ISO Tire Size Standards: Understanding Size Designations and Charts

A tire size such as 205/55 R16 91V is a standardized code. In this code, 205 is the nominal section width in millimetres. 55 is the aspect ratio, which gives sidewall height as a percentage of width. R means radial construction. 16 is the nominal rim diameter in inches. 91V is the service description: a load index and a speed symbol. The first numbers describe the tire's size and shape. The last characters are performance ratings and are not dimensions. ISO tire size standards define this metric sizing logic, so tires from different manufacturers can be identified and compared in a consistent way.

What are ISO tire size standards?

ISO tire size standards are international documents published by the International Organization for Standardization (ISO). They define how tires and rims are designated and measured. The work is handled by the ISO technical committee for tyres, rims and valves (ISO/TC 31). Separate parts of the standards cover different vehicle types, including passenger cars, trucks and buses, motorcycles and bicycles.

Regional organizations apply and extend these principles in their own technical manuals. Examples include ETRTO in Europe, the Tire and Rim Association (TRA) in the United States and JATMA in Japan. For the reader, the practical result is that a tire size designation means essentially the same thing on most modern passenger tires, whatever the brand.

The system is described as metric sizing because width is given in millimetres and shape is given as a ratio. However, the rim diameter on car tires is still expressed as an inch code. Most ISO tire sizes therefore combine two units in one code.

How do you read the numbers on a tire sidewall?

Read the size designation from left to right. Each part answers a different question: how wide the tire is, how tall the sidewall is relative to that width, how the tire is built, which rim it fits, and how much load and speed it is rated for.

Marking (example 205/55 R16 91V)	Name	Unit	Type of value	What it tells you
205	Nominal section width	Millimetres	Dimension	Approximate width of the inflated tire from sidewall to sidewall
55	Aspect ratio (series)	Percent	Ratio	Sidewall height equals about 55% of the section width
R	Construction code	None	Construction	Radial ply construction
16	Rim diameter code	Inches	Dimension (code)	Fits a rim with a 16-inch nominal diameter
91	Load index	Code (maps to kg)	Performance rating	Maximum load per tire under specified conditions (91 = 615 kg)
V	Speed symbol	Code (maps to km/h)	Performance rating	Maximum speed under specified conditions (V = 240 km/h)

Nominal section width: the first number

The nominal section width is the first number in an ISO tire size. It is the tire's width in millimetres when the tire is inflated and mounted on a specified measuring rim, and it includes the sidewalls but not raised lettering or protective ribs. The word nominal matters. A tire marked 205 is not guaranteed to measure exactly 205 mm. The real width varies with the rim width it is mounted on and with the design of each model. The nominal width is mainly useful for comparing sizes. A 225 tire is about 20 mm wider than a 205 tire on comparable rims.

Aspect ratio: why the second number is a percentage

The aspect ratio is the sidewall (section) height expressed as a percentage of the section width. It is not a height in millimetres. In 205/55, the sidewall height is about 55% of 205 mm, or roughly 113 mm. A lower aspect ratio such as 40 means a shorter sidewall relative to width. A higher ratio such as 65 means a taller one.

Because the ratio depends on width, the same aspect ratio gives a taller sidewall on a wider tire. A 225/55 sidewall is taller than a 205/55 sidewall. Changing width alone also changes sidewall height and overall diameter.

Construction letter: R and related markings

The letter after the aspect ratio describes how the tire is built. R stands for radial, which is the construction used by nearly all modern passenger car tires. Some high-speed tires show ZR in the size, for example 225/45 ZR17. ZR is a speed-related marking tied to tires rated above 240 km/h. The service description, such as 94Y, gives the actual speed rating.

Rim diameter and bead seat diameter

The number after the construction letter is the rim diameter code. On passenger car tires it is given in inches, so 16 means the tire fits a 16-inch rim. For calculations, the code is converted at 25.4 mm per inch: 16 in × 25.4 = 406.4 mm.

Strictly, the tire fits the rim at the bead seat diameter. This is the diameter of the part of the rim where the tire bead sits and seals. Standards define the exact bead seat geometry for each rim code, and it can differ slightly from the simple inch conversion. The key rule for fitment is that a tire's rim diameter must exactly match the wheel's rim diameter. A 16-inch tire cannot be mounted on a 17-inch wheel.

