Essential Guide to Surface Roughness: Rz Vs Ra

Table of Contents

Surface roughness is a critical aspect in various manufacturing and has a great impact on the functionality, aesthetics, and performance of products. Ra is the arithmetic average of roughness which measures the deviation of a surface from a mean height. 

Rz is the average maximum height and measures the difference between the highest peak and lowest valley. Components with proper surface roughness may experience less friction and wear,  promoting the functionality of parts and durability. It also promotes quality assurance which leads to customer satisfaction.

Achieving the desired surface roughness involves, selecting appropriate machining, selection of materials, and post-processing techniques. To understand surface roughness better we will look into Ra and Rz in detail and evaluate their consideration and applications.

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RZ vs RA (Image Source: engineering product design) 

Definitions and Measurement 

Several parameters are used to measure surface roughness accurately. The choice of method depends on various factors such as aesthetics. 

Roughness Average (Ra)

It is one of the commonly used parameters for measuring surface roughness. Ra is the arithmetic mean of the profile heights over a specified evaluation period. Stylus-based measurement instruments are used for measuring surface texture. 

Other instruments that can be used are optical profilers. Average roughness is measured in microns or micro inches. One micron equals roughly 40 microinches. A lower Ra value indicates a smoother surface finish while a higher Ra value indicates a rougher surface with a larger deviation from the mean line. 

Ra is used across various industries where surface finish, performance, and functionality are crucial such as automotive and medical devices. It is a good first indicator of the overall height of the surface texture. 

Ten Point Height of Irregularities (Rz)

This is the measure of the vertical distance from the highest peak to the lowest valley within five sampling lengths. It quantifies the height variation of the surface profile focusing on the most extreme irregularities. 

Rz is measured using profilometers or other measurement instruments. The surface of the material is scanned with a stylus recording the profile of surface irregularities. 

To acquire the Rz value the five highest peaks and five lowest valleys within the evaluation length are identified and the average distance between them is calculated. Rz values are expressed in micrometers or microinches.

A higher Rz value indicates greater peak to valley variation suggesting a rougher texture with more irregularities whereas a lower Rz value indicates smoother surface texture with small height variation. 

Rz provides valuable information about extreme height variation present on the surface. This may impact the functionality, performance, and aesthetic properties of products. It is suitable for applications where height variation is critical. An example is the aerospace and automotive industries.

Differences Between Ra and Rz 

Ra and Rz are important parameters for measuring surface roughness but they differ in their approach. Some of the differences include the following,

Ra is calculated by averaging the absolute values of the deviation of the roughness profile from the mean line over a specified evaluation length. This provides a measure of the general surface texture.  

On the other hand, Rz is determined by identifying the five highest peaks and five lowest valleys within the evaluation length and calculating the average distance between them. This gives insight into the height irregularities of the surface.

Ra is commonly used to control surface finish requirements in the manufacturing process such as aesthetic surfaces while Rz is applicable where extreme height variations can affect functionality and performance such as sliding contacts.

Ra focuses on the general texture and smoothness of the surface providing an overall measure of surface roughness. However, Rz offers insight into the maximum peak-to-valley variation on the surface capturing extreme variation.

Measurement Methodologies and Interpretation

Measuring Ra and Rz accurately involves taking into account several key steps. Below are the specific processes that are carried out to calculate these values.

The first step is the preparation stage where the surface to be measured is prepared by ensuring it is clean and free from any contaminant. The next step is setting up the instrument to be used such as a profilometer following the guidelines of the manufacturer. 

To ensure accuracy proper calibration of the instrument is critical. Selecting the evaluation length is the next phase. Choose an appropriate evaluation length based on surface characteristics and standards being followed to ensure the accuracy of the measurements.

Initiate the scanning process with the stylus on the surface. The instrument traces the contour of the surface irregularities. After this step data generated by the instrument is collected and later used in the calculation of the Ra and Rz values.

A lower Ra value represents a smoother surface texture with a smaller average deviation from the mean line. Furthermore, a higher value indicates a rougher surface texture with a larger average deviation. 

Ra is commonly used to assess quality smooth surfaces to promote desired aesthetics requirements. Smoother surfaces with lower Ra values have reduced friction and wear.

A higher Rz value shows greater peak-to-valley variation, suggesting a rougher surface texture with more irregularities. A lower Rz however indicates a smoother surface texture with small peak to valley variation. Rz provides insights into surface irregularities that can affect functionality and performance.

Understanding and accurately interpreting surface parameters such as Ra and Rz, helps ensure the quality and reliability of the products. It also helps in optimizing performance and eventual contribution to customer satisfaction. 

Applications and Considerations

Below are some of the considerations for selecting either Ra or Rz based on part requirements:

Choosing Between Ra and Rz

Choosing between these two parameters depends on various factors such as functional performance and industry standards. Here are some guidelines on which parameter to use.

