Tire Size Decoder
The number that decides whether a tire fits is the bead seat diameter (BSD) — the second number of the ETRTO size molded on almost every tire, like the 559 in 50-559. Inch labels hide it: 26 × 1.75 is 559 mm but 26 × 1-3/4 is 571 mm, and the two don’t interchange. Type any marking below to get the real size, the tube that fits, and the traps to avoid.
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What it means
Decimal-inch size · read as 27.5 x 2.4
Bead seat (BSD)
584mm
The number that decides fit
Tire width
61mm
2.40 in
ETRTO
61-584
Width-bead seat
Inches × 25.4 — makers differ by a few mm (Schwalbe marks 2.40 in as 62 mm; Kenda marks 4.00 in as 98 mm).
584 mm — 27.5 MTB, gravel, French touring, many e-bikes
Also sold as: 650B · 27.5 in · 26 x 1-1/2
Which tube fits
Wheel circumference (for a bike computer)
The formula is the unloaded tire. Published charts differ from each other by up to 4%, because real circumference depends on rim width, pressure and load — for the exact number, roll the wheel one turn with you on the bike and measure.
Watch out for
26 × 1.75 is not 26 × 1-3/4
Decimal 26 × 1.75 is 559 mm (mountain bike); fractional 26 × 1-3/4 is 571 mm (Schwinn cruiser). Mathematically equal labels, incompatible tires. 26 × 1-3/8 (590 or 597) and 26 × 1-1/2 (584) are two more “26-inch” sizes.
27.5 is smaller than 27
27.5 in (650B) is 584 mm, well under 27-inch's 630 mm. Sheldon Brown's rule: “27 × anything except 27 five”.
Worked example
Decoding “27.5 × 2.40” — the common e-bike size
27.5 is a diameter family, and 27.5-inch wheels have a 584 mm bead seat — the same rim as 650B and the old French 26 × 1-1/2. 2.40 is the width in inches: 2.40 × 25.4 ≈ 61 mm, though Schwalbe marks this width as 62 mm. So the ETRTO size is about 62-584, and that’s what to match when buying.
A tube for it is labeled for 584 mm with a width range that includes 62 mm. On the ETRTO chart a 62 mm tire is designed for rims 19 to 29 mm wide inside. Its unloaded circumference is π × (584 + 2 × 62) ≈ 2224 mm; Wahoo publishes 2,213 mm for the same size.
Reference
Bike tire size chart: bead seat diameters and what they're called
Every size a bike, e-bike or kids’ bike is likely to use. If two labels share a bead seat, the tires interchange (width permitting); if not, they don’t.
| BSD | Marked as | Used on | ISO code |
|---|---|---|---|
| 787 mm | 36 in | Unicycles, novelty | — |
| 686 mm | 32 in | Unicycles, novelty | — |
| 642 mm | 700A · 28 x 1-3/8 (some) · 28 x 1-5/8 x 1-1/2 (some) | Obsolete | — |
| 635 mm | 28 x 1-1/2 · 700B · F10 · F25 | Rod-brake roadsters (UK, Netherlands, China, India) | — |
| 630 mm | 27 x 1 · 27 x 1-1/8 · 27 x 1-1/4 · 27 x 1-3/8 | Older road bikes | — |
| 622 mm | 700C · 29 in · 28 in (Europe) · 28 x 1-5/8 x 1-3/8 · 28 x 1.40 (German) | Road, hybrid, gravel, 29er MTB, many e-bikes | 29 (28 for ≤ 50 mm) |
| 609 mm | 27 x 1-1/2 (Danish) | Rare | — |
