Loader Tire Sizes: How to Read Markings, Use Size Charts and Convert Inch and Metric Sizes

Most loader tire sizes are printed on the sidewall in one of two formats: an inch-based size such as 23.5R25 or a metric size such as 750/65R25. In 23.5R25, 23.5 is the nominal section width in inches, R means radial construction, and 25 is the rim diameter in inches. In 750/65R25, 750 is the nominal section width in millimetres, 65 is the aspect ratio (sidewall height as a percentage of width), R means radial, and 25 is the rim diameter in inches. From these numbers you can estimate sidewall height, overall diameter and circumference. Actual dimensions and whether a tire fits a given machine must still be confirmed with the tire manufacturer's data and the equipment specifications.

How do you read loader tire size markings?

A loader tire size describes the tire's nominal width, its construction type and the rim diameter it mounts on. Some formats also include the aspect ratio. Each part of the code has a specific unit, and the units are often mixed. The rim diameter is almost always in inches, even when the width is given in millimetres.

Format	Example	Width	Aspect ratio	Construction	Rim diameter	Typical use
Inch, wide-base	23.5R25 or 23.5-25	23.5 in	Not printed; set by the size series	R = radial, dash = bias	25 in	Wheel loaders
Metric	750/65R25	750 mm	65%	R = radial	25 in	Wheel loaders, low-profile alternatives
Inch width with aspect ratio	12.5/80-18	12.5 in	80%	Dash = bias	18 in	Compact loaders, backhoe loaders
Low-section inch	19.5L-24	19.5 in	L = low section height	Dash = bias	24 in	Backhoe loaders
Inch, skid steer	12-16.5	12 in	Not printed	Dash = bias	16.5 in	Skid steer loaders

Inch sizes for wheel loaders (example: 23.5R25)

An inch size such as 23.5R25 gives the nominal section width (23.5 inches) and the rim diameter (25 inches). The aspect ratio is not written in the code. It is built into the standard wide-base size series, so you cannot calculate an exact sidewall height from the marking alone. Common wide-base wheel loader sizes on 25-inch rims include 17.5-25, 20.5-25, 23.5-25, 26.5-25 and 29.5-25. Each is available in bias (dash) or radial (R) versions.

Metric sizes for wheel loaders (example: 750/65R25)

A metric loader tire size such as 750/65R25 states three numbers directly: section width in millimetres (750 mm), aspect ratio as a percentage (65%), and rim diameter in inches (25 in). Because the aspect ratio is printed, the sidewall height and overall diameter can be estimated from the marking alone. The 65 series is a lower-profile, wider format. It is often offered as an alternative to a narrower wide-base inch size of similar overall diameter.

Skid steer, compact and backhoe loader sizes

Smaller loaders use different rim diameters and formats.

• Skid steer loaders: sizes such as 10-16.5, 12-16.5 and 14-17.5 give the nominal width in inches and a 16.5 in or 17.5 in rim diameter. These half-inch rim diameters use a different rim design from 16 in or 17 in rims, so the two are not interchangeable.
• Compact and backhoe loaders: 12.5/80-18 combines an inch width (12.5 in) with a percentage aspect ratio (80%). 19.5L-24 uses the letter L to indicate a low section height.
• Service marking: many skid steer and industrial tires carry NHS (Not for Highway Service), meaning they are not intended for road use.

What the dash, R and other construction letters mean

The separator between the width and the rim diameter shows the internal construction of the tire:

• R: radial construction.
• Dash (-): bias (cross-ply) construction.
• TL: tubeless.
• TT: tube type.

A bias and a radial tire with the same nominal size are designed to similar dimensional standards. However, they differ in load rating, deflection and actual measured dimensions.

What do the performance markings on a loader tire mean?

Performance markings tell you how much load a loader tire can carry, at what speed, and what tread type it has. They are separate from the size and do not describe dimensions. Two tires with identical size markings can have very different load ratings and tread depths.

Marking	Example	What it means	What it does not tell you
Ply rating (PR)	16PR	A strength and load index, mainly for bias tires. It is not a literal count of plies.	Size or tread depth
Star rating	★, ★★, ★★★	The load capacity level of a radial OTR tire at its specified inflation. More stars indicate a higher load rating.	Dimensions
Load index and speed symbol	e.g. a number followed by A2 or B	The load index is looked up in a load index table to find the maximum load. The speed symbol gives the reference speed: A2 = 10 km/h, A8 = 40 km/h, B = 50 km/h. Loader tires often show two pairs, giving different loads at different speeds.	Size or tread type
TRA tread code (loader/dozer)	L-2, L-3, L-4, L-5, L-3S, L-4S, L-5S	L-2 = traction tread. L-3 = rock tread. L-4 = rock, deep tread. L-5 = rock, extra deep tread. The suffix S = smooth tread.	Exact tread depth in millimetres

The tread code matters for dimensions. A deeper tread (L-4 or L-5) adds material to the outside of the tire, so the actual overall diameter is usually larger than an L-3 tire of the same nominal size.

How do you calculate sidewall height, overall diameter and circumference?

For any loader tire size that includes an aspect ratio, you can calculate the nominal geometry in four steps. Work in one unit throughout. The examples below use millimetres, with 1 inch = 25.4 mm.

1. Find the sidewall height: section width × (aspect ratio ÷ 100). For 750/65R25: 750 × 0.65 = 487.5 mm.
2. Convert the rim diameter to millimetres: rim inches × 25.4. For a 25 in rim: 25 × 25.4 = 635 mm.
3. Find the overall diameter: rim diameter + 2 × sidewall height. 635 + 2 × 487.5 = 1,610 mm, which is about 63.4 in.
4. Find the circumference: overall diameter × π. 1,610 × 3.1416 ≈ 5,058 mm, or about 5.06 m.

