BMX Race Tire Size Chart and Sizing Guide

A BMX race tire size such as 20 x 1.75 gives two things: the nominal wheel size (20 inches) and the nominal tire width (1.75 inches). The number that decides whether a tire fits a rim is the ETRTO (ISO) code, for example 47-406. That code means a tire about 47 mm wide that mounts on a rim with a 406 mm bead seat diameter. Most 20-inch BMX race wheels use the 406 mm bead seat. Fractional sizes such as 20 x 1-1/8 use a different 451 mm bead seat, so they are not interchangeable with decimal 20-inch tires. This guide explains each sidewall marking, gives a BMX race tire size chart, and shows how width and bead seat diameter change overall diameter and circumference.

How do you read the numbers on a BMX race tire sidewall?

A BMX race tire sidewall usually carries up to three kinds of information. Dimensions describe the tire size. Inflation limits give the allowed pressure in psi and bar. Construction details include casing TPI, bead type and rotation direction. Only the dimensional markings tell you about size and fit.

Decimal size: 20 x 1.75

The decimal size is the inch-based designation used by BMX racers and retailers. In 20 x 1.75, the first number is the nominal wheel size in inches and the second is the nominal tire width in inches. Both numbers are labels, not exact measurements. A 20-inch BMX tire is not exactly 20 inches in outside diameter, and a 1.75-inch tire does not always measure 1.75 inches wide once mounted. All decimal 20-inch tires share the same 406 mm bead seat diameter, so any decimal 20-inch tire fits any 20-inch decimal BMX rim in terms of diameter.

ETRTO / ISO size: 47-406

The ETRTO size, standardised in ISO 5775, is the most reliable size marking on a BMX tire. It has two parts:

• First number (47): the nominal tire width in millimetres.
• Second number (406): the bead seat diameter in millimetres. This is the diameter of the rim shelf where the tire bead sits.

The bead seat diameter must match the rim exactly. The width figure is nominal and depends on the rim used. Decimal and ETRTO widths do not always line up with a simple inch-to-millimetre conversion. For example, 1.75 in × 25.4 = 44.45 mm, yet tires sold as 20 x 1.75 may be marked 44-406 or 47-406 depending on the maker. When two tires with the same decimal size carry different ETRTO widths, the ETRTO figure is the better guide.

Fractional size: 20 x 1-1/8 and 20 x 1-3/8

Fractional BMX tire sizes use a separate, older naming system. A 20 x 1-1/8 tire is not the same as a 20 x 1.125 tire. Fractional 20-inch sizes such as 1-1/8 and 1-3/8 normally use a 451 mm bead seat diameter. Decimal 20-inch sizes use 406 mm. That 45 mm difference means a fractional tire cannot be mounted on a decimal rim, and the reverse is also true. Typical ETRTO markings are 28-451 for 20 x 1-1/8 and 37-451 for 20 x 1-3/8.

Pressure, TPI, bead and direction markings

These markings describe how the tire is used and built. They do not describe its size.

• Pressure range: the minimum and maximum inflation, usually in psi and bar, sometimes also in kPa (1 bar ≈ 14.5 psi ≈ 100 kPa). The maximum shown on the tire and the rim maker's limit should both be respected. The lower of the two applies.
• TPI (threads per inch): the density of the casing fabric. A higher TPI usually means a thinner, more supple casing. TPI does not change the tire's size.
• Bead type: wire beads are rigid. Folding beads, often aramid or Kevlar, can be folded and are usually lighter. Both seat on the same bead seat diameter.
• Rotation arrow or front/rear marking: the intended direction of travel or wheel position for directional tread patterns.

Why BMX tires have no aspect ratio, speed rating or load index

BMX race tires do not carry the aspect ratio, speed rating or load index found on car and kart tires. Bicycle tire size systems define width and bead seat diameter only. Sidewall height is not printed, so it has to be estimated. A common approximation is that a bicycle tire's section height is close to its width. That rough rule is used for the calculations below. The only printed performance limit on most BMX race tires is the inflation range.