| 599 mm | 26 x 1.25 · 26 x 1.375 (very old US) | Rare | — |
| 597 mm | 26 x 1-1/4 · 26 x 1-3/8 (Schwinn S-6) | Older British club bikes; Schwinn lightweights | — |
| 590 mm | 26 x 1-3/8 (E.A.3) · 650A | English 3-speeds, department-store and juvenile 10-speeds | — |
| 584 mm | 650B · 27.5 in · 26 x 1-1/2 | 27.5 MTB, gravel, French touring, many e-bikes | 27.5 |
| 583 mm | 700D | Oddball (GT); a 650B tire fits per Sheldon Brown | — |
| 571 mm | 650C · 26 x 1 · 26 x 1-3/4 (Schwinn S-7) | Triathlon and small road bikes; Schwinn cruisers | — |
| 559 mm | 26 in decimal (26 x 1.00 to 26 x 2.6) · 26 x 4.0 fat | MTB, cruisers, fat bikes, many fat e-bikes | 26 |
| 547 mm | 24 x 1-1/4 · 24 x 1-3/8 (Schwinn S-5) | British and Schwinn juvenile | — |
| 541 mm | 600A | European juvenile | — |
| 540 mm | 24 x 1-1/8 · 24 x 1-3/8 (E.5) | British juvenile, most wheelchairs | — |
| 520 mm | 24 x 1 · 24 x 1-1/8 · 24 x 1-3/8 · 24 x 1-3/4 (Schwinn S-7) | Small road bikes | — |
| 507 mm | 24 in decimal (24 x 1.5 to 24 x 2.4) · 24 x 4.0 fat | Kids' MTB, cruisers | 24 |
| 501 mm | 22 x 1-3/8 (British) · 22 x 1.00 | Juvenile | — |
| 457 mm | 22 x 1.75 to 22 x 2.4 | BMX cruiser, juvenile | — |
| 451 mm | 20 x 1-1/8 · 20 x 1-1/4 · 20 x 1-3/8 · OS 20 x 1.6 / 1.75 / 1.85 | Juvenile road, adult BMX racing, some folders | — |
| 440 mm | 500A | Juvenile, folding | — |
| 438 mm | 20 x 1-3/8 (Dutch) | Dutch juvenile | — |
| 419 mm | 20 x 1-3/4 (Schwinn) | Schwinn juvenile | — |
| 406 mm | 20 in decimal (20 x 1.5 to 20 x 2.4) · 20 x 3.0 · 20 x 4.0 fat | BMX, kids, folders, trailers, many folding and fat e-bikes | 20 |
| 400 mm | 18 x 1-3/8 · 18 x 1 | British juvenile | — |
| 369 mm | 17 x 1-1/4 | Moulton | — |
| 355 mm | 18 in decimal | Birdy folders, juvenile | 18 |
| 349 mm | 16 x 1-3/8 | Brompton, older Moulton, recumbent fronts | 16 |
| 305 mm | 16 in decimal (16 x 1.75 to 16 x 2.125) | Kids, folders, trailers | 16 |
| 254 mm | 14 in decimal (14 x 1.75) | Kids | 14 |
| 203 mm | 12-1/2 x anything · 12 in | Kids, scooters | 12.5 |
The six labels that get the wrong tire bought
0126 × 1.75 is not 26 × 1-3/4
Decimal 26 × 1.75 is 559 mm (mountain bike); fractional 26 × 1-3/4 is 571 mm (Schwinn cruiser). Mathematically equal labels, incompatible tires. 26 × 1-3/8 (590 or 597) and 26 × 1-1/2 (584) are two more “26-inch” sizes.
02Three different “20-inch” wheels
Decimal 20 × 1.75 and 20 × 4.0 are 406 mm (BMX, folders, fat e-bikes); 20 × 1-1/8 to 1-3/8 and “OS” BMX sizes are 451 mm; Schwinn's 20 × 1-3/4 is 419 mm.
0327-inch is not 700C
27 × anything is 630 mm; 700C is 622 mm. They look alike and are 8 mm apart — a 700C tire won't seat on a 27-inch rim.
0427.5 is smaller than 27
27.5 in (650B) is 584 mm, well under 27-inch's 630 mm. Sheldon Brown's rule: “27 × anything except 27 five”.
05“28-inch” can be 622 or 635
In Europe 28 in usually means 700C (622 mm), but 28 × 1-1/2 roadster tires are 635 mm, and 28 × 1-5/8 × 1-1/2 appears at 622, 635 and 642.