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Rough Surface (Image Source: Carbide3d)

Functional performance

If the parts operate in dynamic or sliding applications, consider using Rz to get the maximum peak-to-valley height variation. These irregularities can impact friction, wear, and performance of the part. It also provides insight into the effectiveness of sealing surfaces to prevent leaks.

Aesthetics consideration

Ra is more appropriate for parts that require a smooth visual appearance. It is often used to ensure uniformity and consistency in surface finish and enhance the overall aesthetics of the product.

Many industries have established standards for surface roughness requirements based on specific applications. These standards and regulations clarify whether Ra or Rz should be used for surface finish assessment.

In some cases, both Ra and Rz can be used to provide a more comprehensive assessment of surface texture and performance.

Before completing surface roughness specification, conduct testing and validation to ensure that the selected parameter meets the desired performance and quality standards. Refine surface roughness specification based on feedback and testing results.

Practical Implications

The impact of surface roughness parameters is very significant. Here are some of these factors. 

Surface roughness directly affects machining processes. Higher surface roughness can lead to tool wear and higher energy consumption. Proper control of surface roughness parameters is important to optimize machining efficiency.

Surface roughness parameters guide the selection of finishing methods and determine the required level of precision. Consistency and quality control over surface roughness ensure uniform finishing results.

Manufacturers must consider surface roughness requirements to ensure enhanced functionality and performance of parts. Continuous monitoring of surface roughness during manufacturing processes can help detect deviations and the necessary adjustments can be made.

Surface Roughness on Technical Drawings

When specifying surface roughness on technical drawings it is essential to use clear and standardized symbols to ensure clarity and compliance with manufacturing standards. Here are some strategies to achieve this:

Utilize standardized symbols as per ISO standards to represent surface roughness on technical drawings. These symbols are universally recognized by manufacturers worldwide.

Incorporate clear labels of the surface roughness symbol with the proper roughness parameters. This ensures that manufacturers understand the specific surface finish requirements. 

Include detailed notes adjacent to the surface roughness symbol to provide additional information and clarification on the requirements.

Clearly define tolerance zones to indicate acceptable deviation to help ensure consistency and product quality. Specify the evaluation length over which the roughness parameter is measured. This promotes consistency in measurement and interpretation.

Consider the manufacturing process by indicating the preferred method by incorporating symbols. For parts with specific aesthetics provide visual examples to give a clear visual reference to the manufacturer.

By following these strategies for specifying surface roughness on technical drawings you can ensure compliance with manufacturing standards. This leads to the production of high-quality parts with desired surface finish.

Challenges and Conversion Between Ra and Rz 

Converting between Ra and Rz can be challenging due to the difference in their measurement techniques and surface roughness profiles. Here are some challenges and considerations.

Different measurement methods: Ra and Rz are calculated from different measurement methodologies focusing on distinct aspects of surface roughness. This difference makes direct conversion between the two parameters challenging.

Different standards and specifications may define Ra and Rz measurement methods differently. This leads to variations in conversion factors. It is important to refer to specific guidelines being followed to ensure accuracy and consistency.

Surface Profiles can exhibit varying characteristics making Conversation between Ra and Rz challenging. There is no linear relationship between Ra and Rz due to the different ways they quantify surface roughness.

Relying solemnly on conversion ratios can lead to misinterpretation of surface roughness properties. It is important to focus on controlling and optimizing one roughness parameter rather than trying and convert between them.

Best Practices for Measuring Surface Roughness 

Follow standardized measurement methods and equipment such as profilometers to measure surface roughness accurately. This helps to recognize industry standards and specifications.

Select the appropriate evaluation length based on the specific requirement of the application. Use the specified parameter indicated on the drawings. The evaluation length should be selected to capture relevant surface features.

Ensure that measurement equipment is properly calibrated according to manufacturer’s guidelines and standards. This helps to maintain measurement accuracy and reliability.

Conduct multiple measurements at different areas on the surface to account for surface variation. This ensures an accurate representation of surface texture.

Seek feedback and collaboration from various teams involved in the manufacturing process to help refine surface roughness specifications to meet desired requirements.

Through following these practices manufacturers can effectively measure surface roughness, ultimately contributing to the production of high-quality products with desired surface finish.

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Surface Finishes (Image Source: FacFox) 


Understanding and accurately measuring surface roughness based on Ra and Rz are essential for enhanced manufacturing practices. These parameters greatly influence product quality, performance, and customer satisfaction. 

Accurate measurement promotes compliance with industry standards and optimizing manufacturing processes. Surface roughness is greatly influenced by design considerations, material selection, and the machining process.

Manufacturers incorporating best practices in specifying surface roughness is one of the key factors that ensure their success in today’s competitive environment. By fostering effective communication with customers to ensure parts meet precise expectations, you can enhance product quality and customer satisfaction.

Effective communication with customers is key to understanding their precise expectation concerning surface finish. Adhering to best practices in specifying surface roughness empowers you to deliver products that meet customer expectations. This enhances your reputation and reduces costs associated with rework.


Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

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