Articles on: Inventory Replenishment

Fixed Time Period Model


Effective inventory management is crucial for the success of any business. For merchants, ensuring that products are in stock when customers demand them is a balancing act that requires careful planning and strategy. One approach to optimize this process is the Fixed Time Period Model, also known as the periodic review system. In this guide, we will explore how merchants can apply this method to their businesses, particularly when using an Inventory Replenishment by Vendor Lead Time report.

Which Businesses Are Best Suited?

Selecting the right inventory management model is crucial for the success of your Shopify business. The Fixed Time Period Model is particularly well-suited for certain types of merchants and businesses. Let's explore which types of Shopify merchants can benefit the most from implementing the Fixed Time Period Model in their inventory replenishment strategies:

E-commerce Stores with Stable and Predictable Demand:

Best Fit: If your online store sells products with consistent and predictable demand patterns, the Fixed Time Period Model can be highly effective. For example, if you sell everyday household items or products with a steady year-round demand, this model simplifies your inventory management without the need for constant monitoring.

Why It's Ideal: With reliable demand, the Fixed Time Period Model streamlines your inventory management process, helping you maintain consistent stock levels. You can schedule periodic reviews and orders, reducing the need for constant manual intervention.

Businesses with Batch Ordering Opportunities:

Best Fit: Businesses that can benefit from ordering in batches to reduce order processing costs and take advantage of quantity discounts are a great fit for the Fixed Time Period Model. This includes businesses that have strong supplier relationships and need to manage bulk orders efficiently.

Why It's Ideal: This model aligns with batch ordering practices, as it involves placing orders at specific intervals. By doing so, you can save on transaction costs and negotiate better terms with suppliers when placing larger, less frequent orders.

Online Retailers Collaborating with Reliable Suppliers:

Best Fit: If your business works closely with dependable suppliers that consistently meet lead time expectations, the Fixed Time Period Model is a strong choice. This collaboration ensures that you can predict when to expect your orders.

Why It's Ideal: By having reliable suppliers, you can confidently predict lead times and make timely replenishment decisions. The Fixed Time Period Model leverages this reliability, optimizing inventory levels without the fear of unexpected supplier delays.

Small to Medium-Sized Businesses (SMBs) Seeking Simplicity:

Best Fit: Small to medium-sized businesses looking for a straightforward and easy-to-implement inventory management approach can benefit from the Fixed Time Period Model.

Why It's Ideal: The model's simplicity, compared to more complex continuous review systems, makes it accessible for SMBs with limited resources and inventory management expertise. It's a practical choice for businesses focusing on growth and simplicity.

In summary, the Fixed Time Period Model is well-suited for e-commerce businesses with stable and predictable demand, those that can benefit from batch ordering, retailers with dependable supplier relationships, and small to medium-sized businesses seeking a straightforward approach. By identifying your business type and demand patterns, you can leverage the Fixed Time Period Model to optimize your inventory management practices and streamline your Shopify operations.

Understanding the Fixed Time Period Model

In this section, we'll take a closer look at the Fixed Time Period Model, gaining a deeper understanding of its fundamental concepts and components. The Fixed Time Period Model, also known as the periodic review system, offers a structured approach to managing your inventory efficiently.

Review Period (also known as the review cycle or reorder cycle)
At the heart of the Fixed Time Period Model is the concept of a review period. The review period is the pre-defined interval between two consecutive inventory assessments. Think of it as your inventory's heartbeat – it determines how frequently you'll check your stock levels and make replenishment decisions. The review period can vary based on your specific business needs. You might choose to review your inventory daily, weekly, monthly, or at any other interval that aligns with your operation's demands.

Reorder Quantity
The Fixed Time Period Model also involves calculating the reorder quantity. This represents the amount of inventory you should order during each review period. The goal is to order enough to replenish your stock to a predetermined target level. The Economic Order Quantity (EOQ) formula is often used to calculate this quantity efficiently, taking into account factors like holding costs and ordering costs.