0624-inch: 507 vs 520, 540 and 547
Decimal 24-inch mountain bike tires are 507 mm; the fractional 24-inch sizes are 520, 540 or 547 mm. 24 × 1-1/8 alone can be 520 or 540.
Wheel circumference for bike computers (mm)
| Size | ETRTO | π(BSD + 2W) | Cateye / Garmin | Wahoo | Schwalbe |
|---|---|---|---|---|---|
| 16 x 1-3/8 | 37-349 | 1329 | 1300 | 1300 | 1330 |
| 20 x 1.75 | 47-406 | 1571 | 1515 | 1515 | 1580 |
| 20 x 1.95 / 2.00 | 50-406 | 1590 | 1565 | 1565 | 1600 |
| 20 x 4.0 | 100-406 | 1904 | — | — | — |
| 24 x 1.75 | 47-507 | 1888 | 1890 | 1890 | 1900 |
| 26 x 1.95 / 2.00 | 50-559 | 2070 | 2050 | 2050 | 2075 |
| 26 x 2.125 / 2.25 | 57-559 | 2114 | 2070 | 2070 | 2120 |
| 26 x 4.0 | 100-559 | 2384 | — | — | — |
| 26 x 1-3/8 | 37-590 | 2086 | 2068 | 2068 | 2100 |
| 27 x 1-1/4 | 32-630 | 2180 | 2161 | 2161 | 2200 |
| 27.5 x 2.25 | 57-584 | 2193 | 2182 | 2182 | 2215 |
| 27.5 x 2.40 | 62-584 | 2224 | — | 2213 | — |
| 27.5 x 2.80 | 71-584 | 2281 | — | 2279 | — |
| 700 x 25C | 25-622 | 2111 | 2105 | 2109 | 2135 |
| 700 x 28C | 28-622 | 2130 | 2136 | 2130 | 2150 |
| 700 x 32C | 32-622 | 2155 | 2155 | 2152 | 2170 |
| 700 x 38C | 38-622 | 2193 | 2180 | 2190 | — |
| 700 x 45C | 45-622 | 2237 | 2242 | 2234 | — |
| 29 x 2.2 | 56-622 | 2306 | 2298 | 2302 | — |
| 29 x 2.35 | 60-622 | 2331 | 2326 | 2326 | 2340 |
Which rims a tire width is designed for (ETRTO)
| Tire width | Inner rim widths (mm) |
|---|---|
| 18 mm | 13 |
| 20 mm | 13 |
| 23 mm | 13, 15 |
| 25 mm | 13, 15, 17 |
| 28 mm | 15, 17, 19 |
| 32 mm | 15, 17, 19 |
| 35 mm | 17, 19, 21 |
| 37 mm | 17, 19, 21, 23 |
| 40 mm | 17, 19, 21, 23 |
| 42 mm | 17, 19, 21, 23, 25 |
| 44 mm | 17, 19, 21, 23, 25 |
| 47 mm | 17, 19, 21, 23, 25, 27 |
| 50 mm | 17, 19, 21, 23, 25, 27 |
| 52 mm | 17, 19, 21, 23, 25, 27, 29 |
| 54 mm | 19, 21, 23, 25, 27, 29 |
| 57 mm | 19, 21, 23, 25, 27, 29 |
| 60 mm | 19, 21, 23, 25, 27, 29 |
| 62 mm | 19, 21, 23, 25, 27, 29 |
How to use it
In 4 steps
- 1Find the size on the tire's sidewall. Two numbers joined by a dash (like 57-559) is the ETRTO size — use that if it's there.
- 2Type exactly what's printed, including any fraction (26 × 1-3/8 is not 26 × 1.375), or tap an example.
- 3Read the bead seat diameter: that's the number a replacement tire or tube has to match.
- 4If the result says ambiguous, the label is used for more than one rim — find the ETRTO number or measure before you buy.
Why one wheel can have four names
Tire sizes grew up in different countries at different times. Inch sizes came from the US and Britain, French sizes like 700C and 650B from Europe, and the ETRTO system (now ISO 5775) was created to end the confusion by naming a tire with two measurements that don’t lie: its width and the diameter of the rim ledge the bead sits on. That’s why a 29-inch mountain bike, a 700C road bike and a European “28-inch” city bike can all use 622 mm rims — and why two tires both called “26-inch” may be 559, 571, 584, 590 or 597 mm.
