Every bicycle tyre carries markings molded into its rubber sidewall. Those markings dictate whether a replacement tyre will seat safely onto your rim, clear your frame and brakes, or blow off the bead under pressure.
622mm Rim Bead Seat Diameter (BSD). The only 100% accurate measurement across all tyre brands.
25 = Tyre width in mm.
C = French rim code (corresponds to a 622mm rim bead).
2.25″ or 4.0″ = Tyre width in decimal inches.
Why Bicycle Tyre Sizing Is So Confusing
The cycling industry evolved across Europe, the UK, and North America over 130 years, resulting in three conflicting sizing standards:
- The French System (e.g. 700x25C): Used predominantly for road, touring, and gravel bicycles. The number 700 refers to the approximate outside diameter in mm, 25 is width in mm, and the letter C indicates a 622mm rim bead seat.
- The Imperial System (e.g. 29×2.25″, 26×2.0″, 20×4.0″): Used for mountain bikes, BMX, cruisers, and fat tyre e-bikes. Specified in decimal inches.
- The ETRTO / ISO Standard (e.g. 25-622, 100-406): Established by the European Tyre and Rim Technical Organisation. It specifies: Tyre Width (mm) – Rim Bead Seat Diameter (mm).
A 29-inch mountain bike tyre and a 700c road racing tyre have completely different outside heights, yet both fit the exact same 622mm bead seat diameter (BSD). Conversely, an imperial 26×1.75″ tyre (559mm rim) will never fit a vintage French 26×1 3/8″ rim (590mm rim). If the BSD numbers do not match, the tyre will either refuse to fit or blow off the rim under inflation.
Master Bicycle Tyre Size Conversion Chart
Type any marking into the search box or click a category pill to instantly filter dimensions, rim bead diameters, and compatible inner tubes:
| Common / Trade Size | ETRTO (Width-BSD) | Rim Bead (BSD) | Typical Pressure | Discipline | Matching Inner Tube |
|---|---|---|---|---|---|
| 700 x 20C (28 x 0.75″) | 20-622 | 622 mm | 95 – 115 PSI | Vintage Road | 700 x 18–23c Presta |
| 700 x 23C (28 x 0.90″) | 23-622 | 622 mm | 90 – 105 PSI | Road Racing | 700 x 20–25c Presta |
| 700 x 25C (28 x 1.00″) | 25-622 | 622 mm | 80 – 95 PSI | Modern Road | 700 x 23–28c Presta |
| 700 x 28C (28 x 1.10″) | 28-622 | 622 mm | 70 – 85 PSI | Road Endurance | 700 x 25–32c Presta |
| 700 x 32C (28 x 1.25″) | 32-622 | 622 mm | 55 – 70 PSI | All-Road / Commute | 700 x 28–35c Presta |
| 700 x 35C (28 x 1.35″) | 35-622 | 622 mm | 50 – 65 PSI | Touring / Hybrid | 700 x 32–40c Presta/Schrader |
| 700 x 38C (28 x 1.50″) | 38-622 | 622 mm | 45 – 60 PSI | Gravel Adventure | 700 x 35–43c Presta |
| 700 x 40C (28 x 1.60″) | 40-622 | 622 mm | 40 – 55 PSI | Cyclocross / Gravel | 700 x 38–45c Presta |
| 29 x 2.00″ (700 x 50C) | 50-622 | 622 mm | 30 – 42 PSI | 29er XC MTB | 29 x 1.9–2.3″ |
| 29 x 2.25″ | 57-622 | 622 mm | 26 – 38 PSI | 29er Trail MTB | 29 x 2.1–2.4″ |
| 29 x 2.35″ | 60-622 | 622 mm | 24 – 35 PSI | 29er Enduro MTB | 29 x 2.2–2.5″ |
| 29 x 2.40″ | 62-622 | 622 mm | 22 – 32 PSI | 29er DH / E-MTB | 29 x 2.3–2.6″ |
| 27.5 x 2.25″ (650B x 57) | 57-584 | 584 mm | 28 – 40 PSI | Trail MTB | 27.5 x 2.1–2.4″ |
| 27.5 x 2.40″ (650B x 62) | 62-584 | 584 mm | 24 – 34 PSI | Enduro MTB | 27.5 x 2.3–2.6″ |
| 27.5 x 3.00″ (27.5+ Plus) | 75-584 | 584 mm | 16 – 25 PSI | Plus Bike | 27.5 x 2.8–3.2″ Plus |
| 26 x 1.75″ | 47-559 | 559 mm | 35 – 50 PSI | Classic City / MTB | 26 x 1.5–1.95″ |
| 26 x 1.95″ | 50-559 | 559 mm | 32 – 45 PSI | Classic XC MTB | 26 x 1.75–2.125″ |
| 26 x 2.25″ | 57-559 | 559 mm | 30 – 42 PSI | Classic Mountain | 26 x 2.1–2.4″ |
| 20 x 4.00″ (Fat E-Bike) | 100-406 | 406 mm | 18 – 28 PSI | 20″ Fat E-Bike | 20 x 4.0″ Heavy Duty Fat Tube |
