Read agricultural tire size markings

The short answer

The size marking on a tractor tire describes three things: how wide the tire is, how it is built, and what rim diameter it fits. A modern marking such as 750/55 R26 means a tire 750 millimetres wide, with a sidewall height equal to 55 percent of that width, radial construction (R), mounted on a 26-inch rim. A legacy marking such as 8.3 R24 gives the width in inches (8.3) instead of millimetres, followed by the construction letter and the 24-inch rim diameter.

Two conditions change how you read the numbers. First, the units: modern markings give the width in millimetres, but the rim diameter is always in inches, while legacy markings use inches for both. Second, the slash: in modern sizes the number after the slash is the sidewall height as a percentage of width, but in some older sizes it is the rim width in inches. Reading the wrong convention gives you a completely wrong dimension.

How each part of the marking works

Read the marking from left to right. Each element answers a separate question about the tire, and the separators between the numbers carry meaning too.

• Section width: the first number. In modern markings it is in millimetres (750 in 750/55 R26). In legacy markings it is in inches (8.3 in 8.3 R24, 12.4 in 12.4-28). One inch equals 25.4 mm, so 8.3 inches is about 211 mm.
• Aspect ratio: in modern markings, the number after the slash. It expresses sidewall height as a percentage of the width, not as a measurement. In 750/55 R26, the sidewall is 55 percent of 750 mm, or 412.5 mm.
• Construction symbol: R means radial construction. A dash (-) or an X in the same position means diagonal construction, also called bias-ply. So 12.4-28 and 12.4 X28 both describe a diagonal tire.
• Rim diameter: the last number, always in inches, even when the width is metric. It is the diameter of the wheel the tire is designed to mount on.

In practice, a lower aspect ratio means a shorter sidewall relative to the tire's width, and a higher one means a taller sidewall. Because the aspect ratio is a percentage, two tires with the same ratio but different widths do not have the same sidewall height: you have to multiply it out to compare them.

The two slash conventions in older markings

Older agricultural sizes are where most misreadings happen, because the slash does not always mean the same thing.

• 12.4/11-28: the 11 is the rim width in inches, not a percentage. The tire is 12.4 inches wide, the dash indicates diagonal construction, and it fits a 28-inch rim.
• 10.5/65-16: the 65 is the sidewall height as a percentage of the tire width, the same idea as a modern aspect ratio. The tire is 10.5 inches wide, the dash indicates diagonal construction, and it fits a 16-inch rim.

The size of the number after the slash is a useful clue: a small value like 11 reads naturally as a rim width in inches, while a value like 65 reads as a percentage. Treat that as a clue, not a rule. If a marking could be read either way, confirm it against the tire manufacturer's published data for that exact size before relying on it.

Quick reference: example markings decoded

Marking	Width	After the slash	Construction	Rim diameter
750/55 R26	750 mm	Sidewall = 55% of width (412.5 mm)	Radial	26 in
600/65 R28	600 mm	Sidewall = 65% of width (390 mm)	Radial	28 in
8.3 R24	8.3 in (about 211 mm)	None	Radial	24 in
12.4-28 or 12.4 X28	12.4 in (about 315 mm)	None	Diagonal	28 in
12.4/11-28	12.4 in	Rim width = 11 in	Diagonal	28 in
10.5/65-16	10.5 in	Sidewall = 65% of width	Diagonal	16 in

Decoding a marking step by step

A simple worksheet keeps the units and conventions straight. Copy the complete marking from the sidewall first, including any letters and separators, then fill in each field. The example below is an illustrative worked entry for 600/65 R28.

Complete marking   | 600/65 R28
Width and unit     | 600 mm
Slash meaning      | aspect ratio: 65%
Construction       | R = radial
Rim diameter, unit | 28 inches

Sidewall height    = 600 x 0.65 = 390 mm

• 1. Record the full marking exactly as printed, so a dash, X or R is not lost.
• 2. Decide the width unit: a three-digit width such as 600 or 750 is millimetres; a decimal such as 8.3 or 12.4 is inches.
• 3. If there is a slash, decide which convention applies: percentage of width (modern sizes and formats like 10.5/65-16) or rim width in inches (formats like 12.4/11-28).
• 4. Identify construction: R for radial, a dash or X for diagonal.
• 5. Read the last number as the rim diameter in inches.
• 6. Check your work: the rim diameter should match the number stamped on the wheel, and any calculated sidewall height should be smaller than the width whenever the ratio is below 100.

Comparing two sizes with simple calculations

Converting everything to millimetres lets you compare sizes written in different formats. Multiply inches by 25.4, and multiply a metric width by its aspect ratio to get sidewall height. A nominal overall diameter is the rim diameter plus two sidewall heights.

750/55 R26
  Sidewall  = 750 x 0.55        = 412.5 mm
  Rim       = 26 x 25.4         = 660.4 mm
  Overall   = 660.4 + 2 x 412.5 = 1485.4 mm (nominal)

600/65 R28
  Sidewall  = 600 x 0.65        = 390 mm
  Rim       = 28 x 25.4         = 711.2 mm
  Overall   = 711.2 + 2 x 390   = 1491.2 mm (nominal)

This example shows why the marking alone can mislead: the two tires have different widths, ratios and rim sizes, yet their calculated overall diameters differ by only about 6 mm. These figures are derived from the nominal marking, not measured dimensions, and they say nothing about whether a tire is approved for a particular tractor, rim or load. For a replacement or size change, use the tire manufacturer's published dimensions and the tractor maker's approved tire list.