The same method works with inch widths. For 12.5/80-18, the sidewall height is 12.5 × 0.80 = 10 in. The overall diameter is 18 + 2 × 10 = 38 in (about 965 mm). The circumference is 38 × π ≈ 119.4 in.

To estimate revolutions per kilometre, divide 1,000,000 by the circumference in millimetres. For 750/65R25: 1,000,000 ÷ 5,058 ≈ 198 revolutions per km, based on calculated geometry.

Loader tire size conversion: inch to metric

A loader tire size conversion between inch and metric formats starts with the section width: multiply inches by 25.4 to get millimetres, or divide millimetres by 25.4 to get inches. A conversion only shows how close two sizes are. It does not make them interchangeable. Rim diameter normally stays in inches in both systems.

• 17.5 in × 25.4 = 444.5 mm. This is why 17.5R25 is commonly cross-referenced with 445/80R25 in some catalogues.
• 20.5 in × 25.4 = 520.7 mm
• 23.5 in × 25.4 = 596.9 mm
• 26.5 in × 25.4 = 673.1 mm
• 29.5 in × 25.4 = 749.3 mm

When a wide-base inch size is compared with a 65-series metric size, the useful comparison is usually overall diameter, not width. A 65-series tire is wider than the inch size it is compared with, but it has a lower sidewall, so the overall diameter can end up similar. For example, 650/65R25, 750/65R25 and 850/65R25 are commonly discussed as wider alternatives to 20.5R25, 23.5R25 and 26.5R25. Whether a specific alternative is approved for a specific loader depends on the manufacturers' data.

Loader tire size chart: calculated dimensions

The loader tire size chart below shows calculated nominal geometry, not catalogue measurements. For the metric 65-series sizes, the printed aspect ratio is used. The wide-base inch sizes have no printed aspect ratio, so a simplified assumption of 0.80 is used to make a rough comparison possible. Real catalogue diameters for wide-base sizes can differ from these figures and are often larger, especially for deep-tread L-4 and L-5 versions.

Size	Section width (mm)	Aspect ratio used	Sidewall height (mm)	Calculated overall diameter (mm / in)	Calculated circumference (mm)
17.5R25	444.5	0.80 (assumed)	355.6	1,346 / 53.0	4,229
20.5R25	520.7	0.80 (assumed)	416.6	1,468 / 57.8	4,612
650/65R25	650	0.65 (printed)	422.5	1,480 / 58.3	4,650
23.5R25	596.9	0.80 (assumed)	477.5	1,590 / 62.6	4,995
750/65R25	750	0.65 (printed)	487.5	1,610 / 63.4	5,058
26.5R25	673.1	0.80 (assumed)	538.5	1,712 / 67.4	5,378
850/65R25	850	0.65 (printed)	552.5	1,740 / 68.5	5,466

The chart shows the expected pattern. Each 65-series size is clearly wider than the inch size above it, while its calculated overall diameter is close. For example, 650/65R25 is about 129 mm wider than 20.5R25 (650 − 520.7). Under these assumptions, its calculated diameter is only about 12 mm larger (1,480 vs 1,468 mm), a difference of roughly 0.8%.

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

Each of the three size values affects sidewall height, overall diameter and circumference in a predictable way.

• Section width: at a fixed aspect ratio, a wider tire also has a taller sidewall, because sidewall height is a percentage of width. Going from 650/65R25 to 750/65R25 adds 100 mm of width and 65 mm of sidewall height. That raises the calculated overall diameter by 130 mm.
• Aspect ratio: at a fixed width, a lower aspect ratio means a shorter sidewall. Each millimetre of sidewall height adds two millimetres to overall diameter, since the sidewall appears above and below the rim.
• Rim diameter: if the sidewall stays the same, each extra inch of rim diameter adds 25.4 mm to overall diameter. A 25 in and a 29 in rim tire are therefore never interchangeable without a different wheel.
• Circumference: circumference changes in the same proportion as overall diameter. A 1% larger diameter means about 1% more distance travelled per revolution and about 1% fewer revolutions per kilometre.

Calculated geometry vs actual dimensions and fitment

Calculated loader tire dimensions are nominal estimates. Real tires differ for several reasons:

• Tread depth: L-4 and L-5 deep and extra-deep treads add to the overall diameter. Wear reduces it.
• Rim width: the measured section width depends on the rim width the tire is mounted on. Each size has approved rim widths; 23.5-25 tires, for example, are commonly fitted to 19.50-25 rims.
• Load and inflation: under load, the tire deflects, so the static loaded radius and rolling circumference are smaller than the free, unloaded calculation.
• Manufacturer variation: nominal sizes follow industry standards, but each tire line has its own published overall diameter and width.

Fitment compatibility is a separate question from geometry. A replacement size for a loader also has to meet these conditions:

• It matches the rim diameter and an approved rim width.
• It carries the required load at the machine's operating speed.
• It clears fenders, articulation points and the frame.
• It keeps front and rear rolling circumferences matched closely enough for the drivetrain.

Four-wheel-drive loaders are sensitive to front-to-rear diameter mismatch, so tires on both axles are normally kept to the same size and similar wear. Use the size marking and calculations to understand and compare tires. Use the tire manufacturer's data book and the loader maker's approved tire list to confirm what actually fits.