BMX race tire size chart

BMX racing uses several wheel diameters, from small balance-bike sizes up to 24-inch cruiser wheels. Each decimal wheel size has a fixed bead seat diameter. The chart below lists the standard bead seat diameters for each family. Tire widths within each family vary by model and manufacturer.

Nominal size marking	Bead seat diameter (ETRTO)	Example ETRTO marking	Notes
12 x (decimal width)	203 mm	e.g. 47-203	Balance and small youth bikes
16 x (decimal width)	305 mm	e.g. 47-305	Small youth BMX
18 x (decimal width)	355 mm	e.g. 47-355	Youth and some micro race bikes
20 x 1.50 to 20 x 2.125 (decimal)	406 mm	e.g. 40-406, 47-406, 50-406, 54-406	Standard 20-inch BMX race wheel
20 x 1-1/8 (fractional)	451 mm	28-451	Not compatible with 406 mm rims
20 x 1-3/8 (fractional)	451 mm	37-451	Not compatible with 406 mm rims
22 x (decimal width)	457 mm	e.g. 47-457	Less common intermediate size
24 x (decimal width)	507 mm	e.g. 47-507	Cruiser-class BMX

To use the chart, find the nominal size printed on the tire. Then check that the second number of its ETRTO code matches the bead seat diameter of your rim. Widths between families are not related. A 24 x 1.75 and a 20 x 1.75 have the same nominal width but different bead seat diameters.

Why don't fractional and decimal 20-inch BMX tires interchange?

Fractional and decimal 20-inch BMX tires do not interchange because they are built for different bead seat diameters: 451 mm for fractional sizes and 406 mm for decimal sizes. A tire only seats correctly when its bead diameter matches the rim's bead seat diameter. A 451 mm tire is too large to stay on a 406 mm rim. A 406 mm tire is too small to stretch onto a 451 mm rim.

The "20-inch" label is shared only by naming tradition. If a sidewall shows both a fractional or decimal size and an ETRTO code, use the ETRTO bead seat number to check compatibility.

How do width and bead seat diameter change overall diameter and circumference?

A BMX tire's overall diameter is approximately the bead seat diameter plus twice the tire's section height. Because bicycle sidewall height is roughly equal to tire width, a wider tire on the same rim produces a slightly larger overall diameter and circumference. A larger bead seat diameter has a bigger effect than a change in width.

How to estimate a BMX tire's diameter and circumference

1. Read the ETRTO code from the sidewall, for example 47-406.
2. Take the width (47 mm) as an estimate of section height.
3. Add twice that height to the bead seat diameter: 406 + (2 × 47) = 500 mm estimated overall diameter.
4. Multiply the diameter by π (about 3.1416) to get circumference: 500 × 3.1416 ≈ 1571 mm.
5. To convert to inches, divide millimetres by 25.4: 500 mm ≈ 19.7 in.

Calculated size comparison for common BMX race tires

The table applies the same method to several common BMX race tire markings. All figures are calculated estimates based on nominal ETRTO values, not measured dimensions.

ETRTO marking	Typical nominal size	Estimated sidewall height	Estimated overall diameter	Estimated circumference
40-406	20 x 1.50–1.60	40 mm	486 mm (19.1 in)	1527 mm
44-406	20 x 1.75	44 mm	494 mm (19.4 in)	1552 mm
47-406	20 x 1.75	47 mm	500 mm (19.7 in)	1571 mm
50-406	20 x 1.95	50 mm	506 mm (19.9 in)	1590 mm
54-406	20 x 2.125	54 mm	514 mm (20.2 in)	1615 mm
28-451	20 x 1-1/8	28 mm	507 mm (20.0 in)	1593 mm
37-451	20 x 1-3/8	37 mm	525 mm (20.7 in)	1649 mm

Some practical comparisons from the table:

• 20 x 1.75 (47-406) vs 20 x 1.95 (50-406): 3 mm more width adds about 6 mm of diameter and 19 mm of circumference, roughly 1.2% more.
• 20 x 1.75 (47-406) vs 20 x 2.125 (54-406): the wider tire adds about 14 mm of diameter and 44 mm of circumference, roughly 2.8% more.
• 20 x 1-1/8 (28-451) vs 20 x 1.95 (50-406): the much narrower fractional tire ends up with nearly the same estimated diameter, because its larger rim makes up for its lower sidewall.

