Medium-Sized Rocks: Mastering the 1/5th Rule for Efficient Operations

In the world of manufacturing, optimizing operations for efficiency and productivity is a constant endeavor. Lean principles have emerged as a powerful approach to streamline manufacturing processes and reduce waste. A crucial component of Lean Manufacturing is determining the target cycle time for each operation, a concept we’ll delve into in this blog post.

Understanding the Bill of Operations

Before we dive into the rule of thumb for setting target cycle times, let’s briefly explore the concept of a Bill of Operations. Similar to a Bill of Materials (BOM), which lists the components required to create a product, a Bill of Operations outlines the labor tasks needed during the manufacturing process. This is a vital complement to the BOM, as it explains “how” the product comes together.

The Significance of Target Cycle Time

The key to efficient manufacturing lies in setting the right target cycle time for each operation. The target cycle time is the time allocated for an operation to ensure it aligns with customer demand. Here’s a simple formula to calculate it: available production time divided by customer demand.

For instance, if your customer requires 20 units per week and you have 10 hours of available working time each day for 5 days, your Takt Time (target cycle time) will be 2.5 hours. This means one unit should be completed every 2.5 hours to meet the weekly demand.

The 1/5th Rule of Thumb

Now, let’s address the rule of thumb for setting the target cycle time. It’s recommended that the target cycle time for each individual operation should be approximately 1/5th of the Takt Time at the highest anticipated production rate. This rule has a crucial purpose: flexibility.

Imagine trying to distribute a large rock evenly across jars. It’s impossible. Similarly, very small rocks (or grains of sand) can be distributed in any number of jars, but it’s time-consuming. The ideal scenario is to have medium-sized rocks that can be easily distributed to achieve nearly level-loaded jars. This analogy applies to labor content in manufacturing.

By adhering to the 1/5th rule, you ensure that operations are sized in a way that allows for quick adjustments when Takt Time changes. It promotes flexibility in your assembly line, making it easier to adapt to fluctuating customer demands.

Implementing the Rule

Let’s put this rule into practice. Suppose your product can be assembled in 20 man-hours. With a target cycle time of around 12 man-minutes per operation, you’ll need to break the entire build process into roughly 100 operations.

The process of defining these operations involves process mapping and time studies. You’ll document and time each step of the production process, ensuring that each operation aligns with the 1/5th rule. Additionally, each operation should be performed by a single operator in a single cell with a defined set of tools or machines.


Setting the target cycle time for each operation is a crucial step in Lean Manufacturing. The 1/5th rule of thumb ensures that your operations are sized for flexibility, allowing you to adapt to changing customer demands efficiently.

By implementing this rule and following other Lean Manufacturing principles, such as value stream mapping and waste reduction, you can transform your manufacturing processes and achieve higher levels of efficiency and productivity. It’s a journey that requires commitment, but the results are well worth the effort.

Are you looking to streamline your operations and embark on a Lean Manufacturing journey? We’re here to help. Contact us at to explore how Optegrity Solutions can assist you in managing your operations, optimizing your processes, and achieving your manufacturing goals. Let’s start this transformation together.

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