Wheel Measurements Explained: Diameter, Width, Offset and More

The key wheel measurements are rim diameter, rim width, offset or backspacing, bolt pattern and center bore. Rim diameter and rim width are usually in inches. Offset, bolt pattern and center bore are usually in millimetres. The tire sidewall adds three more numbers: section width, aspect ratio and the rim diameter the tire fits. Together these numbers tell you how tall and wide a tire is and where the wheel sits relative to the hub. They do not, on their own, prove that a wheel and tire will fit a particular vehicle.

What do the numbers on a tire sidewall mean?

A tire size such as P225/45R17 94V combines dimensions, a ratio, a construction code and two performance ratings in one string. The first three numbers describe size. The last number and letter describe how much load the tire can carry and how fast it is rated to go.

Tire size example: P225/45R17 94V, part by part

Marking	Meaning	Type of value	Unit
P	Passenger-car tire (P-metric). LT means light truck. No letter usually means a European metric size.	Category code	None
225	Tire section width: the nominal width from sidewall to sidewall, measured on a specified rim	Dimension	Millimetres
45	Aspect ratio: sidewall height as a percentage of section width	Ratio	Percent
R	Radial construction	Construction code	None
17	Rim diameter the tire is made to fit, measured at the bead seats	Dimension	Inches
94	Load index: 94 corresponds to 670 kg (1,477 lb) per tire	Performance rating	Index number
V	Speed rating: V corresponds to 240 km/h (149 mph)	Performance rating	Letter code

Dimensions, ratios, units and ratings: how to tell them apart

Only some sidewall numbers are physical sizes. Mixing them up is the most common reason size comparisons go wrong.

• Dimensions are physical sizes: section width (225 mm) and rim diameter (17 in). One size string mixes metric and imperial units, so convert before you calculate.
• The aspect ratio is a percentage, not a length. A 45 aspect ratio on a 225 mm tire gives a different sidewall height than a 45 aspect ratio on a 255 mm tire.
• Performance ratings say nothing about size. These include the load index, speed rating, maximum load, maximum inflation pressure and, in the United States, the UTQG treadwear, traction and temperature grades.
• Identification codes are not measurements either. The last four digits of the DOT code give the week and year of manufacture, so 2324 means week 23 of 2024.

Other tire size formats you may see

• Flotation sizes such as 31x10.50R15 give the overall diameter (31 in), the section width (10.50 in) and the rim diameter (15 in), all in inches. Overall diameter is printed directly, so you do not need to calculate it.
• Bicycle ETRTO/ISO sizes such as 25-622 give tire width (25 mm) and bead seat diameter (622 mm), both in millimetres. The same tire may also be labelled 700x25C.

How tire size numbers determine sidewall height, diameter and circumference

You can calculate a tire's nominal sidewall height, overall diameter and circumference from its section width, aspect ratio and rim diameter. Use these steps for any metric size:

1. Sidewall height (mm) = section width (mm) × aspect ratio ÷ 100.
2. Rim diameter (mm) = rim diameter (in) × 25.4.
3. Overall diameter (mm) = rim diameter (mm) + 2 × sidewall height. The sidewall counts twice because there is one above and one below the rim.
4. Circumference (mm) = overall diameter × π (about 3.1416).

Worked example for 225/45R17:

• Sidewall height: 225 × 0.45 = 101.25 mm.
• Rim diameter: 17 × 25.4 = 431.8 mm.
• Overall diameter: 431.8 + 202.5 = 634.3 mm, about 24.97 in.
• Circumference: 634.3 × 3.1416 ≈ 1,992.7 mm.

Comparing tire sizes: width, aspect ratio and rim diameter together

A larger rim diameter can still give almost the same overall diameter if the aspect ratio drops. In that case the sidewall gets shorter while the wheel gets larger. This is the idea behind plus-sizing.

Tire size	Sidewall height	Overall diameter	Circumference	Diameter vs 205/55R16
205/55R16	112.8 mm	631.9 mm (24.88 in)	1,985.2 mm	Reference
225/45R17	101.3 mm	634.3 mm (24.97 in)	1,992.7 mm	+0.4%
225/40R18	90.0 mm	637.2 mm (25.09 in)	2,001.8 mm	+0.8%
235/40R18	94.0 mm	645.2 mm (25.40 in)	2,027.0 mm	+2.1%

The table shows three effects. Raising rim diameter while lowering aspect ratio keeps overall diameter close but shortens the sidewall by about 23 mm, from 205/55R16 to 225/40R18. Increasing section width at the same aspect ratio also increases sidewall height, because the ratio is a percentage of width. That is why 235/40R18 is taller than 225/40R18.