Load index and speed symbol: ratings, not dimensions

The load index and speed symbol together form the service description. They state what the tire is rated to carry and how fast it may be driven under the conditions set by the standard. They do not describe size. Two tires can share the same size designation but have different service descriptions, such as 205/55 R16 91V and 205/55 R16 94W.

Load index	Max load per tire	Speed symbol	Max speed
88	560 kg	S	180 km/h
91	615 kg	T	190 km/h
94	670 kg	H	210 km/h
97	730 kg	V	240 km/h
100	800 kg	W	270 km/h
—	—	Y	300 km/h

Markings such as XL or Reinforced mean the tire has a higher load capacity than the standard-load version of the same size. That capacity applies at the appropriate inflation pressure.

What other ISO tire size formats will you see?

Most passenger tires use the format shown above. Some related formats add a prefix or suffix. These change how the load rating is read, but the size logic stays the same.

• P-metric (for example P215/65 R15): A North American variant defined by TRA. The dimensions are read the same way, but P-metric and European metric tires of the same size use different load tables. Their load capacities are not automatically equal.
• C or LT (for example 195/70 R15C 104/102R): Light commercial or light truck tires. They are often shown with two load indexes: the first for single fitment and the second for dual fitment.
• Millimetre rim diameters: A few special tires use rim diameters given in millimetres rather than inches. They are not interchangeable with inch-sized rims.

ISO bicycle tire sizes

ISO bicycle tire sizes, defined in ISO 5775 and based on the ETRTO system, use two numbers in millimetres. The first is the tire width and the second is the bead seat diameter. A tire marked 37-622 is about 37 mm wide and fits a rim with a 622 mm bead seat diameter. This system removes the confusion of traditional names, where tires called 26 inch or 28 inch could fit different rims.

Bead seat diameter (ISO)	Common traditional names
622 mm	700C, 29er, 28 inch
584 mm	650B, 27.5 inch
571 mm	650C
559 mm	26 inch mountain bike
507 mm	24 inch
406 mm	20 inch (BMX and many folding bikes)

For bicycles, the bead seat diameter must match the rim exactly. Width can usually vary within the range suited to the rim and frame clearance.

How do you calculate tire diameter and circumference from the size?

You can estimate a tire's sidewall height, overall diameter and circumference from its size designation with three simple steps. The example uses 205/55 R16.

1. Sidewall height: Multiply the section width by the aspect ratio as a decimal. 205 mm × 0.55 = 112.75 mm.
2. Overall diameter: Convert the rim diameter to millimetres, then add two sidewall heights, one above and one below the rim. 16 × 25.4 = 406.4 mm, and 406.4 + 2 × 112.75 = 631.9 mm.
3. Circumference: Multiply the overall diameter by π (about 3.1416). 631.9 mm × 3.1416 ≈ 1,985 mm, or about 1.99 m per revolution.

From the circumference you can also estimate revolutions per kilometre. Divide 1,000,000 mm by the circumference: 1,000,000 ÷ 1,985 ≈ 504 revolutions per km.

How do width, aspect ratio and rim diameter change tire size?

Each of the three size numbers affects overall diameter differently. A larger rim diameter increases overall diameter directly. A higher aspect ratio increases sidewall height. A larger width also increases sidewall height, because the aspect ratio is a percentage of width.

When people move to a larger wheel and keep overall diameter similar, they usually lower the aspect ratio. This shortens the sidewall to make room for the bigger rim. This approach is often called plus sizing.

Size	Sidewall height	Overall diameter	Circumference	Revolutions per km	Diameter vs 205/55 R16
205/55 R16	112.8 mm	631.9 mm	1,985 mm	504	Reference
225/45 R17	101.3 mm	634.3 mm	1,993 mm	502	+0.4%
195/65 R15	126.8 mm	634.5 mm	1,993 mm	502	+0.4%
205/60 R16	123.0 mm	652.4 mm	2,050 mm	488	+3.2%
225/55 R16	123.8 mm	653.9 mm	2,054 mm	487	+3.5%

The table shows three common patterns:

• Rim size changes with a balancing aspect ratio: 195/65 R15, 205/55 R16 and 225/45 R17 have almost the same calculated overall diameter, within about 0.4%. Each has a different rim size and sidewall height.
• Aspect ratio alone changes: Moving from 205/55 R16 to 205/60 R16 adds about 10 mm of sidewall height on each side. Overall diameter rises by about 20 mm, or 3.2%.
• Width alone changes: Moving from 205/55 R16 to 225/55 R16 makes the tire wider and also taller. The 55% ratio now applies to a larger width, so overall diameter rises by about 3.5%.

