Agricultural Tire Dimensions and Size Calculator

Agricultural tire sizes are printed on the sidewall and describe the tire's nominal dimensions, construction type, and rim diameter. The two main systems are the older standard (inch) system and the newer metric system. A tire marked 18.4-38 is 18.4 inches wide with a 38-inch rim diameter, while 520/85R42 means 520 mm wide, an 85% aspect ratio, radial construction, and a 42-inch rim. These numbers help you calculate the tire's sidewall height, overall diameter, and circumference so you can compare sizes and check fitment.

What Do the Numbers on an Agricultural Tire Mean?

Agricultural tire markings follow either a standard inch format or a metric format. Each number gives a specific dimension, and the units depend on the format.

Standard (Inch) Markings

The older system, still common on many tractor and implement tires, uses two numbers separated by a hyphen or an "x". Example: 18.4-38 or 16.9x28.

• The first number (18.4) is the section width in inches. This is the width of the tire from sidewall to sidewall when mounted and inflated, not the tread width.
• The second number (38) is the rim diameter in inches. This is the diameter of the wheel the tire fits.
• There is no aspect ratio in this system. The sidewall height is traditionally about 80–90% of the section width, giving a tall, flexible sidewall for traction and ride comfort.

Metric Markings

Metric sizes, common on newer radial tractor tires, look like this: 520/85R42.

• 520 = section width in millimeters (mm).
• 85 = aspect ratio. This is the sidewall height expressed as a percentage of the section width. Here, the sidewall height is 85% of 520 mm = 442 mm.
• R = radial construction. The absence of "R" (e.g., 520/85-42) usually means bias-ply.
• 42 = rim diameter in inches.

Some metric tires may also have a load index and speed symbol, but these are less critical for basic dimension comparison.

How to Read Other Markings on Agricultural Tires

Beyond size, you may see additional letters or numbers that indicate construction, load capacity, and intended use.

• R / D / B: Radial, Diagonal (bias), Bias. Radial tires have flexible sidewalls and a smoother ride; bias tires are more rigid and often used for heavy loads.
• Load index: A number that corresponds to the maximum load the tire can carry at a given inflation pressure. Example: 152A8 means a load index of 152 and a speed symbol A8 (40 km/h).
• "IMP" or "Implement": Indicates a tire designed for implements like wagons or planters, often with a lower speed rating.
• "R-1", "R-1W", "R-2": Tread pattern codes. R-1 is the standard agricultural traction tread, R-1W is a deeper tread for wet conditions, R-2 is for rice and very wet soils.

How Width, Aspect Ratio, and Rim Diameter Affect Overall Dimensions

The numbers on the sidewall allow you to calculate the tire's sidewall height, overall diameter, and circumference. These are useful for comparing tire sizes and checking clearance.

• Sidewall height = section width × aspect ratio (for metric) or about 85% of section width (for standard inch sizes).
• Overall diameter = rim diameter + (2 × sidewall height). Remember to convert units: if section width is in mm and rim diameter in inches, convert mm to inches (1 inch = 25.4 mm).
• Circumference = π × overall diameter. This is important for gearing and speedometer accuracy; a difference in circumference means a difference in travel distance per revolution.

Example Calculation

Compare a standard 18.4-38 and a metric 520/85R38.

1. 18.4-38:
   • Width = 18.4 inches
   • Sidewall height ≈ 0.85 × 18.4 = 15.6 inches
   • Overall diameter = 38 + (2 × 15.6) = 69.2 inches
   • Circumference = π × 69.2 ≈ 217.4 inches
2. 520/85R38:
   • Width = 520 mm = 20.47 inches
   • Sidewall height = 0.85 × 520 mm = 442 mm = 17.4 inches
   • Overall diameter = 38 + (2 × 17.4) = 72.8 inches
   • Circumference = π × 72.8 ≈ 228.7 inches

The metric tire is about 3.6 inches larger in overall diameter, which would change ground speed and may require fender clearance checks.

What a Tire Size Calculator Can Tell You

An agricultural tire size calculator automates these geometric calculations. You input the tire size (either standard or metric) and it returns the section width, sidewall height, overall diameter, and circumference, often in both inches and millimeters. Some calculators also let you compare two sizes side by side.

However, the calculator gives nominal dimensions based on the tire's designated size. The actual tire dimensions can vary slightly depending on the manufacturer, rim width, inflation pressure, and load. Always check the manufacturer specifications for the exact tire you plan to use.

Calculated Geometry vs. Actual Tire Dimensions and Vehicle Fitment

While the calculated overall diameter is useful for narrowing down options, it does not guarantee fitment. The vehicle's rim diameter must match the tire's rim diameter exactly. The section width must fit within the available space without rubbing against fenders, chassis, or other components. The overall diameter affects clearances and, if different from the original, can change the tractor's ground speed, gear ratios, and implement timing.

For implements like planters or sprayers, the tire's load capacity is critical. A tire with the right dimensions but insufficient load rating can fail. Therefore, use the size calculator for initial comparison, but always verify with manufacturer data and the equipment's owner's manual.

Comparing Standard and Metric Agricultural Tire Sizes

Many common tire sizes have equivalents between the two systems. Here is a comparison table of a few common agricultural tire sizes, assuming standard sidewall heights for inch sizes (85%) and the specified aspect ratio for metric sizes.

Tire Size	Section Width	Sidewall Height	Overall Diameter	Circumference
16.9-28	16.9 in	14.4 in	56.8 in	178.4 in
420/85R28	16.5 in (420 mm)	14.0 in	56.0 in	175.9 in
18.4-38	18.4 in	15.6 in	69.2 in	217.4 in
520/85R38	20.5 in (520 mm)	17.4 in	72.8 in	228.7 in
20.8-42	20.8 in	17.7 in	77.4 in	243.2 in
580/85R42	22.8 in (580 mm)	19.4 in	80.8 in	253.8 in

Note that these are calculated nominal values. Actual dimensions may differ by ±1–2% depending on the tire model and rim width. When switching between a standard and metric size, a difference of up to a few percent in circumference may be acceptable, but always check with your equipment dealer to ensure proper fit and performance.

Summary of Agricultural Tire Dimension Terms

• Section width: The width of the tire from sidewall to sidewall, in inches or mm.
• Aspect ratio: The sidewall height as a percentage of the section width (metric system only).
• Rim diameter: The diameter of the wheel, in inches.
• Overall diameter: The total diameter of the mounted, inflated tire (rim + two sidewalls).
• Rolling circumference: The distance the tire travels in one revolution; calculated as π × overall diameter.
• Nominal dimensions: The dimensions implied by the size marking, not necessarily the exact measured dimensions.
• Implement tires: Tires designed for towed implements; often have specific load and speed ratings and may have different tread patterns.