Tire tools
Free · instant · embeddableAg tire size conversion calculator
Convert metric agricultural tire codes to traditional inch equivalents and see every derived dimension — section width, sidewall, overall diameter, rolling radius, and circumference. Built for dealers, mechanics, and farmers matching or replacing tires.
Transparent method
How this converter works
Agricultural tires are labeled in two systems. Metric codes such as 520/85R42 spell out the section width in millimeters, the aspect ratio (sidewall height as a percentage of width), and the rim diameter in inches. Older standard-inch codes such as 20.8-42 give only a nominal width and rim size, with the sidewall implied. The calculator parses either format, computes the physical dimensions (section width, sidewall height, overall diameter, rolling radius, and circumference in both inches and millimeters), and suggests the nearest equivalent code in the other notation.
For inch codes it assumes the traditional 85% aspect ratio unless you override it, and it matches widths against the standard agricultural size table (7.5 through 30.5 inches, 190 through 800 mm) the way tire catalogues do.
Treat the outputs as nominal catalogue dimensions. Actual mounted size varies with brand, tread pattern, rim width, and inflation pressure, so verify load capacity and clearance against the manufacturer's data sheet before buying, especially when sizing duals or replacing tires on a mechanical front-wheel-drive tractor.
FAQ
Frequently asked questions
What does a tire code like 520/85R42 mean?
The first number (520) is the section width in millimeters. The second (85) is the aspect ratio: sidewall height as a percentage of the width. R means radial construction, and the last number (42) is the rim diameter in inches. Older inch codes like 20.8-42 give the nominal width and rim diameter in inches instead.
Are metric and inch equivalents exact?
No. The equivalent code shown is the nearest standard catalogue size, not a mathematically exact match. For example, 520/85R42 and 20.8-42 are conventional equivalents even though their computed dimensions differ slightly. Always confirm fitment with the tire manufacturer’s data book before ordering.
Why do inch sizes need an aspect ratio?
Traditional inch codes do not encode sidewall height, so the calculator assumes an 85% aspect ratio, which fits most classic agricultural tires. If you are converting a low-profile size (for example a 70- or 65-series), enter its actual aspect ratio in the override field.
Why does rolling circumference matter?
Rolling circumference determines how far the tractor travels per axle revolution. On mechanical front-wheel-drive tractors, the front and rear circumferences must stay within the manufacturer’s specified lead/lag ratio; a mismatched replacement size can cause driveline wind-up, tire scrubbing, and premature wear.
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