26" vs 27.5" vs 29" Wheels — What's the Difference and Which Inner Tube Do I Need?
If you've ever stood in a bike shop (or searched online) trying to work out which inner tube fits your mountain bike, you're not alone. Wheel sizing is one of the most confusing parts of cycling — partly because the same wheel can have multiple names, and partly because the numbers don't always mean what you'd expect. Here's a clear explanation of what the sizes actually mean and how to make sure you buy the right tube every time.
The Three Main MTB Wheel Sizes
26 Inch
The original mountain bike standard, dominant from the 1980s through to the early 2010s. If you have an older MTB, a hardtail bought before 2013, or a budget bike, there's a good chance it runs 26" wheels. The wheel diameter is approximately 559mm at the rim.
26" wheels are lighter, more manoeuvrable, and easier to accelerate — which is why they're still popular on jump bikes, dirt jump bikes, and some kids' bikes. Spare parts including inner tubes remain widely available.
27.5 Inch (also called 650b)
The middle ground, introduced around 2012 as a compromise between the agility of 26" and the rollover ability of 29". The rim diameter is 584mm. Most mid-range and performance hardtails and full-suspension bikes sold between 2014 and the present run 27.5" wheels.
You'll see both "27.5" and "650b" used interchangeably — they're the same size. If you're searching for tubes and getting confused by 650b results, that's why.
29 Inch (also called 29er)
The largest common MTB wheel size, with a 622mm rim diameter — the same as a standard road bike or hybrid. 29" wheels roll over obstacles more easily, maintain momentum better on rough terrain, and give more traction. They're the dominant standard on cross-country and trail bikes today.
Note: 29" tubes and 700c road bike tubes share the same rim diameter (622mm) but road tyres are much narrower, so the tubes aren't interchangeable — always check the width range on the tube packaging.
Why the Second Number Matters Too
The wheel diameter is only half the equation. Tyres — and therefore tubes — also have a width, shown as the second number on your tyre sidewall. A tyre marked 27.5 x 2.10 is a 27.5" wheel with a 2.10 inch wide tyre.
Inner tubes are designed to fit a range of widths, not a single size. Our Impac 27.5" Inner Tube fits 27.5 x 1.75–2.35, which covers the vast majority of trail and enduro tyres. The Impac 26" Inner Tube fits 26 x 1.75–2.35.
If your tyre width falls within the tube's stated range, it will fit. If it's outside that range — for example a very narrow 27.5 x 1.5 hybrid tyre or a very wide 27.5 x 2.6 plus tyre — you'll need a tube specifically sized for that width.
How to Find Your Wheel Size
Check the tyre sidewall — the size is moulded into the rubber. You'll see either the inch sizing (e.g. 27.5 x 2.10) or the ISO/ETRTO metric sizing (e.g. 54-584, where 584 is the rim diameter in mm and 54 is the tyre width in mm).
ISO/ETRTO rim diameters:
- 559mm = 26 inch
- 584mm = 27.5 inch / 650b
- 622mm = 29 inch / 700c
If the tyre is too worn to read, check the bike manufacturer's website using your frame model and year, or measure the wheel diameter with a tape measure from the ground to the centre of the axle and double it.
Can I Mix Wheel Sizes?
No — a 26" tyre will not fit a 27.5" rim, and a 27.5" tube will not work in a 26" tyre. The rim diameters are different and the bead won't seat correctly. Always match the tube to the wheel size printed on your tyre.
Quick Reference
| Wheel size | Also known as | ISO rim diameter | Our tube |
|---|---|---|---|
| 26" | 26 inch | 559mm | Impac 26" Tube — 26 x 1.75–2.35 |
| 27.5" | 650b | 584mm | Impac 27.5" Tube — 27.5 x 1.75–2.35 |
| 29" | 29er, 700c | 622mm | Contact us — we can advise |
FAQs
My bike says 650b on the frame — what tube do I need?
A 27.5" tube. 650b and 27.5" are the same size — check your tyre sidewall for the width and match it to the tube's stated range.
Can I use a 29" tube in a 700c road tyre?
The rim diameter is the same (622mm) but road tyres are much narrower than MTB tyres. A 29" MTB tube will be too wide for a narrow road tyre and won't seat properly. Use a tube sized for your tyre width.
My tyre says 26 x 1.95 — will the Impac 26" tube fit?
Yes — 1.95 falls within the 1.75–2.35 range of the Impac 26" tube.
What's the difference between Schrader and Presta valves?
Both our Impac tubes use a 40mm Schrader valve — the chunky valve type, same as a car tyre, standard on mountain bikes. See our full guide: Schrader vs Presta — What's the Difference?
Still Not Sure? We Can Help
If you're unsure which tube fits your bike, drop us a message or pop into the shop in Hadfield. We stock Impac 26" and 27.5" inner tubes, available with free UK delivery or local collection.