What a diameter difference means in practice

A change in overall diameter changes how far the tire travels per revolution. The percentage change in circumference equals the percentage change in diameter.

• Distance per revolution: a 225/45R17 travels about 1.99 m per turn, or roughly 502 revolutions per kilometre in calculated terms.
• Speedometer reading: suppose 235/40R18 tires (645.2 mm) replace 225/45R17 tires (634.3 mm). The tire is about 1.7% larger, so an indicated 100 km/h corresponds to roughly 101.7 km/h of actual speed, other factors unchanged.
• Ground clearance: the axle rises by half the diameter difference. Here that is about 5.5 mm.

Calculated geometry versus actual tire dimensions

Calculated sizes are nominal figures. They are not exact measurements of a real mounted tire.

• Section width changes with rim width. The same tire measures wider on a wider rim and narrower on a narrower rim. The nominal figure applies to a specified measuring rim.
• Brands and models differ. Two tires with the same size marking can have slightly different real diameters and widths because of tread depth, tread design and construction. Manufacturers publish actual specifications for each model.
• Load and wear change the dimensions. A loaded tire has a shorter rolling radius than its free diameter, and tread wear reduces diameter over time.
• Geometry is not fitment. Matching overall diameter does not confirm clearance to suspension, brakes or bodywork. It also does not confirm the correct load index, or that the tire is approved for the rim width.

What do the markings on a wheel mean?

Wheel markings describe the rim's diameter, width, flange shape, offset, bolt pattern and center bore. A typical stamping such as 17x7.5J ET45 5x114.3 CB67.1 reads as follows:

Marking	Meaning	Unit
17	Rim diameter, measured at the bead seats where the tire beads sit	Inches
7.5	Rim width, measured between the inner faces of the rim flanges at the bead seats	Inches
J	Rim flange profile (shape and height of the flange)	Letter code
ET45	Wheel offset of +45 mm. ET stands for the German term Einpresstiefe.	Millimetres
5x114.3	Bolt pattern: 5 studs on a 114.3 mm diameter circle	Count × millimetres
CB67.1	Center bore: diameter of the central hole that fits over the hub	Millimetres

These markings are usually cast or stamped inside the wheel barrel, on the back of a spoke or on the hub mounting pad. Not every wheel shows every value.

Rim width, overall wheel width and tire section width: what is the difference?

These three widths measure different things, and confusing them leads to wrong offset and fitment calculations.

• Rim width (bead seat width) is the nominal width on the wheel marking, such as 7.5 in. It is measured between the inner edges of the rim flanges, where the tire bead seats.
• Overall rim width (overall wheel width) is measured from the outer edge of one flange to the outer edge of the other. On typical car wheels it is about 1 in more than the nominal rim width, because each flange adds width. Check your own wheel, because flange designs vary.
• Tire section width is the tire's own width, such as 225 mm. Each tire size is approved for a range of rim widths, so the tire size does not tell you the exact rim width.

How to measure rim width with the tire off

Rim width is measured between the inner faces of the rim flanges, across the bead seats. Follow these steps for an unmounted wheel:

1. Lay the wheel flat on a stable surface.
2. Find the bead seats, the flat ledges just inside each flange where the tire bead sits.
3. Measure from the inside edge of one flange to the inside edge of the other, across the barrel, using a tape measure, ruler or large calipers.
4. Record the result in inches. A reading close to 7.5 in matches a 7.5 in nominal rim width.
5. To get overall wheel width, measure again from the outer edge of one flange to the outer edge of the other. You need this figure for backspacing and offset calculations.

How to measure wheel width with the tire on

With a mounted tire you cannot reach the bead seats, so you either read the wheel's stamped size or measure the overall width across the flanges and estimate the nominal width from it. The same method works whether you call it a tire or a tyre.

1. Look for the stamping first. Check the inside of the barrel, the back of the spokes and the hub mounting face for a marking such as 17x7.5J. This is the most reliable way to determine wheel width with the tire on.
2. If there is no marking, use two straightedges. Place one straightedge across the outer flange lips and another across the inner flange lips. Use bars just longer than the rim diameter so they rest on the metal flanges and not on the bulging tire sidewall.
3. Measure between the straightedges at the rim, perpendicular to them. This is the overall wheel width.
4. Estimate nominal rim width by subtracting the flange allowance, typically about 1 in on car wheels. For example, a 9 in overall width usually indicates an 8 in rim.
5. Treat the result as an estimate. Flange profiles differ, and a reading taken over a mounted tire is less precise than a direct bead seat measurement.

