the bead seat matches, width is the next question. A given rim has a sensible range of tyre widths it can carry safely, governed by the rim's internal width. As a rough guide, a tyre should be somewhere between roughly 1.4 and 2.4 times the internal rim width — too narrow and the tyre profile becomes light-bulb-shaped and unstable; too wide and it can roll or burp on the rim.

The practical takeaway: a 19 mm internal road rim is happy with tyres from about 25 mm to 32 mm, while a 25 mm internal gravel rim suits roughly 38 mm to 50 mm. Always confirm with your rim manufacturer's stated range if you have it.

Tyre width vs rim width

Here is a detail that surprises riders: the width printed on the tyre is a nominal figure measured on a reference rim. Fit the same tyre to a wider rim and it will measure wider in reality; on a narrow rim it will measure narrower. A tyre marked 28 mm can easily sit at 30 mm on a modern wide rim. This matters because it changes the outer diameter, and therefore the rolling circumference your computer relies on (more on that below).

Clearance

Even when bead seat and rim width are correct, the tyre still has to fit in the frame and fork. Leave at least a few millimetres of space on each side and above the tyre for mud, flex and any wheel build imperfection — a common recommendation is around 4-6 mm of clearance. A tyre that just barely clears when the wheel is true can rub badly once it picks up grit, so be conservative, especially on gravel and winter bikes.

Rolling circumference for your bike computer

Your speed and distance readings depend entirely on the wheel circumference you program in, so a wrong number means wrong data on every ride. You can measure it directly or calculate it. To calculate, start from the bead-seat diameter, add twice the tyre's section height to get the outer diameter, then multiply by pi.

  • Outer diameter (mm) ≈ bead-seat diameter + (2 × section height)
  • Rolling circumference (mm) ≈ π × outer diameter

Take a 700x28c tyre. Bead seat is 622 mm; a 28 mm tyre has a section height a little under its width, but as a working figure assume roughly 28 mm. Outer diameter ≈ 622 + (2 × 28) = 678 mm. Rolling circumference ≈ π × 678 ≈ 2130 mm. That tallies neatly with the ~2105-2155 mm figures bike-computer tables list for 700x25 to 700x32 tyres.

The most accurate method, though, is the roll-out test: mark the valve position, roll the bike forward one full wheel revolution with your weight on it, mark the floor, and measure the distance. This captures the real, loaded outer diameter on your actual rim — which, as we saw, can differ from the nominal numbers. Use the tyre converter on the Convert.bike home page to turn any ETRTO or inch size into an outer diameter and circumference in seconds, then sanity-check against a roll-out.

Circumference feeds into more than just your computer. It also sets your effective rolling diameter for gear inches, ratios and development, and it underpins the maths behind our bike speed, cadence and gearing calculator. Change tyre size dramatically and your gearing and speed readings shift with it.

Next steps

Start by finding the ETRTO code on your current tyres and noting the bead-seat number — that single figure unlocks everything else. Confirm your rim's internal width and check your frame clearance before ordering anything new. When you have a candidate tyre, run its size through the home-page tyre converter to see the outer diameter and rolling circumference, then program that figure into your computer or, better still, verify it with a roll-out test. From there, explore how your new tyre affects gear development and overall wheel sizing so the whole drivetrain picture stays consistent. Match the bead seat, respect the clearance, and tyre shopping stops being a gamble.