Updated August 2026
Two tyres both marked 26 inches won't necessarily fit the same bike. The 26-inch rim on a Helkama Aino or Oiva is 584 millimetres in diameter, while the 26-inch rim on a mountain bike is 559.
This is the most common reason a tyre ordered online doesn't fit. The solution is to forget inches and read one figure on the tyre's sidewall.
The short answer
Choose a tyre by its ETRTO marking, not by inch size. The ETRTO figure appears on the sidewall in the form 47-622, for example: the first number is the tyre's width in millimetres and the second is the rim diameter in millimetres. The second number has to match exactly. Width can be varied, as long as the tyre fits the frame and under the mudguard.
Why inch size isn't enough
Over the decades, rims of different sizes have been made under the same inch marking. Inches describe roughly the tyre's outer diameter, which also depends on the tyre's height – not the measurement that determines whether it fits.
The decisive measurement is rim diameter, given in millimetres under the ETRTO standard. When that number matches, the tyre fits.
A classic trap: the markings 28 x 1 5/8 x 1 1/2 (44-635) and 28 x 1 5/8 x 1 1/2 NL (40-622) look almost identical, but they are made for different rim sizes and are not interchangeable.
Tyre size chart: inches and ETRTO
The most common inch sizes in Finland and their millimetre equivalents:
| Inch size | ETRTO | Where it's used |
|---|---|---|
| 12 1/2" | 203 | The smallest children's bikes |
| 16" | 305 | Children's bikes |
| 20" | 406 | Children's bikes and BMX |
| 22" | 484 | Older Jopo bikes |
| 24" | 507 | Junior bikes |
| 26" | 559 | Mountain bikes and most modern junior bikes |
| 26" | 584 | Helkama Aino, Oiva and Jääkäri, and SA-INT bikes |
| 27.5" | 584 | 27.5" mountain bikes, also known as 650B |
| 27" | 630 | Older road bikes |
| 28" | 622 | Hybrids, road and gravel bikes |
| 28" | 635 | Older, often Swedish standard bikes |
| 29" | 622 | 29" mountain bikes |
Note two interesting points in the chart. 26" appears twice – 559 and 584 – and these do not fit each other's rims. And 28" and 29" are the same rim size (622): the difference is only in the tyre's width and height, so a narrow 28" road tyre can be fitted to a 29" mountain bike rim.
Changing the width
You can generally fit tyres of different widths to the same rim, as long as the diameter stays the same. For example, a 40-622 tyre can typically be replaced with a 32-622.
Three things need checking before increasing width, though:
- Frame and fork. Can the tyre rotate freely at every point?
- Mudguards. This is usually what runs out first, because a wider tyre is also taller.
- Rim width. Too narrow a rim for a wide tyre makes the tyre unstable in corners.
Leave at least a few millimetres of clearance between tyre and frame. In winter that space fills with slush and ice, so year-round riding needs more.
Inner tubes
Inner tubes use the same markings as outer tyres, but the sizes are flexible. The marking 32/47-609/642 means the tube suits outer tyres from 32 to 47 mm wide and 609 to 642 mm in diameter – so it fits, for instance, the 40-622 tyre common on hybrids.
Three valve types
| Valve | How to spot it | Where |
|---|---|---|
| Schrader (car valve) | Thick, same as on a car | Mountain bikes, children's bikes, hybrids |
| Dunlop / Woods | Traditional, with a black cap | Older city bikes |
| Presta | Thin, with a threaded tip | Road, gravel and modern mountain bikes |
A Presta valve is thinner than the others, and its tip must be unscrewed by hand before inflating. Important: you usually can't fit another valve type to a rim drilled for Presta, because the hole is smaller. The other way round works, but you'll need an adapter ring so the thin valve doesn't move around in the hole.
Tyre pressure
The recommended pressure is printed on the tyre's sidewall. Inner tubes don't have their own pressure recommendations – pressure is always determined by the outer tyre.
The basic principle: the wider the tyre, the lower the pressure. A narrow road tyre runs at high pressure, a wide off-road tyre considerably lower.
Rider weight also matters: a heavier rider needs higher pressure in the same tyre. In winter it's worth running lower pressure than in summer, because a larger contact patch improves grip.
What about tubeless?
Tubeless means fitting the tyre without an inner tube. Sealing tape goes between rim and tyre, and sealant inside the tyre plugs small holes by itself while you ride.
Benefits: fewer punctures, lower running pressure and better grip. Drawbacks: fitting is more laborious, the sealant needs replacing every few months, and both tyre and rim have to be tubeless compatible.
We also do tubeless setups in our workshop. See prices in our service price list.
Frequently asked questions
How do I find out my bike's tyre size?
Look at the sidewall of your current tyre. You'll find an ETRTO marking in the form 40-622, for example. The second number is the rim diameter, and it has to match on the new tyre.
Why doesn't a 26" tyre fit my 26" bike?
Because 26 inches refers to two different rim sizes: 559 mm on mountain bikes and 584 mm on, among others, the Helkama Aino, Oiva and Jääkäri. Check the ETRTO figure on the sidewall.
Will a 28" tyre fit a 29" rim?
Yes. Both have a rim size of 622 mm, so they are interchangeable. The difference is in the tyre's width and height.
Can I fit a wider tyre?
You can, if the diameter stays the same and the tyre fits the frame and under the mudguard. Remember that a wider tyre is also taller.
Which inner tube fits my bike?
Inner tube sizes are flexible. Choose a tube whose width and diameter ranges cover your outer tyre's dimensions, and make sure the valve type is right.
Can I change the valve type?
Going from Presta to something thicker usually isn't possible, because the rim hole is too small. The other way round works with an adapter ring.
How often should tyre pressures be checked?
Every couple of weeks. A tyre loses pressure gradually even without a puncture.
Summary
Forget inches and read the ETRTO figure. When the second number on the sidewall matches your bike's rim, the tyre fits – and width can be adjusted to suit how you ride.
If you're unsure, take a photo of your current tyre's sidewall and send it to us – we'll work out the right size for you: myynti@emiliasport.fi or +358 50 470 4686. We also fit tyres at our workshop in Tampere.
Browse our range: tyres, studded tyres and tubeless accessories.
Read also: How many studs does a winter bike tyre need? and How long does a bike chain last?
Written by the Emilia Sport team
Emilia Sport has been a bike shop since 1978. Our own workshop in Tampere fits tyres daily to every type of bike, including older special sizes. We're happy to help by phone too: myynti@emiliasport.fi / +358 50 470 4686.
