The basic size of steel refers to its length, width, height, diameter, radius, inner diameter, outer diameter, and wall thickness. The legal units of measurement for steel length are meter (m), centimeter (cm), and millimeter (mm). Although inches (“) are commonly used in practice, they are not considered a legal unit of measurement.

## 1. Size range of steel

Specifying the size range of steel is an effective way to conserve materials. The range scale refers to the minimum size of the length or the length multiplied by the width, or the range of sizes for length and width that can be delivered. The production unit can produce and supply steel according to the specified size requirements.

## 2. Random length (ordinary length)

A product size (length or width) that falls within the standard range but does not have a fixed size is referred to as indefinite. Random length, also known as normal length, refers to metal materials that can be delivered within a specified length range. For example, ordinary round steel with a length of no more than 25 mm, typically ranging from 4-10 meters, can be delivered within this range.

## 3. Fixed length

Fixed length refers to cutting the metal materials into a specific size according to the order requirements. When delivering fixed length materials, the length specified in the order contract must be met. For example, if the contract specifies a fixed length of 5 meters, the delivered materials must be 5 meters long, and materials shorter or longer than 5 meters are considered non-compliant. In reality, it may not be possible to deliver materials exactly 5 meters long, so a positive deviation is allowed but negative deviation is not.

## 4. Double length

Double length refers to cutting metal materials into integral multiples of a specified size according to the order requirements. When delivering materials in double length, the length of the delivered materials must be an integral multiple of the length specified in the order contract (known as single length) plus a saw bite.

For example, if the single length specified in the contract is 2 meters, the length would be 4 meters when cut into double length, 6 meters when cut into three length, with one or two saw bites added respectively. The amount of saw bite is specified by the standard. Only positive deviation is allowed and negative deviation is not allowed when delivering double length materials.

## 5. Short feet

Short length refers to materials that are shorter than the lower limit of the random length specified in the standard, but not shorter than the shortest allowable length. For example, the standard for water and gas transmission steel pipes allows for 10% of the 2-4 meter steel pipes to be shorter in each batch. 4 meters is the lower limit of the random length, while the minimum allowable length is 2 meters.

## 6. Narrow rule

A narrow rule refers to a material whose width is less than the lower limit of the width specified in the standard, but not less than the narrowest allowed width. When delivering materials in a narrow size, care must be taken to adhere to the narrow size ratio and the narrowest size specified in the relevant standards. The diagram shows the process of producing steel.

Fig. 1 The steel production process.

## 7. Examples of steel length measurement

### 7.1 Length dimension of section steel

- There are two standard lengths for railway tracks: 12.5 meters and 25 meters.
- The dimensions of round steel, wire rod, and steel wire are calibrated in millimeters (mm) of diameter (D).
- The dimensions of square steel are calibrated in millimeters (mm) of side length (a).
- The dimensions of hexagonal and octagonal steel are calibrated in millimeters (mm) of the opposite side distance (s).
- The dimensions of flat steel are calibrated in millimeters (mm) of width (b) and thickness (d).
- The dimensions of I-beam and channel steel are calibrated in millimeters (mm) of waist height (h), leg width (b), and waist thickness (d).
- The dimensions of equilateral angle steel are calibrated in millimeters (mm) of equal side width (b) and side thickness (d). The size of unequal angle steel is calibrated in millimeters (mm) of side width (b, B) and side thickness (D).
- The dimensions of H-beam are calibrated in millimeters (mm) of web height (h), flange width (b), web thickness (t1), and flange thickness (t2).

### 7.2 Length measurement of steel plate and strip

- The thickness (d) of a steel plate is typically calibrated in millimeters (mm). The width (b) and thickness (d) of a steel strip are also calibrated in millimeters (mm).
- There are various sizes for single steel plates, such as hot rolled steel plates: 1 millimeter thick with a width of 600 millimeters and a length of 2000 millimeters; 650 x 2000 millimeters; 700 x 1420 millimeters; 750 x 1500 millimeters; 900 x 1800 millimeters; 1000 x 2000 millimeters, etc.

### 7.3 Length measurement of steel pipe

- The diameter (D), inner diameter, and wall thickness (s) of a steel pipe are typically calibrated in millimeters (mm).
- Different types of steel pipes have various sizes. For example, seamless steel pipes have an outer diameter of 50 millimeters and a wall thickness of 2.5-10 millimeters. There are 29 different types with a wall thickness of 5 millimeters and an outer diameter of 32-195 millimeters. Another example is a welded steel pipe with a nominal diameter of 25 millimeters, including 3.25 millimeter ordinary steel pipe and 4 millimeter thickened steel pipe.