How to interpret forklift press-on and pneumatic tire size markings

The short answer

Forklift tire sizes use several formats, and the same position in a marking does not always mean the same thing. A press-on tire, the solid tire pressed onto a smooth steel band, normally lists three dimensions in this order: outer diameter, width and rim diameter. A pneumatic tire, or a solid tire built to a pneumatic size, usually starts with the section width and ends with the rim diameter. Many of those markings do not state the outer diameter at all. Before you read any number, identify the format. Then decode each position by the rules of that format.

Some terms used below:

Outer diameter (OD): the overall height of the tire from tread to tread.
Width (section width): how wide the tire is across.
Rim diameter: the diameter of the wheel or band the tire mounts on. It is not the size of the tire itself.
Aspect ratio: sidewall height expressed as a percentage of width. It appears in some pneumatic metric sizes.

Press-on tire markings

Inch press-on sizes read as OD × width × rim diameter. Continental's material handling technical data book uses 16×6×10½ as its example: the tire is 16 in tall, 6 in wide and fits a 10½ in rim.

Metric press-on sizes follow the same three-part logic in millimetres. In Continental's example 200/75-100, the outer diameter is 200 mm, the width is 75 mm and the rim diameter is 100 mm. The middle number is easy to misread. In this press-on format, 75 is a width in millimetres. It is not an aspect ratio, even though the slash makes it look like a pneumatic metric size. The rim value is a useful clue. A rim figure such as 100 is plainly millimetres, while pneumatic forklift sizes give the rim in inches as small numbers such as 8 or 12. Confirm the format against the manufacturer's notation guide.

Pneumatic tire markings

Pneumatic sizes come in a width-first style and an OD-first style. The letter R means radial construction. A dash in that position usually indicates a non-radial (cross-ply) size.

Radial, inch width: 7.00 R12 means a 7 in section width, radial construction and a 12 in rim diameter. Continental states that this format implies an aspect ratio of 100%.
Radial, metric width: 180/70 R8 means a 180 mm section width, a 70% aspect ratio, radial construction and an 8 in rim diameter. The width is metric but the rim is in inches.
Cross-ply, width first: 7.00-12 means a 7 in width and a 12 in rim diameter. The outer diameter is not encoded.
Cross-ply, OD first: 18×7-8 means an 18 in outer diameter, a 7 in width and an 8 in rim diameter.

Neither radial format above states the outer diameter directly. When you need that figure, take it from the manufacturer's dimension table for the specific product.

Quick reference

Format	Example	First value	Middle value	Last value	OD stated?
Press-on, inch	16×6×10½	OD 16 in	Width 6 in	Rim 10½ in	Yes
Press-on, metric	200/75-100	OD 200 mm	Width 75 mm	Rim 100 mm	Yes
Pneumatic radial, inch	7.00 R12	Width 7 in	R = radial (100% aspect)	Rim 12 in	No
Pneumatic radial, metric	180/70 R8	Width 180 mm	Aspect 70%, R = radial	Rim 8 in	No
Pneumatic cross-ply, width first	7.00-12	Width 7 in	—	Rim 12 in	No
Pneumatic cross-ply, OD first	18×7-8	OD 18 in	Width 7 in	Rim 8 in	Yes

Both 16×6×10½ and 18×7-8 have three numbers and lead with the outer diameter. On those markings, tire type and separators help you tell press-on and pneumatic formats apart. The number of figures alone will not.

Other sidewall markings are not size

Sidewalls also carry the manufacturer name, product name, construction type and other codes. Continental's illustrated 7.00 R12 radial also shows 136 A5 and 16 PR. Record these separately from the size. They describe load and service characteristics and ply rating, not dimensions. Two tires with the same size designation can carry different codes. Continental directs load-capacity questions to the actual technical data for the product, so look up load capacity there rather than inferring it from the size.

Nominal size versus catalog dimensions

A size designation is a nominal label. The measured dimensions of a real product can differ from it, and can differ between products with the same label. Sentry's solid tire catalog shows this. The products are solid pneumatic-style tires, meaning solid rubber built to fit pneumatic sizes, not air-filled tires.

Sentry K-pattern 7.00-12: listed width 6.70 in, overall diameter 26.00 in.
Sentry S-pattern 7.00×12: listed width 6.70 in, overall diameter 26.30 in.

Both carry "7.00" and "12" in their size, yet the listed width is 6.70 in, not 7.00 in. The two overall diameters also differ by 0.30 in. The same tables include a separate column labelled "Rim Size (in)" showing 5.00 for both products. That field is plainly not the 12 in rim diameter from the designation, so do not swap one for the other. Keep each catalog field under its original label.

Estimating outer diameter from a pneumatic size

When a marking gives width, aspect ratio and rim diameter, you can calculate a nominal outer diameter. This illustrative arithmetic uses the formula below. It does not replace the manufacturer's measured dimensions.

Nominal OD = rim diameter + 2 × (width × aspect ratio)

180/70 R8:
  sidewall = 180 mm × 0.70        = 126 mm
  rim      = 8 in × 25.4          = 203.2 mm
  OD       = 203.2 + 2 × 126      = 455.2 mm (about 17.9 in)

7.00 R12 (100% aspect ratio):
  sidewall = 7 in × 1.00          = 7 in
  OD       = 12 + 2 × 7           = 26 in

The 26 in estimate for a 7.00 size lands close to Sentry's listed 26.00 in and 26.30 in. The 0.30 in spread between those two products, and the gap between the nominal 7 in width and the listed 6.70 in, show why the calculation is a starting point. When the actual dimension matters, use the published figure for that exact product.

A worksheet for recording markings

A simple record keeps decoded values, catalog values and approvals from blending together. Use these columns:

Manufacturer and product name
Construction (press-on, radial, cross-ply, solid pneumatic-style)
Exact designation, copied character for character
Additional markings (for example 136 A5, 16 PR)
Notation reference used to decode it
Decoded outer diameter, width and rim diameter, with units
Catalog dimensions, each under its original field label

Work in this order:

1. Copy the full marking exactly as printed. Include separators such as ×, /, - and R, because they help identify the format.
2. Match the marking to its documented format and note which reference you used.
3. Fill in only the dimensions the notation actually encodes. If the format does not state the outer diameter, leave that cell blank.
4. Enter catalog measurements separately, keeping the manufacturer's field names.

What decoding does not tell you

Decoding a size tells you what the notation means. It does not tell you whether a tire suits a particular forklift. A matching designation does not guarantee matching dimensions, load capacity or rim fit, as the Sentry and Continental examples show. Treat any replacement as a separate decision. Check it against the equipment manufacturer's requirements, the wheel or rim specification, and the current dimensional and load data for the exact replacement product. Your tire supplier or forklift service provider can confirm it.