The Complete Guide to Bicycle Inner Tube Sizes

Getting a flat tire is inevitable. Knowing how to find the right replacement tube is a skill every cyclist needs — and it's easier than it looks once you understand the system. This guide covers how to read your tire, what the numbers mean, which valve you need, and what to look for if you ride a cargo bike, e-bike, or folding bike.

---

Step 1: Read Your Tire Sidewall

Every bicycle tire has its size molded into the sidewall rubber. Flip your bike and look at the side of the tire. You'll find a number like one of these:

• 700x28c — road bike, hybrid, or city bike
• 26x2.1 — mountain bike
• 20x1-3/8 — folding bike (common on Brompton)
• 27.5x2.25 — modern mountain bike
• 29x2.1 — 29er MTB (same diameter as 700c road)
• 40-622 — ISO/ETRTO format (explained below)

The first number is always the wheel diameter, the second is the tire width.

---

Understanding the Three Tire Size Systems

Inch System (e.g., 26x2.1)

Common on mountain bikes and older bikes. Diameter in inches, width in inches. Note: "26 inch" MTB and "26 inch" cruiser wheels are actually different sizes at the bead — always verify with the ISO number if in doubt.

French / Road System (e.g., 700x28c)

Used on road bikes, hybrids, gravel, and city bikes. The "700c" is the approximate diameter, the number after "x" is width in millimeters. Note: 700c and 29" wheels have the same bead diameter (622mm) — the names are different but a 700x40c tube fits a 29x1.5 tire.

ISO / ETRTO System (e.g., 40-622)

The most precise system. First number is tire width in mm, second is bead seat diameter in mm. Always unambiguous — two tires with the same ISO number are always compatible.

• 622mm — 700c road / 29" MTB
• 584mm — 650b / 27.5" MTB
• 559mm — 26" MTB
• 406mm — 20" cargo bikes (Tern, Urban Arrow rear)
• 349mm — 16" Brompton (C, P, T, G Line)
• 305mm — 16" kids bikes

---

Valve Types: Presta vs. Schrader

Your rim has a hole drilled for one specific valve type — you must match it.

Presta Valve

Tall, narrow, all-metal with a small threaded lock nut at the top. Common on road bikes, high-end city bikes, and many e-bikes. Rim hole is narrow (~6mm).

Schrader Valve

Wider, looks exactly like a car tire valve. Common on mountain bikes, kids bikes, and cargo bikes. Works with any gas station air compressor. Rim hole is wider (~8mm).

---

Size Reference by Bike Type

Brompton Folding Bikes

All current models (C Line, P Line, T Line, G Line) — 16x1-3/8 (349mm bead), Schrader valve. Note: Brompton 16" is different from standard 16" kids bikes (305mm).

Tern Cargo Bikes

GSD, HSD — 20x2.15 or 20x2.35 (406mm bead), Schrader. Check sidewall for exact width.

Urban Arrow Cargo Bikes

Front and rear are different sizes — use the quick-select in the finder above. Front: 20" (406mm). Rear: 26" (559mm).

Road Bikes

700c (622mm ISO), width typically 23c–32c. Almost always Presta valve.

Mountain Bikes

26" (559mm), 27.5" (584mm), or 29" (622mm). Usually Schrader valve.

Kids Bikes

12" (203mm), 16" (305mm), 20" (406mm), 24" (507mm). Almost always Schrader valve.

---

Frequently Asked Questions

Can I use a tube that's a different size than my tire?

The diameter must match exactly. Width can vary — tubes come in ranges like 700x25-32c. A tube slightly narrower than your tire is fine; much wider is not recommended.

What's the difference between a 20" BMX tube and a Brompton tube?

Completely different. Brompton uses 349mm bead; standard 20" BMX/cargo uses 406mm. Always verify by ISO number, not the inch label.

Do e-bikes need special inner tubes?

No, but heavier-gauge butyl is recommended since e-bikes are heavier and faster. Match your tire size closely and check valve length — many cargo e-bikes have thicker rims.

What PSI should I inflate my tube to?

Check your tire sidewall — it lists the recommended range. Narrower road tires run higher pressure; wider cargo and MTB tires run lower. E-bikes benefit from slightly higher pressure due to added weight.

---

Still Not Sure? We Can Help.

Clever Cycles has been selling and servicing bikes in Portland since 2000. We specialize in Brompton folding bikes, cargo bikes, e-bikes, and city commuters — and we stock tubes for all of them. Stop in at 900 SE Hawthorne Blvd and we'll figure it out together. No appointment needed for a tube question.