What a diameter difference means in practice

A change in overall diameter changes how far the vehicle travels per wheel revolution. On a vehicle whose speedometer and odometer work from wheel rotation, a tire with a larger diameter makes the vehicle travel farther per revolution than the instruments assume.

For example, replacing 205/55 R16 with 205/60 R16 increases the calculated diameter by about 3.2%. When the speedometer shows 100 km/h, the calculated true speed would be about 103 km/h. This example ignores the speedometer's own calibration allowance and tread wear. The same proportional effect applies to recorded distance.

How do you read an ISO tire size chart?

An ISO tire size chart lists tire sizes alongside their calculated or specified dimensions. These usually include section width, sidewall height, overall diameter, circumference and sometimes revolutions per kilometre. Its purpose is to let you compare sizes side by side without calculating each one.

1. Find your current size: Locate the exact designation from the sidewall or the vehicle's tire placard, such as 205/55 R16.
2. Check what each column represents: Confirm whether the chart shows calculated nominal values or the manufacturer's published dimensions, and whether diameters are in millimetres or inches.
3. Compare diameter and circumference: Look at sizes with the same rim diameter if you are keeping your wheels. Look at sizes with a different rim diameter only if you are changing wheels.
4. Calculate the percentage difference: Subtract the original diameter from the new diameter, divide by the original and multiply by 100. For 205/60 R16 versus 205/55 R16: (652.4 − 631.9) ÷ 631.9 × 100 ≈ 3.2%.
5. Treat the result as a dimensional comparison only: A chart shows how sizes differ geometrically. It does not tell you whether a size is approved or physically fits a particular vehicle.

Why calculated tire dimensions differ from actual dimensions

A calculated tire size is a geometric estimate built from nominal values. A real tire's measured dimensions can differ for several reasons:

• Nominal values are rounded designations: The marked width and aspect ratio identify a size class. They are not exact measurements of a specific tire. Standards define design dimensions and permitted tolerances, and individual models from different manufacturers can vary within them.
• Rim width affects section width: Section width is defined on a specified measuring rim. Mounting the same tire on a narrower or wider approved rim makes the actual width slightly smaller or larger.
• Load and inflation change the shape: Calculated diameter describes an inflated tire with no load. Under vehicle weight, the tire deflects at the contact patch. Its loaded radius and effective rolling circumference are therefore smaller than the simple calculation suggests.
• Tread wear reduces diameter: A worn tire has a smaller overall diameter than a new tire of the same size.
• Bead seat geometry: The calculation uses the inch code converted to millimetres. The standardized bead seat diameter can differ slightly from that figure.

For precise figures on a specific tire, use the manufacturer's published data for that model. Treat calculated values as a reliable tool for comparing sizes relative to each other.

Why matching diameter does not guarantee fitment

Two ISO tire sizes with nearly identical calculated diameters are geometrically similar. That does not make them interchangeable on a given vehicle. Fitment compatibility depends on factors the size calculation does not cover:

• Rim diameter and rim width: The tire's rim diameter must match the wheel exactly. The wheel's width must fall within the range approved for that tire size.
• Load index and speed symbol: The replacement tire must meet the load and speed requirements specified for the vehicle. This is true even if the dimensions match.
• Physical clearance: A wider tire or a different sidewall height can contact suspension parts, brake components or body panels when steering or under suspension compression.
• Vehicle approval and systems: The vehicle manufacturer lists approved sizes. Some vehicles' driving aids and instrument calibration assume specific rolling dimensions, and local regulations may restrict alternative sizes.

An ISO tire size chart and the calculations above answer one question: how much does one size differ from another in width, sidewall height, diameter and circumference? Whether a size is suitable for a specific car is a separate question. The answer comes from the vehicle's specifications, the wheel dimensions and the tire's service description.