| 20 x 4.25″ (Fat Cruiser) | 108-406 | 406 mm | 16 – 25 PSI | 20″ Fat Cruiser | 20 x 4.0–4.25″ Fat Tube |
| 26 x 4.00″ (Fat Bike) | 100-559 | 559 mm | 10 – 20 PSI | 26″ Fat Bike | 26 x 4.0″ Fat Tube |
| 26 x 4.70″ (Monster Fat) | 120-559 | 559 mm | 8 – 15 PSI | Monster Fat Bike | 26 x 4.5–4.8″ Fat Tube |
| 29 x 4.30″ (Monster Fat) | 110-622 | 622 mm | 12 – 22 PSI | 29+ Monster Fat | 29 x 4.0″ Fat Tube |
| 20 x 1.75″ | 44-406 | 406 mm | 40 – 60 PSI | BMX / Junior | 20 x 1.75–2.125″ Schrader |
| 20 x 2.25″ | 57-406 | 406 mm | 50 – 70 PSI | Street BMX | 20 x 2.1–2.4″ Schrader |
| 24 x 1.75″ | 47-507 | 507 mm | 35 – 50 PSI | Junior 24″ Bike | 24 x 1.75–2.125″ Schrader |
Rim Width vs Tyre Width: Can You Fit Wider Tyres?
A frequent workshop inquiry is: “Can I replace my 25mm tyres with 28mm or 32mm?”
The answer depends on two factors: Frame & Brake Clearance and Internal Rim Bed Width.
Mounting an excessively wide tyre onto a narrow rim creates a “lightbulb” profile, causing the sidewalls to squirm during sharp turns and risking unseating. Conversely, stretching a narrow tyre across a wide rim exposes the metal rim bead to potholes and pinch flats.
Ideal Tyre Widths: 20mm to 28mm (Road & Racing).
Maximum Safe Width: 32mm. Rims wider than this risk lateral tyre squirm.
Ideal Tyre Widths: 28mm to 50mm (Gravel, All-Road, Cross-Country).
Maximum Safe Width: 2.35″ (60mm). Provides stable square sidewall support.
Ideal Tyre Widths: 3.8″ to 4.8″ (Fat Bikes & Fat E-Bikes).
Minimum Safe Width: 3.0″. Never mount standard narrow tyres onto wide fat rims.
Electric Bike Tyre Considerations: Why Standard Rubber Fails
Electric bikes subject tyres to wear rates double or triple those of pedal cycles:
- Higher Total Mass: A typical 20-inch fat tyre e-bike weighs 28kg to 35kg before adding rider weight and cargo. This elevates sidewall stress and increases pinch puncture rates over potholes.
- Instant Geared Hub Torque: Bafang and geared rear hub motors generate immediate torque from a standstill, rapidly wearing the centre tread strip of soft compound tyres.
- ECE-R75 Certification: For speed pedelecs or high-powered e-bikes capable of assisted speeds up to 50 km/h (31 mph), tyres must carry European ECE-R75 homologation. These tyres feature thicker puncture belts and reinforced carcasses.
Workshop Stock: Recommended Tyres & Heavy-Duty Tubes
Looking for replacement rubber for your fat tyre e-bike, hybrid, or commuter? We stock the most durable, puncture-resistant options in our UK workshop:
20 x 4.0 Off-Road Knobby Fat Tyres
Heavy-duty 100-406 casing with deep tread lugs for mud, gravel, and wet tarmac. Outstanding puncture resistance on electric utility bikes.
20 x 4.0 Slick White Wall Tyres
Smooth rolling low-resistance tread for road commuting, retro styling, and reduced battery energy consumption per charge.
Heavy-Duty Fat Bike Inner Tubes (20 x 4.0)
Thick butyl rubber with reinforced valve stem collars. Eliminates pinch flats caused by low pressures and rough curbs.
Nylon Rim Strips & Protectors (20×4 / 26×4)
Prevents spoke punctures and rim cut-outs from abrading your inner tubes under heavy load cycling.
Frequently Asked Technical Questions
Can I put a 700x28C tyre on a bike that came with 700x25C?
Are 29-inch and 700C wheels the same size?
Why does my 20-inch fat bike tyre say 100-406?
What valve type should I choose: Presta or Schrader?
Schrader (car-style 8mm valve): Standard on electric fat bikes, BMX, kids’ bikes, and hybrid commuters. Easy to inflate at automotive fuel stations.