Do not use tire section width to work out rim width. A 225 mm tire may be approved for several rim widths, so the tire size alone cannot tell you which rim it is mounted on.

Measuring wheel width on the car

You can roughly measure overall wheel width with the wheel still on the car by holding a straightedge against the outer flange and measuring back to the inner flange. Brakes, suspension parts and the tire sidewall make the inner flange hard to reach. For dependable width and backspacing figures, remove the wheel or read the stamped markings.

What are wheel offset and backspacing?

Wheel offset is the distance in millimetres from the wheel centerline to the hub mounting face. Backspacing is the distance in inches from the hub mounting face to the inner edge of the wheel. Both describe where the wheel sits relative to the hub. Offset references the middle of the wheel, while backspacing references its inner edge.

Positive, zero and negative offset

• Positive offset: the mounting face is toward the outer (street) side of the wheel centerline. More of the wheel sits inboard, under the vehicle.
• Zero offset: the mounting face sits exactly on the wheel centerline.
• Negative offset: the mounting face is toward the inner side of the centerline, so the wheel sits further outward.

These positions follow directly from the geometry. At the same width, lowering offset by 10 mm moves the whole wheel 10 mm outward, both the inner and the outer edge. Widening a wheel from 7.5 in to 8.5 in at the same offset adds 0.5 in (12.7 mm) on each side of the centerline.

How to convert between offset and backspacing

Offset and backspacing are linked through the overall wheel width. Convert all values to the same unit first.

• Offset (in) = backspacing (in) − overall wheel width (in) ÷ 2
• Backspacing (in) = overall wheel width (in) ÷ 2 + offset (in)
• Multiply inches by 25.4 to get millimetres.

Example: a 17x8 wheel measures 9 in overall and has 5.6 in of backspacing. The centerline sits 4.5 in from either outer edge. Offset = 5.6 − 4.5 = 1.1 in, which is about +28 mm. If you used the nominal 8 in width instead, the centerline would land at 4.0 in and the offset would come out wrong by 0.5 in.

How to measure wheel width, backspacing and offset

You can find backspacing and offset with two straightedges and a tape measure, ideally with the wheel off the vehicle. A mounted tire can stay on if the straightedges rest on the flange lips.

1. Remove the wheel and place it face down on blocks, so the outer face does not rest on the hub area.
2. Lay a straightedge across the inner (back) flange lips.
3. Measure straight down from the straightedge to the hub mounting face. This is the backspacing.
4. Measure the overall wheel width from outer flange edge to outer flange edge, as described above.
5. Divide the overall width by 2 to locate the wheel centerline.
6. Subtract half the overall width from the backspacing to get offset in inches, then multiply by 25.4 for millimetres. A positive result is positive offset, and a negative result is negative offset.
7. Compare your result with any ET marking on the wheel. A large mismatch usually means a measuring error, such as using nominal instead of overall width.

How these measurements apply to spoked wheels

Spoked wheels use the same basic dimensions: bead seat diameter, rim width, and the wheel's position relative to the hub. Some of these values come from the rim and lacing rather than from a one-piece casting.

• Rim diameter and width are measured at the bead seats, as on any wheel. Bicycle rims often use the ETRTO format, such as 622x19 (622 mm bead seat diameter, 19 mm inner width). Motorcycle rims usually show width and diameter in inches, such as 2.50x17.
• Wheel position (offset or dish) on a spoked wheel depends on the hub and on how the spokes pull the rim relative to the hub flanges. It is not set by a fixed mounting face as on a cast wheel.
• Spoke length depends on a separate rim measurement, the effective rim diameter at the spoke nipple seats. That is not the same as the bead seat diameter printed on the rim or tire.

What dimensional differences can and cannot tell you

Tire and wheel measurements let you compare sizes and spot obvious mismatches, but they cannot guarantee fitment on their own.

• They can tell you the nominal sidewall height, overall diameter and circumference of a tire size. They also show how far a wheel's inner and outer edges move when width or offset changes, and whether bolt pattern and rim diameter match.
• They cannot tell you the exact dimensions of a specific mounted tire, real clearance to brakes, suspension and bodywork, or whether the center bore and load ratings suit the vehicle.
• For a final decision, check the actual tire specifications from the manufacturer, the tire's approved rim width range, the vehicle's specified sizes, and a physical clearance check.