How tire size affects BMX gearing

A change in tire diameter changes how far a BMX bike travels per pedal revolution. That distance is the rollout. With a 44-tooth chainring and a 16-tooth rear cog (ratio 2.75):

• 47-406 tire: 1571 mm × 2.75 ≈ 4.32 m per crank revolution, or about 54.1 gear inches (19.7 in × 2.75).
• 50-406 tire: 1590 mm × 2.75 ≈ 4.37 m per crank revolution, or about 54.8 gear inches.

The difference is about 5 cm per crank revolution. Some BMX gearing charts calculate with a nominal 20-inch diameter rather than the actual tire. Check which basis a chart uses before comparing your numbers with it.

How accurate are calculated BMX tire dimensions?

Calculated BMX tire dimensions are useful for comparing tires with each other, but they are not exact measurements. A mounted tire's real width and diameter depend on several factors:

• Rim internal width: a wider rim spreads the beads further apart and usually makes the tire measure wider and slightly lower. A narrower rim does the opposite.
• Inflation pressure: higher pressure expands the casing slightly, and pressure also changes how much the tire compresses under the rider.
• Casing and tread design: knob height, tread profile and casing shape vary between models carrying the same ETRTO marking.
• Wear: worn tread reduces overall diameter over time.
• Manufacturer tolerances: two tires labelled 50-406 from different brands may not measure the same.

For an exact figure, measure the mounted and inflated tire. You can measure width across the widest point with calipers. For circumference, mark the tire and the floor, roll the wheel one full turn with the rider seated, and measure the distance between the marks.

How do you check BMX tire and rim compatibility?

A BMX race tire fits correctly when its bead seat diameter matches the rim, its width suits the rim's internal width, and the bike has enough clearance. Calculated geometry alone does not confirm fitment. Work through these checks in order:

1. Match the bead seat diameter. The second number of the tire's ETRTO code (for example 406) must equal the rim's bead seat diameter. This check has no tolerance.
2. Check width against rim internal width. Compare the tire's ETRTO width with the rim or tire manufacturer's recommended range. A very wide tire on a narrow rim, or a narrow tire on a wide rim, can affect bead security and handling.
3. Check frame and fork clearance. Use the estimated diameter and width as a starting point, then confirm the real clearance at the chainstays, seatstays and fork crown. Allow room for mud and wheel flex.
4. Check brake alignment. Rim brakes are positioned for one bead seat diameter. Moving from a 406 mm to a 451 mm rim shifts the braking surface outward by about 22.5 mm, half the 45 mm difference, so the brake may no longer line up with the rim.
5. Confirm pressure limits. Inflate within both the tire's printed range and the rim's maximum, whichever is lower.
6. Check race class rules. BMX racing classes, such as cruiser categories built around 24-inch wheels, are defined by wheel size. Confirm your tire size against the current rules of the governing body for the class you race.

What do size differences between BMX race tires really mean?

Between BMX race tires on the same 406 mm rim, a change in nominal width mainly changes tire volume, contact patch and weight. Overall diameter changes only slightly, usually by a few millimetres. Between tires with different bead seat diameters, the difference is a compatibility issue rather than a performance choice, because the tire will not mount on the wrong rim.

When comparing BMX race tire sizes, read the ETRTO code first. Use the decimal or fractional size as a familiar label. Treat calculated diameter and circumference as estimates for comparison, and confirm real fit on the rim and bike before racing.