Fixed time period inventory management model


One of the key advantages of the Fixed Time Period Model is its simplicity. Unlike continuous review systems that require constant monitoring, this model involves periodic, scheduled reviews. Merchants can allocate their resources more efficiently because they don't need to constantly check inventory levels. This makes it an accessible choice for businesses of varying sizes.

Batch Ordering
The Fixed Time Period Model promotes batch ordering. Instead of placing orders frequently, you order in larger quantities at less frequent intervals. This approach can lead to reduced order processing costs and the potential for taking advantage of quantity discounts offered by suppliers.

Reduced Monitoring
As mentioned earlier, you're not required to continually monitor your inventory levels. Instead, you schedule periodic reviews. This not only simplifies the process but can also help reduce stress and operational workload.


Variable Demand
One of the significant challenges of the Fixed Time Period Model is its limited responsiveness to variable demand patterns. If your product demand fluctuates significantly, this model might not adapt well to those changes. The periodic reviews could lead to either overstocking or stockouts, depending on the timing of the reviews and the fluctuations in demand.

Safety Stock
To address the challenge of variable demand, you may need to maintain a safety stock. Safety stock is an additional inventory buffer that you hold to account for unexpected spikes in demand or delays in supplier lead times. While safety stock is essential, it increases your carrying costs and can tie up capital that might be used more effectively elsewhere.

Unpredictable Lead Times
The Fixed Time Period Model assumes consistent lead times for ordering and replenishing inventory. However, if your suppliers' lead times vary, you may find yourself with inadequate stock on hand when you need it. This unpredictability can challenge the model's effectiveness.

Calculating the Reorder Quantity

To calculate the order point effectively, several factors must be taken into account. These factors are instrumental in ensuring that you order just the right amount of inventory to maintain a balance between stockouts and overstock. Here are the key considerations:

Average Units Sold per day: One of the primary factors is your average demand during the review period. It's crucial to understand the typical rate at which your products are sold or used during the review interval.

Reorder Cycle: as known as the review period, is the number of days between two consecutive inventory assessments.

Lead Time: Lead time is the time it takes for your suppliers to deliver inventory once an order is placed. A dependable estimate of lead time is essential for determining when to initiate an order. It's crucial to consider not only the average lead time but also potential variations or delays.

Safety Stock Days: In the Fixed Time Period Model, safety stock is essential to account for demand variability and uncertainties in lead times. Safety stock acts as a cushion, preventing stockouts during periods of unexpected high demand or supplier delays. The amount of safety stock you maintain should be carefully calculated to strike a balance between risk and carrying costs.

Reorder Quantity Formula

Now, let's talk about the actual calculation of the reorder quantity. The reorder quantity formula is central to this process:

Reorder Quantity = [Average Units Sold per day] x ( [Reorder Cycle] + [Lead Time] + [Safety Stock Days] ) - ( [Inventory On Hand] + [Incoming Inventory] )

This quantity is typically determined based on factors such as average demand during the review period, lead time, and safety stock requirements. The reorder quantity is meant to cover the expected demand during the time it takes for new stock to arrive (lead time) and the time between two inventory reviews (reorder cycle) as well.

Example: Calculating the Reorder Quantity

Let's continue with the example of your online retail store selling smartphone accessories, but this time we'll calculate the Order Point using safety stock measured in days instead of a fixed quantity. Here are the key figures:

- Average Units Sold per day: 50 units per day
- Reorder Cycle: 30 days
- Lead Time (time it takes for your supplier to deliver): 10 days
- Safety Stock Days (measured in days): 5 days (to account for unexpected fluctuations in demand or supplier delays)
- Inventory On Hand: 600 units
- Incoming Inventory: 500 units

Now, let's calculate the Reorder Quantity using the formula:

- Reorder Quantity = [Average Units Sold per day] x ( [Reorder Cycle] + [Lead Time] + [Safety Stock Days] ) - ( [Inventory On Hand] + [Incoming Inventory] )
- Reorder Quantity = (50 units/day) x ( 30 days + 10 days + 5 days) - (600 units + 500 units)
- Reorder Quantity = 1,150 units

In this revised example, your Reorder Point is 1,150 units. This quantity will cover the sales demand during the reorder cycle plus the vendor lead time as well.

