Tire tools
Embed code availableAg tire size conversion calculator
This converter converts metric agricultural tire codes to standard-inch equivalents, and inch codes to metric. It lists section width, sidewall height, overall diameter, rim diameter, rolling radius, and circumference for the entered size.
Method
How tire dimensions are calculated
Agricultural tires are labeled in two systems. A metric code such as 520/85R42 gives the section width in millimeters, the aspect ratio (sidewall height as a percentage of width), and the rim diameter in inches. A standard-inch code such as 20.8-42 gives the nominal section width and rim diameter in inches. It does not state the sidewall height.
Sidewall height = section width × aspect ratio. Overall diameter = rim diameter + 2 × sidewall height. Rolling radius = overall diameter ÷ 2. Circumference = π × overall diameter. Each value is shown in inches and millimeters.
For inch codes the calculator uses an 85% aspect ratio unless another value is entered. The equivalent code is the nearest width in the standard agricultural size table (7.5 to 30.5 inches, 190 to 800 mm), matched the same way tire catalogues match sizes.
The outputs are nominal catalogue dimensions. Actual mounted size varies with brand, tread pattern, rim width, and inflation pressure. Check 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. 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. For a low-profile size (for example a 70- or 65-series), enter the 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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Rated horsepower, operating weight, and tire sizes for each model are listed in the tractor spec database. Models can also be browsed by manufacturer.