The short version
- Only the ETRTO number matters.
28-622means a 28 mm wide tire on a 622 mm bead seat diameter rim. - The second number must match your rim exactly. 622 tire on 622 rim. No exceptions, no adapters.
- 700C, 29″ and 28″ are all 622 mm. Different names, same bead seat.
- Tire width should be about 1.4–2.4× the internal rim width. Outside that band, handling and safety suffer.
Why the old systems broke
Two incompatible traditions collided in the twentieth century, and neither described the thing that actually determines fitment.
The inch system (British and American) quoted the approximate outside diameter of the inflated tire. Since a fat tire on a small rim and a thin tire on a large rim can both measure about 26 inches, one label ended up covering several rims. Worse, the industry then used two incompatible notations: 26×1.375 (decimal) and 26×1⅜ (fraction) are numerically the same value and are different, non-interchangeable sizes — 559 mm and 590 mm bead seats respectively.
The French system quoted a target outside diameter in millimetres plus a letter for width class — A was narrow, B medium, C wide — all engineered to reach roughly the target diameter. So 700A, 700B and 700C all aimed at a 700 mm outside diameter but used different rims: 642, 635 and 622 mm respectively. Only 700C survived in volume, which is why the letter now looks like meaningless decoration.
26×1.75 is 47-559 — a mountain bike tire.
26×1¾ is 44-597 — an old cruiser tire.
A 38 mm difference in bead seat diameter. The tire will either refuse to seat or blow off the rim under pressure. This is not a theoretical problem; it is the single most common bicycle tire purchasing error.
ETRTO / ISO 5775: the fix
The European Tyre and Rim Technical Organisation designation — identical in substance to ISO 5775 — describes the two things that actually govern compatibility, both in millimetres:
28 – 622
- 28 = nominal inflated tire width in mm (this varies with rim width — see below)
- 622 = bead seat diameter (BSD) in mm — the diameter of the rim surface the tire bead sits on
The BSD must match your rim exactly. The width is a recommendation that must fall inside the rim's supported range. Every tire sold in Europe, and most sold anywhere, prints the ETRTO number on the sidewall — usually in small type near the more prominent marketing size.
The complete BSD reference chart
| BSD (mm) | Also called | Where you find it |
|---|---|---|
| 622 | 700C · 29″ · 28″ · 28×1⅝ | Road, gravel, hybrid, touring, 29er MTB — the dominant modern size |
| 584 | 650B · 27.5″ · 26×1½ | Gravel, all-road, 27.5″ MTB, French randonneur |
| 559 | 26″ (decimal, e.g. 26×2.1) | Classic MTB, folding bikes, many e-bikes and cargo bikes |
| 571 | 650C · 26×1″ | Older triathlon and small-frame road bikes |
| 590 | 26×1⅜Prime; (fractional) | British three-speeds, older city bikes — not 559 |
| 597 | 26×1¼ / 26×1¾ | Vintage cruisers and roadsters |
| 630 | 27″ | Pre-1985 road bikes; larger than 700C |
| 507 | 24″ (decimal) | Junior MTB, some cargo and recumbent bikes |
| 451 | 20″ (fractional) | BMX race, high-performance folders |
| 406 | 20″ (decimal) | BMX freestyle, Brompton-class folders, many kids' bikes |
| 349 | 16″ (fractional) | Brompton |
| 305 | 16″ (decimal) | Children's bikes, some folders |
Both are 622 mm. The distinction is width and application, not diameter. A 29″ MTB tire (say 57-622) and a road tire (25-622) share a bead seat and nothing else: they need different rim widths, different frame clearance, and different brakes. If the ETRTO matches but the tire is three times wider than anything you have run, check the frame before you buy.
Rim width: the second half of compatibility
Fitting a tire is necessary but not sufficient. The internal rim width determines the shape the tire takes when inflated — its actual width, its profile, and how it behaves in a corner. A tire that is too narrow for its rim forms a squared-off, light-bulb profile with a vulnerable exposed sidewall. A tire that is too wide sits on a rim too narrow to support it and rolls under load, feeling vague and risking a burp or blow-off.
| Internal rim width | Sensible tire range | Typical application |
|---|---|---|
| 15 mm | 23–32 mm | Older road wheels |
| 19 mm | 25–40 mm | Modern road / light gravel |
| 21–23 mm | 28–50 mm | Gravel and all-road — the current mainstream |
| 25–27 mm | 35–2.1″ (53 mm) | Adventure gravel, XC crossover |
| 30 mm | 2.1–2.5″ | Trail and enduro MTB |
| 35 mm | 2.4–2.8″ | Enduro and downhill |
| 80–100 mm | 3.8–5.0″ | Fat bikes |
The stated tire width is a nominal value. A tire labelled 28 mm might measure 27 mm on a 17 mm internal rim and 30 mm on a 25 mm internal rim. As a rough guide, each 1 mm of extra internal rim width adds about 0.4 mm of measured tire width. That matters when frame clearance is tight — measure the inflated tire rather than trusting the label.