Decimal and fractional inch sizes are the biggest trap. 26 × 1.75 and 26 × 1-3/4 are mathematically equal and completely incompatible: the decimal one is a 559 mm mountain-bike size, the fractional one a 571 mm cruiser size. Sheldon Brown called it the most common cause of mismatched tires.
Widths are looser than diameters. Makers measure differently — Schwalbe marks 2.40 inches as 62 mm, Kenda marks 4.00 inches as 98 mm — and the same tire measures wider on a wider rim, about 0.4 mm per millimeter of rim. Electric scooters use motorcycle-style sizes instead (60/70-6.5 on a Segway Ninebot MAX), which bicycle tires and tubes don’t fit.
FAQ
Tire size: questions
Is 26 × 1.75 the same as 26 × 1-3/4?
No. 26 × 1.75 (decimal) is a 559 mm mountain-bike size; 26 × 1-3/4 (fractional) is a 571 mm Schwinn cruiser size. The labels are mathematically equal, but the rims differ by 12 mm and the tires don't interchange.
Is 27.5-inch the same as 650B?
Yes. Both are 584 mm bead seat — so is the old 26 × 1-1/2. A 27.5 × 2.4 mountain-bike tire and a 650B × 47 gravel tire fit the same rim, width permitting.
Is 29-inch the same as 700C?
Yes — both are 622 mm bead seat, and so is the European "28-inch" (28 × 1-5/8 × 1-3/8). The names describe outside diameter with different tire widths; the rim is the same. 27-inch is different: 630 mm.
What size is a Segway Ninebot MAX scooter tire?
60/70-6.5 — a motorcycle-style marking meaning 60 mm wide, a sidewall 70% as tall as the width, on a 6.5-inch rim. That works out to about 249 mm (roughly 10 inches) outside diameter. Bicycle tires and tubes don't fit it.
Can I put a wider tire on my rim?
Often, within a range. On the ETRTO chart a rim 19 mm wide inside is designed for tires from 28 to 62 mm, and a 25 mm rim for 42 to 62 mm. The other limit is clearance: the tire needs room in the frame, fork and fenders, and it grows 1–2 mm as it wears in.
What do TL and TLR mean on a tire?
TL means tubeless; TLR (tubeless-ready) means the tire can run tubeless with sealant, or with a normal tube. Only tubeless-ready tires are approved for hookless rims.
Something not answered here? Just ask — I’d rather talk it through than have you guess.
Sources
Where these numbers come from
- [1]Sheldon Brown & John Allen, “Tire Sizing Systems” (opens in a new tab)The cross-reference of every bead seat diameter and the labels used for it.
- [2]ISO 5775-1:2023 (official preview) (opens in a new tab)The ETRTO/ISO designation (width-bead seat) and the overall-diameter codes.
- [3]Schwalbe Tech Info 2015 (opens in a new tab)Size list, ETRTO tire/rim combinations, measured circumferences.
- [4]Schwalbe tubes (opens in a new tab)How tubes are labeled: width range over bead seat (No. 13 = 40–62/559).
- [5]Kenda 20-inch tubes (opens in a new tab)Separate tubes for 406 and 451 mm “20-inch” wheels.
- [6]Schwalbe valve FAQ (opens in a new tab)Presta needs a 6 mm rim hole, Schrader 8 mm.
- [7]Continental, tire/rim combinations (opens in a new tab)A tire measures about 0.4 mm wider per 1 mm of extra rim width.
- [8]Cateye tire circumference chart (2015) (opens in a new tab)Published circumferences (Garmin publishes identical figures).
- [9]Wahoo tire circumference chart (opens in a new tab)A second published set of circumferences.
- [10]Michelin, “How to Read a Motorcycle Tire” (opens in a new tab)Width/aspect-rim markings used on 10-inch scooter tires.
- [11]Segway 60/70-6.5 replacement tire (opens in a new tab)The Ninebot MAX's 10-inch tire size.
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