Applying the Fixed Time Period Model in Assisty

Fixed time period inventory replenishment report sample

In Assisty, we've designed an inventory management system that simplifies the application of the Fixed Time Period Model. With our intuitive interface and powerful tools, you can optimize your inventory replenishment process efficiently. Let's explore how our system, and in particular, the "Inventory Replenishment by Vendor Lead Time" report, can revolutionize your inventory management.

Introducing the Inventory Replenishment by Vendor Lead Time Report

Our "Inventory Replenishment by Vendor Lead Time" report is at the heart of the Fixed Time Period Model in Assisty. This report provides you with the insights and data you need to make informed decisions about when and how much to reorder. Here's what you can expect from this report:

Order Point Calculation: The report calculates the order point based on your specified parameters, such as average demand, lead times, and safety stock.

Replenishment Quantity Suggestion: When your current stock falls below the calculated order point, the report suggests the replenishment quantity you should order. This feature ensures that you always have the right amount of inventory on hand to meet customer demand.

Email Scheduling: We understand that consistent inventory management is crucial. Therefore, our system allows you to create schedules to automatically send the replenishment suggestion data to your email. You can set up weekly or monthly reports to stay on top of your inventory management without manual intervention.

Collaboration with Vendors: Many of our customers have found value in sharing these reports with their suppliers. By scheduling the reports to be sent to your vendors, you enable them to better prepare for the supply, fostering stronger collaboration and efficiency in your supply chain.

Export to CSV or Google Spreadsheet: To further streamline your procurement process, Assisty enables you to export the replenishment suggestions into CSV or Google Spreadsheet files. This means you can easily share the data with your vendors as a comprehensive product list to purchase. It's an efficient way to keep everyone on the same page and ensure your supply chain runs smoothly.


In the realm of inventory management, precision and efficiency are paramount. This guide has walked you through the Fixed Time Period Model and how it can revolutionize the way you handle inventory replenishment in your business. As we conclude, let's recap the key takeaways and emphasize the benefits of applying this model.

The Fixed Time Period Model simplifies the replenishment process by setting regular review intervals, calculating order points, and suggesting replenishment quantities when your inventory falls below these points. It's a versatile approach that can benefit a wide range of businesses, but it shines brightest under specific circumstances:

Stable and Predictable Demand: If your business deals with products that have consistent and predictable demand patterns, this model simplifies your inventory management process without the need for constant oversight.

Batch Ordering Opportunities: Businesses that can make the most of batch ordering to reduce costs and capitalize on quantity discounts will find the Fixed Time Period Model a valuable tool in their inventory management arsenal.

Reliable Supplier Relationships: Collaborating with dependable suppliers who consistently meet lead time expectations ensures that you can make replenishment decisions with confidence, leveraging the Fixed Time Period Model's optimization potential.

SMBs Seeking Simplicity: Small to medium-sized businesses, with an eye on simplicity and straightforward inventory management, can benefit from the model's ease of implementation.

The Fixed Time Period Model streamlines the replenishment process, reduces stockouts, and provides a cost-effective approach to inventory management, allowing you to allocate your resources more efficiently and foster better supplier relationships.

Incorporate this model into your inventory management strategy, and don't hesitate to explore the features and reports offered by Assisty, which can further enhance your inventory management efforts.

In the dynamic world of e-commerce and retail, efficiency is often the key to success. By adopting the Fixed Time Period Model, you take a step closer to achieving that efficiency, and you ensure that your business meets customer demands consistently, avoids unnecessary carrying costs, and fosters better supplier collaboration.

Start your journey towards more efficient inventory management today, and see how Assisty can empower your business to thrive in a competitive market by streamlining your inventory replenishment, fostering better supplier relationships, and optimizing your operations.

Updated on: 01/12/2023

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