Hookless (tubeless straight-side) rims lack the bead hook that mechanically retains the tire, relying on friction and bead interference instead. That imposes real constraints: typically a maximum inflation pressure around 72.5 psi (5 bar), a minimum tire width, and an approved tire compatibility list from the rim maker. Exceeding the pressure limit can blow the tire off the rim at speed. Check the manufacturer's compatibility list before fitting anything to a hookless wheel — this is not a place for improvisation.
Pressure: the received wisdom was wrong
For decades the advice was that harder equals faster, and 120 psi road tires were normal. Rolling resistance testing on real road surfaces overturned that. Above a certain point, higher pressure increases impedance losses — energy absorbed by the bike and rider vibrating over surface texture — faster than it reduces hysteresis losses in the casing. The result is a distinct optimum, and it is much lower than tradition suggested.
| Tire width | Front | Rear |
|---|---|---|
| 25 mm | 75 psi / 5.2 bar | 80 psi / 5.5 bar |
| 28 mm | 62 psi / 4.3 bar | 68 psi / 4.7 bar |
| 32 mm | 50 psi / 3.4 bar | 55 psi / 3.8 bar |
| 38 mm | 38 psi / 2.6 bar | 42 psi / 2.9 bar |
| 45 mm | 30 psi / 2.1 bar | 33 psi / 2.3 bar |
| 2.25″ MTB | 21 psi / 1.4 bar | 24 psi / 1.7 bar |
| 2.5″ MTB | 18 psi / 1.2 bar | 21 psi / 1.4 bar |
Two structural rules apply regardless. Run the front 3–5 psi lower than the rear, because the rear carries roughly 60% of system weight. And if you run inner tubes rather than tubeless, add about 5–10% to protect against pinch flats, since a tube has no sealant and a much lower impact threshold. Convert between units with our pressure calculator.
The rest of the sidewall
| Marking | Meaning |
|---|---|
| TPI (e.g. 60, 120, 320) | Threads per inch in the casing. Higher TPI means finer threads, a more supple casing and lower rolling resistance — but thinner, more cut-prone material. 60 TPI is durable; 120 TPI is the performance default; 320 TPI is race-day. |
| Folding / Wire | Bead material. Folding beads use aramid (Kevlar) and save roughly 50–90 g per tire; wire beads are cheaper and heavier. |
| TLR / Tubeless Ready | Airtight casing and tubeless-compatible bead, requiring sealant and a tubeless rim. UST is a stricter standard needing no sealant in principle. |
| MAX / recommended pressure | The moulded maximum is a safety ceiling, not a target. The optimum is usually well below it. |
| Rotation arrow | Directional tread. Front and rear tires of the same model are sometimes designed to run in opposite directions — check for separate front/rear markings. |
| Compound / puncture belt names | Brand-specific (GreenGuard, K-Guard, Vectran, R3). Generally the more protection, the heavier and slower the tire. |
Valve types
- Presta (French) — 6 mm, threaded lock nut, standard on road and modern MTB. Needs a valve hole of 6.5 mm; fitting Presta into a Schrader-drilled rim requires a grommet.
- Schrader (American) — 8 mm, identical to car valves, so any petrol station pump works. Common on city bikes, kids' bikes and many e-bikes.
- Woods / Dunlop — 8 mm outer body with a Presta-style core, still widespread in the Netherlands, Germany and much of Asia. Inflates on a Presta pump head.
Valve length matters with deep-section rims: allow at least 15–20 mm of valve protruding past the rim for the pump head to seat. If you run tubes rather than tubeless, our inner tube materials guide covers butyl, latex and TPU trade-offs in detail.
How to buy the right tire in four steps
- Read the ETRTO on your current tire. Small print on the sidewall, format
NN-NNN. Note the second number. - Check the rim. Most rims print their own ETRTO, e.g.
622×21C— bead seat then internal width. - Confirm the new tire's BSD matches exactly and its width falls in the rim's supported range.
- Check frame and fork clearance. Aim for at least 4 mm each side on a road bike and 6–8 mm on gravel or MTB, for mud and for the wheel flexing under load.
Frequently asked questions
What does 28-622 mean?
A 28 mm wide tire for a rim with a 622 mm bead seat diameter — the size most people call 700C or 29 inch.
Is 700C the same as 29 inch?
Same rim, 622 mm. Different naming convention and vastly different typical widths. A 29″ MTB tire will not fit a road frame despite sharing a bead seat diameter.
Why are there so many 26 inch sizes?
Because “26 inch” described the inflated tire's outside diameter, not the rim. At least 559, 584, 590 and 597 mm bead seats have all been sold as 26 inch, and they are mutually incompatible.
What tire width fits my rim?
Roughly 1.4–2.4× the internal rim width. A 21 mm internal rim comfortably handles 29–50 mm tires. Hookless rims have narrower approved ranges — check the manufacturer's list.
Can I fit a wider tire on my existing wheels?
Usually yes if the rim supports the width and the frame has clearance. Wider tires at correspondingly lower pressure roll about as fast on real roads and are dramatically more comfortable. Measure clearance at the chainstays, seatstays and fork crown.
Does a 27 inch tire fit a 700C wheel?
No. 27″ is 630 mm and 700C is 622 mm — an 8 mm difference that will not seat safely. Many 1970s and 80s road bikes are 630 mm; check before ordering.